%% Books @book{fds4165, Author = {S. Chandrasekharan}, Title = {CRITICAL BEHAVIOR AT THE QCD PHASE TRANSITION WITH TWO MASSLESS QUARK FLAVORS}, Booktitle = {Continuous Advances in QCD}, Publisher = {World Scientific}, Editor = {Andrei V. Smilga}, Year = {1994}, Key = {fds4165} } @article{Chandrasekharan:1994kx, Author = {Chandrasekharan, S.}, Title = {CRITICAL BEHAVIOR AT THE QCD PHASE TRANSITION WITH TWO MASSLESS QUARK FLAVORS}, Booktitle = {Continuous Advances in QCD}, Publisher = {World Scientific}, Editor = {Smilga, A.V.}, Year = {1994}, Key = {Chandrasekharan:1994kx} } @book{fds4130, Author = {S. Chandrasekharan}, Title = {SOLVING SIGN PROBLEMS WITH MERON ALGORITHMS}, Series = {Springer Proc. Phys. 86, pp 28-42}, Booktitle = {Computer Simulations in Condensed Matter Physics XIII}, Publisher = {Springer}, Editor = {D.P. Landau and S.P.Lewis and H.-B.Shuttler}, Year = {2000}, Month = {January}, Key = {fds4130} } @book{fds4128, Author = {S. Chandrasekharan}, Title = {NOVEL QUANTUM MONTE CARLO ALGORITHMS FOR FERMIONS}, Booktitle = {Quantum Monte Carlo: Recent Advances and Common Problems in Condensed Matter Physics and Field Theory}, Publisher = {EDIZIONI ETS}, Editor = {M. Compostrini and M.P. Lomardo and F. Paderiva}, Year = {2001}, Abstract = {Recent research shows that the partition function for a class of models involving fermions can be written as a statistical mechanics of clusters with positive definite weights. This new representation of the model allows one to construct novel algorithms. We illustrate this through models consisting of fermions with and without spin. A Hubbard type model with both attractive and repulsive interactions becomes tractable using the new approach. Precision results in the two dimensional attractive model confirm a superfluid phase transition in the Kosterlitz-Thouless universality class.}, Key = {fds4128} } @book{fds16196, Author = {S. Chandrasekharan and U.-J. Wiese}, Title = {AN INTRODUCTION TO CHIRAL SYMMETRY ON THE LATTICE}, Booktitle = {Prog. Part. Nucl. Phys. Vol. 53, issue 1,}, Year = {2004}, Key = {fds16196} } %% Papers Published @article{fds245733, Author = {Chandrasekharan, S}, Title = {Fermions with a domain-wall mass: explicit greens function and anomaly cancellation}, Journal = {Nuclear Physics B Proceedings Supplements}, Volume = {34}, Number = {C}, Pages = {579-582}, Year = {1994}, ISSN = {0920-5632}, Abstract = {We calculate the explicit Greens function for fermions in 2+1 dimensions, with a domain wall mass. We then show a calculation demonstrating the anomaly cancellation when such fermions move in the background of an abelian gauge field. © 1994.}, Key = {fds245733} } @article{Chandrasekharan:1993ag, Author = {Chandrasekharan, Shailesh}, Title = {Anomaly cancellation in (2+1)-dimensions in the presence of a domain wall mass}, Journal = {Phys. Rev. D}, Volume = {49}, Pages = {1980-1987}, Year = {1994}, url = {http://arxiv.org/pdf/hep-th/9311050}, Abstract = {http://arxiv.org/abs/hep-th/9311050}, Key = {Chandrasekharan:1993ag} } @article{Chandrasekharan:1994ae, Author = {Chandrasekharan, S.}, Title = {Fermions with a domain wall mass: Explicit Greens function and anomaly cancellation}, Journal = {Nucl. Phys. Proc. Suppl.}, Volume = {34}, Pages = {579-582}, Year = {1994}, Key = {Chandrasekharan:1994ae} } @article{fds245735, Author = {Chandrasekharan, S}, Title = {Anomaly cancellation in 2+1 dimensions in the presence of a domain wall mass.}, Journal = {Physical Review D}, Volume = {49}, Number = {4}, Pages = {1980-1987}, Year = {1994}, Month = {February}, ISSN = {0556-2821}, url = {http://www.ncbi.nlm.nih.gov/pubmed/10017182}, Abstract = {A fermion in 2+1 dimensions, with a mass function which depends on one spatial coordinate and passes through a zero (a domain wall mass), in the background of an Abelian gauge field is considered. In this model, originally proposed in a non-Abelian version by Callan and Harvey, the gauge variation of the effective gauge action mainly consists of two terms. One comes from the induced Chern-Simons term and the other from the chiral fermions, bound to the (1+1)-dimensional wall, and they are expected to cancel each other. Though there exist arguments in favor of this, based on the possible forms of the effective action valid far from the wall and some facts about theories of chiral fermions in 1+1 dimensions, a complete calculation is lacking. In this paper we present an explicit calculation of this cancellation at one loop which is valid even close to the wall. We show that integrating out the ``massive'' modes of the theory does produce the Chern-Simons term, as appreciated previously. In addition, we show that it generates a term that softens the high energy behavior of the (1+1)-dimensional effective chiral theory thereby resolving an ambiguity present in a general (1+1)-dimensional theory.}, Key = {fds245735} } @article{fds245734, Author = {Chandrasekharan, S}, Title = {Critical behavior of the chiral condensate at the QCD phase transition}, Journal = {Nuclear Physics B Proceedings Supplements}, Volume = {42}, Number = {1-3}, Pages = {475-477}, Year = {1995}, ISSN = {0920-5632}, url = {http://dx.doi.org/10.1016/0920-5632(95)00284-G}, Abstract = {We study the critical behavior of the chiral condensate near the QCD phase transition in the background of two fixed light dynamical (sea) quarks. We study the condensate for 5.245 ≤ β ≤ 5.3 and 10-10 ≤ mval ≤ 10 (in lattice units) on a 163 × 4 lattice using staggered fermions with msea fixed at 0.01. © 1995 Elsevier Science B.V. All rights reserved.}, Doi = {10.1016/0920-5632(95)00284-G}, Key = {fds245734} } @article{Chandrasekharan:1994cq, Author = {Chandrasekharan, Shailesh}, Title = {Critical behavior of the chiral condensate at the QCD phase transition}, Journal = {Nucl. Phys. Proc. Suppl.}, Volume = {42}, Pages = {475-477}, Year = {1995}, url = {http://arxiv.org/pdf/hep-lat/9412070}, Abstract = {http://arxiv.org/abs/hep-lat/9412070}, Key = {Chandrasekharan:1994cq} } @article{Chandrasekharan:1995nf, Author = {Chandrasekharan, S and Huang, S}, Title = {Z_{3}twisted chiral condensates in QCD at finite temperatures}, Journal = {Physical Review D - Particles, Fields, Gravitation and Cosmology}, Volume = {53}, Number = {9}, Pages = {5100-5104}, Year = {1996}, url = {http://arxiv.org/pdf/hep-ph/9512323}, Abstract = {It was recently observed in a lattice QCD measurement that the chiral condensate in the quenched approximation shows dramatically different behavior in the three Z3-equivalent deconfined phases. We argue that this phenomenon can be understood qualitatively as an effect of Z3 twists on fermionic fields. Quarks under these Z3 twists become global anyons and, hence, display different thermodynamic properties. We further show that the lattice data can be roughly modeled by a Nambu-Jona-Lasinio-type Lagrangian with a minimal coupling to a constant gauge field A0=2πnT/3 (with n=0,±1), which arises naturally from the nontrivial phase of the Polyakov line.}, Key = {Chandrasekharan:1995nf} } @article{Chandrasekharan:1995gt, Author = {Chandrasekharan, Shailesh and Christ, Norman H.}, Title = {Dirac Spectrum, Axial Anomaly and the QCD Chiral Phase Transition}, Journal = {Nucl. Phys. Proc. Suppl.}, Volume = {47}, Pages = {527-534}, Year = {1996}, url = {http://arxiv.org/pdf/hep-lat/9509095}, Abstract = {http://arxiv.org/abs/hep-lat/9509095}, Key = {Chandrasekharan:1995gt} } @article{fds245732, Author = {Chandrasekharan, S and Christ, N}, Title = {Dirac spectrum, axial anomaly and the QCD chiral phase transition}, Journal = {Nuclear Physics B Proceedings Supplements}, Volume = {47}, Number = {1-3}, Pages = {527-534}, Year = {1996}, Month = {March}, url = {http://dx.doi.org/10.1016/0920-5632(96)00115-6}, Abstract = {The QCD phase transition is studied on 163 and 323 × 4 lattices both with and without quark loops. We introduce a new zero-flavor or quenched species of quark ζ and study the resulting chiral condensate, ζ̄ζ as a function of the ζ mass, mζ. By examining ζ̄ζ for 10-10 ≤ mζ ≤ 10 we gain considerable information about the spectrum of Dirac eigenvalues. A comparison of ma = 0.01 and 0.025 shows little dependence of the Dirac spectrum on such a light, dynamical quark mass, after an overall shift in β is removed. The presence of sufficient small eigenvalues to support anomalous chiral symmetry breaking in the high temperature phase is examined quantitatively. In an effort to enhance these small eigenvalues, ζ̄ζ is also examined in the pure gauge theory in the region of the deconfinement transition with unexpected results. Above the critical temperature, the three Z3 phases show dramatically different chiral behavior. Surprisingly, the real phase shows chiral symmetry, suggesting that a system with one flavor of staggered fermion at Nt = 4 will possess a chiral a phase transition - behavior not expected in the continuum limit.}, Doi = {10.1016/0920-5632(96)00115-6}, Key = {fds245732} } @article{fds245731, Author = {Chandrasekharan, S and Huang, S}, Title = {Z3 twisted chiral condensates in QCD at finite temperatures.}, Journal = {Physical Review D}, Volume = {53}, Number = {9}, Pages = {5100-5104}, Year = {1996}, Month = {May}, ISSN = {0556-2821}, url = {http://www.ncbi.nlm.nih.gov/pubmed/10020507}, Abstract = {It was recently observed in a lattice QCD measurement that the chiral condensate in the quenched approximation shows dramatically different behavior in the three Z3-equivalent deconfined phases. We argue that this phenomenon can be understood qualitatively as an effect of Z3 twists on fermionic fields. Quarks under these Z3 twists become global anyons and, hence, display different thermodynamic properties. We further show that the lattice data can be roughly modeled by a Nambu-Jona-Lasinio-type Lagrangian with a minimal coupling to a constant gauge field A0=2Ï€nT/3 (with n=0,Â±1), which arises naturally from the nontrivial phase of the Polyakov line.}, Key = {fds245731} } @article{fds245727, Author = {Bietenholz, W and Brower, R and Chandrasekharan, S and Wiese, U-J}, Title = {Perfect lattice actions for staggered fermions}, Journal = {Nuclear Physics B}, Volume = {495}, Number = {1-2}, Pages = {285-305}, Year = {1997}, Abstract = {We construct a perfect lattice action for staggered fermions by blocking from the continuum. The locality, spectrum and pressure of such perfect staggered fermions are discussed. We also derive a consistent fixed point action for free gauge fields and discuss its locality as well as the resulting static quark-antiquark potential. This provides a basis for the construction of (classically) perfect lattice actions for QCD using staggered fermions. © 1997 Elsevier Science B.V.}, Key = {fds245727} } @article{fds245728, Author = {Chandrasekharan, S}, Title = {A large N chiral transition on a plaquette}, Journal = {Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics}, Volume = {395}, Number = {1-2}, Pages = {83-88}, Year = {1997}, Abstract = {We construct a model of a chiral transition using the well known large N transition in two dimensional U(N) lattice gauge theory. Restricting the model to a single plaquette, we introduce Grassmann variables on the corners of the plaquette with the natural phase factors of staggered fermions and couple them to the U(N) link variables. The classical theory has a continuous chiral symmetry which is broken at strong couplings, but is restored for weak couplings in the N → ∞ limit. * This work is supported in part by funds provided by the U.S. Department of Energy (D.O.E.) under cooperative research agreement #DF-FC02-94ER40818.}, Key = {fds245728} } @article{fds245729, Author = {Chandrasekharan, S and Wiese, U-J}, Title = {Quantum link models: A discrete approach to gauge theories}, Journal = {Nuclear Physics B}, Volume = {492}, Number = {1-2}, Pages = {455-471}, Year = {1997}, Abstract = {We construct lattice gauge theories in which the elements of the link matrices are represented by non-commuting operators acting in a Hubert space. These quantum link models are related to ordinary lattice gauge theories in the same way as quantum spin models are related to ordinary classical spin systems. Here U (1) and SU (2) quantum link models are constructed explicitly. As Hamiltonian theories quantum link models are non-relativistic gauge theories with potential applications in condensed matter physics. When formulated with a fifth Euclidean dimension, universality arguments suggest that dimensional reduction to four dimensions occurs. Hence, quantum link models are also reformulations of ordinary quantum field theories and are applicable to particle physics, for example to QCD. The configuration space of quantum link models is discrete and hence their numerical treatment should be simpler than that of ordinary lattice gauge theories with a continuous configuration space. © 1997 Published by Elsevier Science B.V.}, Key = {fds245729} } @article{fds245730, Author = {Bietenholz, W and Brower, R and Chandrasekharan, S and Wiese, U-J}, Title = {Progress on perfect lattice actions for QCD}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {53}, Number = {1-3}, Pages = {921-934}, Year = {1997}, Abstract = {We describe a number of aspects in our attempt to construct an approximately perfect lattice action for QCD. Free quarks are made optimally local on the whole renormalized trajectory and their couplings are then truncated by imposing 3-periodicity. The spectra of these short ranged fermions are excellent approximations to continuum spectra. The same is true for free gluons. We evaluate the corresponding perfect quark-gluon vertex function, identifying in particular the "perfect clover term". First simulations for heavy quarks show that the mass is strongly renormalized, but again the renormalized theory agrees very well with continuum physics. Furthermore we describe the multigrid formulation for the non-perturbative perfect action and we present the concept of an exactly (quantum) perfect topological charge on the lattice.}, Key = {fds245730} } @article{Bietenholz:1997kr, Author = {Bietenholz, W. and Brower, R. and Chandrasekharan, S. and Wiese, U. J.}, Title = {Perfect lattice topology: The quantum rotor as a test case}, Journal = {Phys. Lett. B}, Volume = {407}, Pages = {283-289}, Year = {1997}, url = {http://arxiv.org/pdf/hep-lat/9704015}, Abstract = {http://arxiv.org/abs/hep-lat/9704015}, Key = {Bietenholz:1997kr} } @article{Bietenholz:1996qc, Author = {Bietenholz, W. and Brower, R. and Chandrasekharan, S. and Wiese, U. J.}, Title = {Perfect lattice actions for staggered fermions}, Journal = {Nucl. Phys. B}, Volume = {495}, Pages = {285-305}, Year = {1997}, url = {http://arxiv.org/pdf/hep-lat/9612007}, Abstract = {http://arxiv.org/abs/hep-lat/9612007}, Key = {Bietenholz:1996qc} } @article{Chandrasekharan:1996ih, Author = {Chandrasekharan, S. and Wiese, U. J.}, Title = {Quantum link models: A discrete approach to gauge theories}, Journal = {Nucl. Phys. B}, Volume = {492}, Pages = {455-474}, Year = {1997}, url = {http://arxiv.org/pdf/hep-lat/9609042}, Abstract = {http://arxiv.org/abs/hep-lat/9609042}, Key = {Chandrasekharan:1996ih} } @article{Bietenholz:1996pf, Author = {Bietenholz, W. and Brower, R. and Chandrasekharan, S. and Wiese, U. J.}, Title = {Progress on perfect lattice actions for QCD}, Journal = {Nucl. Phys. Proc. Suppl.}, Volume = {53}, Pages = {921-934}, Year = {1997}, url = {http://arxiv.org/pdf/hep-lat/9608068}, Abstract = {http://arxiv.org/abs/hep-lat/9608068}, Key = {Bietenholz:1996pf} } @article{Chandrasekharan:1996en, Author = {Chandrasekharan, Shailesh}, Title = {A large N chiral transition on a plaquette}, Journal = {Phys. Lett. B}, Volume = {395}, Pages = {83-88}, Year = {1997}, url = {http://arxiv.org/pdf/hep-th/9610225}, Abstract = {http://arxiv.org/abs/hep-th/9610225}, Key = {Chandrasekharan:1996en} } @article{fds303654, Author = {Brower, R and Chandrasekharan, S and Wiese, U-J}, Title = {QCD as a Quantum Link Model}, Journal = {Phys. Rev. D}, Volume = {60}, Pages = {094502}, Year = {1997}, Month = {April}, url = {http://arxiv.org/abs/hep-th/9704106v1}, Abstract = {QCD is constructed as a lattice gauge theory in which the elements of the link matrices are represented by non-commuting operators acting in a Hilbert space. The resulting quantum link model for QCD is formulated with a fifth Euclidean dimension, whose extent resembles the inverse gauge coupling of the resulting four-dimensional theory after dimensional reduction. The inclusion of quarks is natural in Shamir's variant of Kaplan's fermion method, which does not require fine-tuning to approach the chiral limit. A rishon representation in terms of fermionic constituents of the gluons is derived and the quantum link Hamiltonian for QCD with a U(N) gauge symmetry is expressed in terms of glueball, meson and constituent quark operators. The new formulation of QCD is promising both from an analytic and from a computational point of view.}, Doi = {10.1103/PhysRevD.60.094502}, Key = {fds303654} } @article{fds245726, Author = {Bietenholz, W and Brower, R and Chandrasekharan, S and Wiese, U-J}, Title = {Perfect lattice topology: the quantum rotor as a test case}, Journal = {Physics Letters B}, Volume = {407}, Number = {3-4}, Pages = {283-289}, Year = {1997}, Month = {September}, url = {http://dx.doi.org/10.1016/S0370-2693(97)00742-9}, Abstract = {Lattice actions and topological charges that are classically and quantum mechanically perfect (i.e. free of lattice artifacts) are constructed analytically for the quantum rotor. It is demonstrated that the Manton action is classically perfect while the Villain action is quantum perfect. The geometric construction for the topological charge is only perfect at the classical level. The quantum perfect lattice topology associates a topological charge distribution, not just a single charge, with each lattice field configuration. For the quantum rotor with the classically perfect action and topological charge, the remaining cut-off effects are exponentially suppressed. © 1997 Published by Elsevier Science B.V.}, Doi = {10.1016/S0370-2693(97)00742-9}, Key = {fds245726} } @article{fds245666, Author = {Brower, R and Chandrasekharan, S and Wiese, U-J}, Title = {Green’s functions from quantum cluster algorithms11This work is supported in part by funds provided by the US Department of Energy (DOE) under cooperative research agreement DE-FC02-94ER40818.}, Volume = {261}, Number = {3}, Pages = {520-533}, Year = {1998}, Abstract = {We show that cluster algorithms for quantum models have a meaning independent of the basis chosen to construct them. Using this idea, we propose a new method for measuring with little effort a whole class of Green’s functions, once a cluster algorithm for the partition function has been constructed. To explain the idea, we consider the quantum XY model and compute its two point Green’s function in various ways, showing that all of them are equivalent. We also provide numerical evidence confirming the analytic arguments. Similar techniques are applicable to other models. In particular, in the recently constructed quantum link models, the new technique allows us to construct improved estimators for Wilson loops and may lead to a very precise determination of the glueball spectrum.}, Key = {fds245666} } @article{fds245724, Author = {Beard, BB and Brower, RC and Chandrasekharan, S and Chen, D and Tsapalis, A and Wiese, U-J}, Title = {D-theory: Field theory via dimensional reduction of discrete variables}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {63}, Number = {1-3}, Pages = {775-789}, Year = {1998}, Abstract = {A new non-perturbative approach to quantum field theory - D-theory - is proposed, in which continuous classical fields are replaced by discrete quantized variables which undergo dimensional reduction. The 2-d classical O(3) model emerges from the (2 + 1)-d quantum Heisenberg model formulated in terms of quantum spins. Dimensional reduction is demonstrated explicitly by simulating correlation lengths up to 350,000 lattice spacings using a loop cluster algorithm. In the framework of D-theory, gauge theories are formulated in terms of quantum links - the gauge analogs of quantum spins. Quantum links are parallel transporter matrices whose elements are non-commuting operators. They can be expressed as bilinears of anticommuting fermion constituents. In quantum link models dimensional reduction to four dimensions occurs, due to the presence of a 5-d Coulomb phase, whose existence is confirmed by detailed simulations using standard lattice gauge theory. Using Shamir's variant of Kaplan's fermion proposal, in quantum link QCD quarks appear as edge states of a 5-d slab. This naturally protects their chiral symmetries without fine-tuning. The first efficient cluster algorithm for a gauge theory with a continuous gauge group is formulated for the U(1) quantum link model. Improved estimators for Wilson loops are constructed, and dimensional reduction to ordinary lattice QED is verified numerically.}, Key = {fds245724} } @article{fds245725, Author = {Orginos, K and Bietenholz, W and Brower, R and Chandrasekharan, S and Wiese, U-J}, Title = {The perfect Quark-Gluon vertex function}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {63}, Number = {1-3}, Pages = {904-906}, Year = {1998}, Abstract = {We evaluate a perfect quark-gluon vertex function for QCD in coordinate space and truncate it to a short range. We present preliminary results for the charmonium spectrum using this quasi-perfect action.}, Key = {fds245725} } @article{fds245737, Author = {Brower, R and Chandrasekharan, S and Wiese, U-J}, Title = {Green's functions from quantum cluster algorithms}, Journal = {Physica A: Statistical Mechanics and its Applications}, Volume = {261}, Number = {3-4}, Pages = {520-533}, Year = {1998}, Abstract = {We show that cluster algorithms for quantum models have a meaning independent of the basis chosen to construct them. Using this idea, we propose a new method for measuring with little effort a whole class of Green's functions, once a cluster algorithm for the partition function has been constructed. To explain the idea, we consider the quantum XY model and compute its two point Green's function in various ways, showing that all of them are equivalent. We also provide numerical evidence confirming the analytic arguments. Similar techniques are applicable to other models. In particular, in the recently constructed quantum link models, the new technique allows us to construct improved estimators for Wilson loops and may lead to a very precise determination of the glueball spectrum. © 1998 Published by Elsevier Science B.V. All rights reserved.}, Key = {fds245737} } @article{Beard:1997ic, Author = {Beard, B. B. and others}, Title = {D-theory: Field theory via dimensional reduction of discrete variables}, Journal = {Nucl. Phys. Proc. Suppl.}, Volume = {63}, Pages = {775-789}, Year = {1998}, url = {http://arxiv.org/pdf/hep-lat/9709120}, Abstract = {http://arxiv.org/abs/hep-lat/9709120}, Key = {Beard:1997ic} } @article{Brower:1998kg, Author = {Brower, R. and Chandrasekharan, S. and Wiese, U. J.}, Title = {Green's functions from quantum cluster algorithms}, Journal = {Physica A}, Volume = {261}, Pages = {520}, Year = {1998}, url = {http://arxiv.org/pdf/cond-mat/9801003}, Abstract = {http://arxiv.org/abs/cond-mat/9801003}, Key = {Brower:1998kg} } @article{Orginos:1997fh, Author = {Orginos, K. and Bietenholz, W. and Brower, R. and Chandrasekharan, S. and Wiese, U. -J.}, Title = {The perfect quark-gluon vertex function}, Journal = {Nucl. Phys. Proc. Suppl.}, Volume = {63}, Pages = {904-906}, Year = {1998}, url = {http://arxiv.org/pdf/hep-lat/9709100}, Abstract = {http://arxiv.org/abs/hep-lat/9709100}, Key = {Orginos:1997fh} } @article{fds303655, Author = {Chandrasekharan, S}, Title = {Ginsparg-Wilson Fermions: A study in the Schwinger Model}, Journal = {Phys.Rev. D}, Volume = {59}, Pages = {094502}, Year = {1998}, Month = {October}, url = {http://arxiv.org/abs/hep-lat/9810007v2}, Abstract = {Qualitative features of Ginsparg-Wilson fermions, as formulated by Neuberger, coupled to two dimensional U(1) gauge theory are studied. The role of the Wilson mass parameter in changing the number of massless flavors in the theory and its connection with the index of the Dirac operator is studied. Although the index of the Dirac operator is not related to the geometric definition of the topological charge for strong couplings, the two start to agree as soon as one goes to moderately weak couplings. This produces the desired singularity in the quenched chiral condensate which appears to be very difficult to reproduce with staggered fermions. The fermion determinant removes the singularity and reproduces the known chiral condensate and the meson mass within understandable errors.}, Doi = {10.1103/PhysRevD.59.094502}, Key = {fds303655} } @article{Chandrasekharan:1998em, Author = {Chandrasekharan, S}, Title = {Ginsparg-Wilson fermions: A study in the Schwinger model}, Journal = {Physical Review D - Particles, Fields, Gravitation and Cosmology}, Volume = {59}, Number = {9}, Pages = {1-8}, Year = {1999}, url = {http://arxiv.org/pdf/hep-lat/9810007}, Abstract = {The qualitative features of Ginsparg-Wilson fermions, as formulated by Neuberger, coupled to two-dimensional U(1) gauge theory are studied. The role of the Wilson mass parameter in changing the number of massless flavors in the theory and its connection with the index of the Dirac operator is studied. Although the index of the Dirac operator is not related to the geometric definition of the topological charge for strong couplings, the two start to agree as soon as one goes to moderately weak couplings. This produces the desired singularity in the quenched chiral condensate which appears to be very difficult to reproduce with staggered fermions. The fermion determinant removes the singularity and reproduces the known chiral condensate and the meson mass within understandable errors. ©1999 The American Physical Society.}, Key = {Chandrasekharan:1998em} } @article{fds245673, Author = {Chandrasekharan, S}, Title = {Lattice QCD with Ginsparg-Wilson fermions}, Journal = {Physical Review D - Particles, Fields, Gravitation and Cosmology}, Volume = {60}, Number = {7}, Pages = {1-6}, Year = {1999}, url = {http://arxiv.org/abs/hep-lat/9805015v3}, Abstract = {Lattice QCD using fermions whose Dirac operator obeys the Ginsparg-Wilson relation is perhaps the best known formulation of QCD with a finite cutoff. It reproduces all the low energy QCD phenomenology associated with chiral symmetry at finite lattice spacings. In particular it explains the origin of massless pions due to spontaneous chiral symmetry breaking and leads to new ways to approach the U(1) problem on the lattice. Here we show these results in the path integral formulation and derive for the first time in lattice QCD a known formal continuum relation between the chiral condensate and the topological susceptibility. This relation leads to predictions for the critical behavior of the topological susceptibility near the phase transition and can now be checked in Monte Carlo simulations even at finite lattice spacings. ©1999 The American Physical Society.}, Doi = {10.1103/PhysRevD.60.074503}, Key = {fds245673} } @article{Brower:1997ha, Author = {Brower, R and Chandrasekharan, S and Wiese, U-J}, Title = {QCD as a quantum link model}, Journal = {Physical Review D}, Volume = {60}, Number = {9}, Pages = {DUMMY42}, Year = {1999}, ISSN = {0556-2821}, url = {http://arxiv.org/pdf/hep-th/9704106}, Abstract = {QCD is constructed as a lattice gauge theory in which the elements of the link matrices are represented by non-commuting operators acting in a Hubert space. The resulting quantum link model for QCD is formulated with a fifth Euclidean dimension, whose extent resembles the inverse gauge coupling of the resulting fourdimensional theory after dimensional reduction. The inclusion of quarks is natural in Shamir's variant of Kaplan's fermion method, which does not require fine-tuning to approach the chiral limit. A rishon representation in terms of fermionic constituents of the gluons is derived and the quantum link Hamiltonian for QCD with a U(N) gauge symmetry is expressed in terms of glueball, meson and constituent quark operators. The new formulation of QCD is promising both from an analytic and from a computational point of view. ©1999 The American Physical Society.}, Key = {Brower:1997ha} } @article{Bhattacharya:1998ue, Author = {Bhattacharya, T and Chandrasekharan, S and Gupta, R and Lee, W and Sharpe, S}, Title = {Non-perturbative renormalization constants using Ward identities}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {73}, Number = {1-3}, Pages = {276-278}, Year = {1999}, url = {http://arxiv.org/pdf/hep-lat/9810018}, Abstract = {We extend the application of vector and axial Ward identities to calculate bA, bP and bT, coefficients that give the mass dependence of the renormalization constants of the corresponding bilinear operators in the quenched theory. The extension relies on using operators with non-degenerate quark masses. It allows a complete determination of the O(a) improvement coefficients for bilinears in the quenched approximation using Ward Identities alone. Only the scale dependent normalization constants Z0P (or Z0S) and ZT are undetermined. We present results of a pilot numerical study using hadronic correlators.}, Key = {Bhattacharya:1998ue} } @article{fds245736, Author = {Bhattacharya, T and Chandrasekharan, S and Gupta, R and Lee, W and Sharpe, S}, Title = {Non-perturbative renormalization constants using Ward identities^{1}}, Journal = {Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics}, Volume = {461}, Number = {1-2}, Pages = {79-88}, Year = {1999}, Abstract = {We extend the application of axial Ward identities to calculate bA, bp and bT, coefficients that give the mass dependence of the renormalization constants of the corresponding bilinear operators in the quenched theory. The extension relies on using operators with non-degenerate quark masses. It allows a complete determination of the O(a) improvement coefficients for bilinears in the quenched approximation using Ward Identities alone. Only the scale dependent normalization constants ZP0 (or ZS0) and ZT are undetermined. We present results of a pilot numerical study using hadronic correlators. © 1999 Elsevier Science B.V. All rights reserved.}, Key = {fds245736} } @article{fds245738, Author = {Chandrasekharan, S}, Title = {Confinement, chiral symmetry breaking and continuum limits in quantum link models}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {73}, Number = {1-3}, Pages = {739-741}, Year = {1999}, Abstract = {Using the example of compact U(1) lattice gauge theory we argue that quantum link models can be used to reproduce the physics of conventional Hamiltonian lattice gauge theories. In addition to the usual gauge coupling g, these models have a new parameter j which naturally cuts-off large electric flux quanta on each link while preserving exact U(1) gauge invariance. The j → ∞ limit recovers the conventional Hamiltonian. At strong couplings, the theory shows confinement and chiral symmetry breaking for all non-trivial values of j. The phase diagram of the 3+1 dimensional theory suggests that a coulomb phase is present at large but finite j. Setting g = 0, a new approach to the physics of compact U(1) gauge theory on the lattice emerges. In this case the parameter j takes over the role of the gauge coupling, and j → ∞ describes free photons.}, Key = {fds245738} } @article{fds245739, Author = {Chandrasekharan, S and Chen, D and Christ, N and Lee, W and Mawhinney, R and Vranas, P}, Title = {Anomalous chiral symmetry breaking above the QCD phase transition}, Journal = {Physical Review Letters}, Volume = {82}, Number = {12}, Pages = {2463-2466}, Year = {1999}, Abstract = {We study the anomalous breaking of UA(1) symmetry just above the QCD phase transition for zero and two flavors of quarks, using a staggered fermion, lattice discretization. The properties of the QCD phase transition are expected to depend on the degree of UA(1) symmetry breaking in the transition region. For the physical case of two flavors, we carry out extensive simulations on a 163 × 4 lattice, measuring a difference in susceptibilities which is sensitive to UA(1) symmetry and which avoids many of the staggered fermion discretization difficulties. The results suggest that anomalous effects are at or below the 15% level. © 1999 The American Physical Society.}, Key = {fds245739} } @article{fds245741, Author = {Chandrasekharan, S and Wiese, U-J}, Title = {Meron-Cluster Solution of Fermion Sign Problems}, Journal = {Physical Review Letters}, Volume = {83}, Number = {16}, Pages = {3116-3119}, Year = {1999}, Abstract = {We present a general strategy to solve the notorious fermion sign problem using cluster algorithms. The method applies to various systems in the Hubbard model family as well as to relativistic fermions. Here it is illustrated for nonrelativistic lattice fermions. A configuration of fermion world lines is decomposed into clusters that contribute independently to the fermion permutation sign. A cluster whose flip changes the sign is referred to as a meron. Configurations containing meron clusters contribute 0 to the path integral, while all other configurations contribute 1. The cluster representation describes the partition function as a gas of clusters in the zero-meron sector.}, Key = {fds245741} } @article{Chandrasekharan:1998wg, Author = {Chandrasekharan, Shailesh}, Title = {Lattice QCD with Ginsparg-Wilson fermions}, Journal = {Phys. Rev. D}, Volume = {60}, Pages = {074503}, Year = {1999}, url = {http://arxiv.org/pdf/hep-lat/9805015}, Abstract = {http://arxiv.org/abs/hep-lat/9805015}, Key = {Chandrasekharan:1998wg} } @article{Chandrasekharan:1999zt, Author = {Chandrasekharan, S. and Scarlet, B. and Wiese, U. J.}, Title = {Meron-Cluster Simulation of Quantum Spin Ladders in a Magnetic Field}, Year = {1999}, url = {http://arxiv.org/pdf/cond-mat/9909451}, Abstract = {http://arxiv.org/abs/cond-mat/9909451}, Key = {Chandrasekharan:1999zt} } @article{Chandrasekharan:1999cm, Author = {Chandrasekharan, Shailesh and Wiese, Uwe-Jens}, Title = {Meron-cluster solution of a fermion sign problem}, Journal = {Phys. Rev. Lett.}, Volume = {83}, Pages = {3116-3119}, Year = {1999}, url = {http://arxiv.org/pdf/cond-mat/9902128}, Abstract = {http://arxiv.org/abs/cond-mat/9902128}, Key = {Chandrasekharan:1999cm} } @article{Chandrasekharan:1998yx, Author = {Chandrasekharan, Shailesh and others}, Title = {Anomalous chiral symmetry breaking above the QCD phase transition}, Journal = {Phys. Rev. Lett.}, Volume = {82}, Pages = {2463-2466}, Year = {1999}, url = {http://arxiv.org/pdf/hep-lat/9807018}, Abstract = {http://arxiv.org/abs/hep-lat/9807018}, Key = {Chandrasekharan:1998yx} } @article{Bhattacharya:1999uq, Author = {Bhattacharya, Tanmoy and Chandrasekharan, Shailesh and Gupta, Rajan and Lee, Weon-Jong and Sharpe, Stephen R.}, Title = {Non-perturbative renormalization constants using Ward identities}, Journal = {Phys. Lett. B}, Volume = {461}, Pages = {79-88}, Year = {1999}, url = {http://arxiv.org/pdf/hep-lat/9904011}, Abstract = {http://arxiv.org/abs/hep-lat/9904011}, Key = {Bhattacharya:1999uq} } @article{Chandrasekharan:1998ck, Author = {Chandrasekharan, Shailesh}, Title = {Confinement, chiral symmetry breaking and continuum limits in quantum link models}, Journal = {Nucl. Phys. Proc. Suppl.}, Volume = {73}, Pages = {739-741}, Year = {1999}, url = {http://arxiv.org/pdf/hep-lat/9809084}, Abstract = {http://arxiv.org/abs/hep-lat/9809084}, Key = {Chandrasekharan:1998ck} } @article{fds303642, Author = {Chandrasekharan, S and Scarlet, B and Wiese, U-J}, Title = {Meron-Cluster Simulation of Quantum Spin Ladders in a Magnetic Field}, Year = {1999}, Month = {September}, url = {http://arxiv.org/abs/cond-mat/9909451v1}, Abstract = {Numerical simulations of numerous quantum systems suffer from the notorious sign problem. Meron-cluster algorithms lead to an efficient solution of sign problems for both fermionic and bosonic models. Here we apply the meron concept to quantum spin systems in an arbitrary external magnetic field, in which case standard cluster algorithms fail. As an example, we simulate antiferromagnetic quantum spin ladders in a uniform external magnetic field that competes with the spin-spin interaction. The numerical results are in agreement with analytic predictions for the magnetization as a function of the external field.}, Key = {fds303642} } @article{Chandrasekharan:1999vz, Author = {Chandrasekharan, S}, Title = {Fermion cluster algorithms}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {83-84}, Number = {1-3}, Pages = {774-776}, Year = {2000}, url = {http://arxiv.org/pdf/hep-lat/9909007}, Abstract = {Cluster algorithms have been recently used to eliminate sign problems that plague Monte-Carlo methods in a variety of systems. In particular such algorithms can also be used to solve sign problems associated with the permutation of fermion world lines. This solution leads to the possibility of designing fermion cluster algorithms in certain cases. Using the example of free non-relativistic fermions we discuss the ideas underlying the algorithm.}, Key = {Chandrasekharan:1999vz} } @article{fds245740, Author = {Chandrasekharan, S and Cox, J and Holland, K and Wiese, U-J}, Title = {Meron-cluster simulation of a chiral phase transition with staggered fermions}, Journal = {Nuclear Physics B}, Volume = {576}, Number = {1-3}, Pages = {481-500}, Year = {2000}, Abstract = {We examine a (3 + 1)-dimensional model of staggered lattice fermions with a four-fermion interaction and ℤ(2) chiral symmetry using the Hamiltonian formulation. This model cannot be simulated with standard fermion algorithms because those suffer from a very severe sign problem. We use a new fermion simulation technique - the meron-cluster algorithm - which solves the sign problem and leads to high-precision numerical data. We investigate the finite temperature chiral phase transition and verify that it is in the universality class of the 3-d Ising model using finite-size scaling. © 2000 Elsevier Science B.V. All rights reserved.}, Key = {fds245740} } @article{fds245742, Author = {Chandrasekharan, S and Osborn, JC}, Title = {Critical behavior of a chiral condensate with a meron cluster algorithm}, Journal = {Physics Letters B}, Volume = {496}, Number = {1-2}, Pages = {122-128}, Year = {2000}, ISSN = {0370-2693}, url = {http://dx.doi.org/10.1016/S0370-2693(00)01294-6}, Abstract = {A new meron cluster algorithm is constructed to study the finite temperature critical behavior of the chiral condensate in a (3 + 1)-dimensional model of interacting staggered fermions. Using finite size scaling analysis the infinite volume condensate is shown to be consistent with the behavior of the form (Tc - T)0.314(7) for temperatures less than the critical temperature and m1/4.87(10) at the critical temperature confirming that the critical behavior belongs to the 3-d Ising universality class within one to two sigma deviation. The new method, along with improvements in the implementation of the algorithm, allows the determination of the critical temperature Tc more accurately than was possible in a previous study. © 2000 Elsevier Science B.V.}, Doi = {10.1016/S0370-2693(00)01294-6}, Key = {fds245742} } @article{Chandrasekharan:1999ys, Author = {Chandrasekharan, S. and Cox, J. and Holland, K. and Wiese, U. J.}, Title = {Meron-cluster simulation of a chiral phase transition with staggered fermions}, Journal = {Nucl. Phys. B}, Volume = {576}, Pages = {481-500}, Year = {2000}, url = {http://arxiv.org/pdf/hep-lat/9906021}, Abstract = {http://arxiv.org/abs/hep-lat/9906021}, Key = {Chandrasekharan:1999ys} } @article{Chandrasekharan:2000gk, Author = {Chandrasekharan, Shailesh and Wiese, Uwe-Jens}, Title = {SO(10) unification of color superconductivity and chiral symmetry breaking?}, Year = {2000}, url = {http://arxiv.org/pdf/hep-ph/0003214}, Abstract = {http://arxiv.org/abs/hep-ph/0003214}, Key = {Chandrasekharan:2000gk} } @article{Chandrasekharan:2000rm, Author = {Chandrasekharan, Shailesh and Osborn, James C.}, Title = {Critical behavior of a chiral condensate with a meron cluster algorithm}, Journal = {Phys. Lett. B}, Volume = {496}, Pages = {122-128}, Year = {2000}, url = {http://arxiv.org/pdf/hep-lat/0010036}, Abstract = {http://arxiv.org/abs/hep-lat/0010036}, Key = {Chandrasekharan:2000rm} } @article{Chandrasekharan:2000dj, Author = {Chandrasekharan, S}, Title = {A Chiral Phase Transition using a Fermion Cluster Algorithm}, Journal = {Chin.J.Phys.}, Volume = {38}, Pages = {696-706}, Year = {2000}, Month = {January}, url = {http://arxiv.org/abs/hep-lat/0001003v1}, Abstract = {The recent solution to the fermion sign problem allows, for the first time, the use of cluster algorithm techniques to compute certain fermionic path integrals. To illustrate the underlying ideas behind the progress, a cluster algorithm is constructed to study the chiral phase transition in a strongly interacting staggered fermion model with an arbitrary mass term in 3+1 dimensions. Unlike conventional methods there is no difficulty in the cluster method to approach the chiral (massless) limit. Results using the new algorithm confirm that the chiral transition falls under the expected universality class.}, Key = {Chandrasekharan:2000dj} } @article{fds303644, Author = {Chandrasekharan, S and Wiese, U-J}, Title = {SO(10) Unification of Color Superconductivity and Chiral Symmetry Breaking?}, Year = {2000}, Month = {March}, url = {http://arxiv.org/abs/hep-ph/0003214v1}, Abstract = {Motivated by the SO(5) theory of high-temperature superconductivity and antiferromagnetism, we ask if an SO(10) theory unifies color superconductivity and chiral symmetry breaking in QCD. The transition to the color superconducting phase would then be analogous to a spin flop transition. While the spin flop transition generically has a unified SO(3) description, the SO(5) and SO(10) symmetric fixed points are unstable, at least in (4 - epsilon) dimensions, and require the fine-tuning of one additional relevant parameter. If QCD is near the SO(10) fixed point, it has interesting consequences for heavy ion collisions and neutron stars.}, Key = {fds303644} } @article{Chandrasekharan:2000fr, Author = {Chandrasekharan, S and Osborn, J}, Title = {Solving sign problems with meron algorithms}, Journal = {Springer Proceedings in Physics}, Volume = {86}, Pages = {28-42}, Year = {2001}, ISSN = {0930-8989}, url = {http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000165950700004&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=47d3190e77e5a3a53558812f597b0b92}, Key = {Chandrasekharan:2000fr} } @article{fds245743, Author = {Chandrasekharan, S}, Title = {QCD at a finite density of static quarks}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {94}, Number = {1-3}, Pages = {71-78}, Year = {2001}, url = {http://dx.doi.org/10.1016/S0920-5632(01)00936-7}, Abstract = {Recently, cluster methods have been used to solve a variety of sign problems including those that arise in the presence of fermions. In all cases an analytic partial re-summation over a class of configurations in the path integral was necessary. Here the new ideas are illustrated using the example of QCD at a finite density of static quarks. In this limit the sign problem simplifies since the fermionic part decouples. Furthermore, the problem can be solved completely when the gauge dynamics is replaced by a Potts model. On the other hand in QCD with light quarks the solution will require a partial re-summation over both fermionic and gauge degrees of freedom. The new approach points to unexplored directions in the search for a solution to this more challenging sign problem. © 2001 Elsevier Science B.V. All rights reserved.}, Doi = {10.1016/S0920-5632(01)00936-7}, Key = {fds245743} } @article{fds245744, Author = {Alford, M and Chandrasekharan, S and Cox, J and Wiese, U-J}, Title = {Solution of the complex action problem in the Potts model for dense QCD}, Journal = {Nuclear Physics B}, Volume = {602}, Number = {1-2}, Pages = {61-86}, Year = {2001}, url = {http://dx.doi.org/10.1016/S0550-3213(01)00068-2}, Abstract = {Monte Carlo simulations of lattice QCD at non-zero baryon chemical potential μ suffer from the notorious complex action problem. We consider QCD with static quarks coupled to a large chemical potential. This leaves us with an SU(3) Yang-Mills theory with a complex action containing the Polyakov loop. Close to the deconfinement phase transition the qualitative features of this theory, in particular its Z(3) symmetry properties, are captured by the 3-d 3-state Potts model. We solve the complex action problem in the Potts model by using a cluster algorithm. The improved estimator for the μ-dependent part of the Boltzmann factor is real and positive and is used for importance sampling. We localize the critical endpoint of the first order deconfinement phase transition line and find consistency with universal 3-d Ising behavior. We also calculate the static quark-quark, quark-antiquark, and antiquark-antiquark potentials which show screening as expected for a system with non-zero baryon density.}, Doi = {10.1016/S0550-3213(01)00068-2}, Key = {fds245744} } @article{fds245745, Author = {Chandrasekharan, S and Chudnovsky, V and Schlittgen, B and Wiese, U-J}, Title = {Flop transitions in cuprate and color superconductors: From SO(5) to SO(10) unification?}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {94}, Number = {1-3}, Pages = {449-452}, Year = {2001}, url = {http://dx.doi.org/10.1016/S0920-5632(01)01002-7}, Abstract = {The phase diagrams of cuprate superconductors and of QCD at non-zero baryon chemical potential are qualitatively similar. The Néel phase of the cuprates corresponds to the chirally broken phase of QCD, and the high-temperature superconducting phase corresponds to the color superconducting phase. In the SO(5) theory for the cuprates the SO(3)s spin rotational symmetry and the U(1)em gauge symmetry of electromagnetism are dynamically unified. This suggests that the SU(2)L ⊗ SU(2)R ⊗ U(1)B chiral symmetry of QCD and the SU(3)c color gauge symmetry may get unified to SO(10). Dynamical enhancement of symmetry from SO(2)s ⊗ Z(2) to SO(3)s is known to occur in anisotropic antiferromagnets. In these systems the staggered magnetization flops from an easy 3-axis into the 12-plane at a critical value of the external magnetic field. Similarly, the phase transitions in the SO(5) and SO(10) models are flop transitions of a "superspin". Despite this fact, a renormalization group flow analysis in 4 - ε dimensions indicates that a point with full SO(5) or SO(10) symmetry exists neither in the cuprates nor in QCD. © 2001 Elsevier Science B.V. All rights reserved.}, Doi = {10.1016/S0920-5632(01)01002-7}, Key = {fds245745} } @article{Alford:2001ug, Author = {Alford, Mark G. and Chandrasekharan, S. and Cox, J. and Wiese, U. J.}, Title = {Solution of the complex action problem in the Potts model for dense QCD}, Journal = {Nucl. Phys. B}, Volume = {602}, Pages = {61-86}, Year = {2001}, url = {http://arxiv.org/pdf/hep-lat/0101012}, Abstract = {http://arxiv.org/abs/hep-lat/0101012}, Key = {Alford:2001ug} } @article{Chandrasekharan:2001cj, Author = {Chandrasekharan, Shailesh}, Title = {Novel quantum Monte Carlo algorithms for fermions}, Year = {2001}, url = {http://arxiv.org/pdf/hep-lat/0110018}, Abstract = {http://arxiv.org/abs/hep-lat/0110018}, Key = {Chandrasekharan:2001cj} } @article{Chandrasekharan:2000fd, Author = {Chandrasekharan, S. and Chudnovsky, V. and Schlittgen, B. and Wiese, U. J.}, Title = {Flop transitions in cuprate and color superconductors: From SO(5) to SO(10) unification?}, Journal = {Nucl. Phys. Proc. Suppl.}, Volume = {94}, Pages = {449-452}, Year = {2001}, url = {http://arxiv.org/pdf/hep-lat/0011054}, Abstract = {http://arxiv.org/abs/hep-lat/0011054}, Key = {Chandrasekharan:2000fd} } @article{Chandrasekharan:2000ew, Author = {Chandrasekharan, Shailesh}, Title = {QCD at a finite density of static quarks}, Journal = {Nucl. Phys. Proc. Suppl.}, Volume = {94}, Pages = {71-78}, Year = {2001}, url = {http://arxiv.org/pdf/hep-lat/0011022}, Abstract = {http://arxiv.org/abs/hep-lat/0011022}, Key = {Chandrasekharan:2000ew} } @article{fds303656, Author = {Chandrasekharan, S and Osborn, JC}, Title = {Kosterlitz-Thouless Universality in a Fermionic System}, Volume = {66}, Pages = {045113}, Year = {2001}, Month = {September}, url = {http://arxiv.org/abs/cond-mat/0109424v1}, Abstract = {A new extension of the attractive Hubbard model is constructed to study the critical behavior near a finite temperature superconducting phase transition in two dimensions using the recently developed meron-cluster algorithm. Unlike previous calculations in the attractive Hubbard model which were limited to small lattices, the new algorithm is used to study the critical behavior on lattices as large as $128\times 128$. These precise results for the first time show that a fermionic system can undergo a finite temperature phase transition whose critical behavior is well described by the predictions of Kosterlitz and Thouless almost three decades ago. In particular it is confirmed that the spatial winding number susceptibility obeys the well known predictions of finite size scaling for $T<T_c$ and up to logarithmic corrections the pair susceptibility scales as $L^{2-\eta}$ at large volumes with $0\leq\eta\leq 0.25$ for $0\leq T\leq T_c$.}, Doi = {10.1103/PhysRevB.66.045113}, Key = {fds303656} } @article{fds303645, Author = {Chandrasekharan, S}, Title = {Novel Quantum Monte Carlo Algorithms for Fermions}, Year = {2001}, Month = {October}, url = {http://arxiv.org/abs/hep-lat/0110018v1}, Abstract = {Recent research shows that the partition function for a class of models involving fermions can be written as a statistical mechanics of clusters with positive definite weights. This new representation of the model allows one to construct novel algorithms. We illustrate this through models consisting of fermions with and without spin. A Hubbard type model with both attractive and repulsive interactions becomes tractable using the new approach. Precision results in the two dimensional attractive model confirm a superfluid phase transition in the Kosterlitz-Thouless universality class.}, Key = {fds303645} } @article{PhysRevB.66.045113, Author = {Chandrasekharan, S and Osborn, JC}, Title = {Kosterlitz-Thouless universality in a fermionic system}, Journal = {Physical Review. B, Condensed Matter}, Volume = {66}, Number = {4}, Pages = {451131-451135}, Year = {2002}, ISSN = {0163-1829}, Abstract = {An extension of the attractive Hubbard model is constructed to study the critical behavior near a finite-temperature superconducting phase transition in two dimensions using the recently developed meron-cluster algorithm. Unlike previous calculations in the attractive Hubbard model which were limited to small lattices, the algorithm is used to study the critical behavior on lattices as large as 128 × 128. These precise results show that a fermionic system can undergo a finite temperature phase transition whose critical behavior is well described by the predictions of Kosterlitz and Thouless almost three decades ago. In particular it is confirmed that the spatial winding number susceptibility obeys the well known predictions of finite size scaling for T <Tc and up to logarithmic corrections the pair susceptibility scales as L2-η at large volumes with 0 ≤ η ≤ 0.25 for 0 ≤T≤T.}, Key = {PhysRevB.66.045113} } @article{Chandrasekharan:2001dd, Author = {Chandrasekharan, S}, Title = {Superconductivity and chiral symmetry breaking with fermion clusters}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {106-107}, Pages = {1025-1027}, Year = {2002}, url = {http://arxiv.org/pdf/hep-lat/0110125}, Abstract = {Cluster variables have recently revolutionized numerical work in certain models involving fermionic variables. This novel representation of fermionic partition functions is continuing to find new applications. After describing results from a study of a two dimensional Hubbard type model that confirm a superconducting transition in the Kosterlitz-Thouless universality class, we show how a cluster type algorithm can be devised to study the chiral limit of strongly coupled lattice gauge theories with staggered fermions.}, Doi = {10.1016/S0920-5632(01)01917-X}, Key = {Chandrasekharan:2001dd} } @article{fds245746, Author = {Brower, R and Chandrasekharan, S and Negele, JW and Wiese, U-J}, Title = {Physical observables from lattice QCD at fixed topology}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {106-107}, Pages = {581-583}, Year = {2002}, url = {http://dx.doi.org/10.1016/S0920-5632(01)01784-4}, Abstract = {Because present Monte Carlo algorithms for lattice QCD may become trapped in a given topological charge sector when one approaches the continuum limit, it is important to understand the effect of calculating at fixed topology. In this work, we show that although the restriction to a fixed topological sector becomes irrelevant in the infinite volume limit, it gives rise to characteristic finite size effects due to contributions from all θ-vacua. We calculate these effects and show how to extract physical results from numerical data obtained at fixed topology.}, Doi = {10.1016/S0920-5632(01)01784-4}, Key = {fds245746} } @article{fds245747, Author = {Chandrasekharan, S and Scarlet, B and Wiese, U-J}, Title = {From spin ladders to the 2D O(3) model at non-zero density}, Journal = {Computer Physics Communications}, Volume = {147}, Number = {1-2}, Pages = {388-393}, Year = {2002}, url = {http://dx.doi.org/10.1016/S0010-4655(02)00311-9}, Abstract = {The numerical simulation of various field theories at non-zero chemical potential suffers from severe complex action problems. In particular, QCD at non-zero quark density can presently not be simulated for that reason. A similar complex action problem arises in the 2D O(3) model - a toy model for QCD. Here we construct the 2D O(3) model at non-zero density via dimensional reduction of an antiferromagnetic quantum spin ladder in a magnetic field. The complex action problem of the 2D O(3) model manifests itself as a sign problem of the ladder system. This sign problem is solved completely with a meron cluster algorithm. © 2002 Elsevier Science B.V. All rights reserved.}, Doi = {10.1016/S0010-4655(02)00311-9}, Key = {fds245747} } @article{fds245748, Author = {Chandrasekharan, S}, Title = {Unexpected results in the chiral limit with staggered fermions}, Journal = {Physics Letters B}, Volume = {536}, Number = {1-2}, Pages = {72-78}, Year = {2002}, ISSN = {0370-2693}, url = {http://dx.doi.org/10.1016/S0370-2693(02)01816-6}, Abstract = {A cluster algorithm is constructed and applied to study the chiral limit of the strongly coupled lattice Schwinger model involving staggered fermions. The algorithm is based on a novel loop representation of the model. Finite size scaling of the chiral susceptibility based on data from lattices of size up to 64 × 64 indicates the absence of long range correlations at strong couplings. Assuming that there is no phase transition at a weaker coupling, the results imply that all mesons acquire a mass at non-zero lattice spacings. Although this does not violate any known physics, it is surprising since typically one expects a single pion to remain massless at non-zero lattice spacings in the staggered fermion formulation. © 2002 Elsevier Science B.V. All rights reserved.}, Doi = {10.1016/S0370-2693(02)01816-6}, Key = {fds245748} } @article{Brower:2001cz, Author = {Brower, R. and Chandrasekharan, S. and Negele, J. W. and Wiese, U. J.}, Title = {Physical observables from lattice QCD at fixed topology}, Journal = {Nucl. Phys. Proc. Suppl.}, Volume = {106}, Pages = {581-583}, Year = {2002}, url = {http://arxiv.org/pdf/hep-lat/0110121}, Abstract = {http://arxiv.org/abs/hep-lat/0110121}, Key = {Brower:2001cz} } @article{Chandrasekharan:2001ya, Author = {Chandrasekharan, S. and Scarlet, B. and Wiese, U. J.}, Title = {From spin ladders to the 2-d O(3) model at non-zero density}, Journal = {Comput. Phys. Commun.}, Volume = {147}, Pages = {388-393}, Year = {2002}, url = {http://arxiv.org/pdf/hep-lat/0110215}, Abstract = {http://arxiv.org/abs/hep-lat/0110215}, Key = {Chandrasekharan:2001ya} } @article{Chandrasekharan:2002ex, Author = {Chandrasekharan, Shailesh}, Title = {Unexpected results in the chiral limit with staggered fermions}, Journal = {Phys. Lett. B}, Volume = {536}, Pages = {72-78}, Year = {2002}, url = {http://arxiv.org/pdf/hep-lat/0203020}, Abstract = {http://arxiv.org/abs/hep-lat/0203020}, Key = {Chandrasekharan:2002ex} } @article{fds245676, Author = {Chandrasekharan, S and Pepe, M and Steffen, FD and Wiese, U-J}, Title = {Nonlinear realization of chiral symmetry on the lattice}, Journal = {Journal of High Energy Physics}, Volume = {7}, Number = {12}, Pages = {831-863}, Year = {2003}, ISSN = {1029-8479}, Abstract = {We formulate lattice theories in which chiral symmetry is realized nonlinearly on the fermion fields. In this framework the fermion mass term does not break chiral symmetry. This property allows us to use the Wilson term to remove the doubler fermions while maintaining exact chiral symmetry on the lattice. Our lattice formulation enables us to address non-perturbative questions in effective field theories of baryons interacting with pions and in models involving constituent quarks interacting with pions and gluons. We show that a system containing a non-zero density of static baryons interacting with pions can be studied on the lattice without encountering complex action problems. In our formulation one can also decide non-perturbatively if the chiral quark model of Georgi and Manohar provides an appropriate low-energy description of QCD. If so, one could understand why the non-relativistic quark model works. © SISSA/ISAS 2004.}, Key = {fds245676} } @article{Chandrasekharan:2002gp, Author = {Chandrasekharan, S}, Title = {Chiral limit of staggered fermions at strong couplings: A loop representation}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {119}, Pages = {929-931}, Year = {2003}, url = {http://arxiv.org/pdf/hep-lat/0208071}, Abstract = {The partition function of two dimensional massless staggered fermions interacting with U(N) gauge fields is rewritten in terms of loop variables in the strong coupling limit. We use this representation of the theory to devise a non-local Metropolis algorithm to calculate the chiral susceptibility. For small lattices our algorithm reproduces exact results quite accurately. Applying this algorithm to large volumes yields rather surprising results. In particular we find mπ ≠ 0 for all N and it increases with N. Since the talk was presented we have found reasons to believe that our algorithm breaks down for large volumes questioning the validity of our results.}, Doi = {10.1016/S0920-5632(03)01722-5}, Key = {Chandrasekharan:2002gp} } @article{fds245716, Author = {Chandrasekharan, S}, Title = {Connections between quantum chromodynamics and condensed matter physics}, Journal = {Pramana - Journal of Physics}, Volume = {61}, Number = {5}, Pages = {901-910}, Year = {2003}, Abstract = {Features of QCD can be seen qualitatively in certain condensed matter systems. Recently some of the analyses that originated in condensed matter physics have found applications in QCD. Using examples we discuss some of the connections between the two fields and show how progress can be made by exploiting this connection. Some of the challenges that remain in the two fields are quite similar. We argue that recent algorithmic developments call for optimism in both fields.}, Key = {fds245716} } @article{fds245718, Author = {Chandrasekharan, S and Strouthos, CG}, Title = {Kosterlitz-Thouless universality in dimer models}, Journal = {Physical Review D}, Volume = {68}, Number = {9}, Pages = {091502}, Year = {2003}, ISSN = {0556-2821}, url = {http://dx.doi.org/10.1103/PhysRevD.68.091502}, Abstract = {Using the monomer-dimer representation of strongly coupled U(N) lattice gauge theories with staggered fermions, we study finite temperature chiral phase transitions in 2+1 dimensions. A new cluster algorithm allows us to compute monomer-monomer and dimer-dimer correlations at zero monomer density (chiral limit) accurately on large lattices. This makes it possible to show convincingly, for the first time, that these models undergo a finite temperature phase transition which belongs to the Kosterlitz-Thouless universality class. We find that the phase transition persists for all values of N, but occurs at different values of the critical temperature Tc. Further, when T /Tc is held fixed, the mean field analysis often used in the large N limit breaks down. © The American Physical Society.}, Doi = {10.1103/PhysRevD.68.091502}, Key = {fds245718} } @article{fds245719, Author = {Chandrasekharan, S and Jiang, F-J}, Title = {Chiral limit of strongly coupled lattice QCD at finite temperatures}, Journal = {Physical Review D}, Volume = {68}, Number = {9}, Pages = {091501}, Year = {2003}, ISSN = {0556-2821}, url = {http://dx.doi.org/10.1103/PhysRevD.68.091501}, Abstract = {We use the recently proposed directed-path algorithm to study the chiral limit of a strongly coupled lattice QCD with staggered quarks at finite temperatures. The new algorithm allows us to compute the chiral susceptibility and the pion decay constant accurately on large lattices for massless quarks. In the low temperature phase we find clear evidence for the singularities predicted by chiral perturbation theory. We also show convincingly that the chiral phase transition is of second order and belongs to the O(2) universality class. © The American Physical Society.}, Doi = {10.1103/PhysRevD.68.091501}, Key = {fds245719} } @article{fds245721, Author = {Chandrasekharan, S and Cox, J and Osborn, JC and Wiese, U-J}, Title = {Meron-cluster approach to systems of strongly correlated electrons}, Journal = {Nuclear Physics B}, Volume = {673}, Number = {3}, Pages = {405-436}, Year = {2003}, url = {http://dx.doi.org/10.1016/j.nuclphysb.2003.08.041}, Abstract = {Numerical simulations of strongly correlated electron systems suffer from the notorious fermion sign problem which has prevented progress in understanding if systems like the Hubbard model display high-temperature superconductivity. Here we show how the fermion sign problem can be solved completely with meron-cluster methods in a large class of models of strongly correlated electron systems, some of which are in the extended Hubbard model family and show s-wave superconductivity. In these models we also find that on-site repulsion can even coexist with a weak chemical potential without introducing sign problems. We argue that since these models can be simulated efficiently using cluster algorithms they are ideal for studying many of the interesting phenomena in strongly correlated electron systems. © 2003 Elsevier B.V. All rights reserved.}, Doi = {10.1016/j.nuclphysb.2003.08.041}, Key = {fds245721} } @article{fds245722, Author = {Adams, DH and Chandrasekharan, S}, Title = {Chiral limit of strongly coupled lattice gauge theories}, Journal = {Nuclear Physics B}, Volume = {662}, Number = {1-2}, Pages = {220-246}, Year = {2003}, url = {http://dx.doi.org/10.1016/S0550-3213(03)00350-X}, Abstract = {We construct a new and efficient cluster algorithm for updating strongly coupled U(N) lattice gauge theories with staggered fermions in the chiral limit. The algorithm uses the constrained monomer-dimer representation of the theory and should also be of interest to researchers working on other models with similar constraints. Using the new algorithm we address questions related to the chiral limit of strongly coupled U(N) gauge theories beyond the mean field approximation. We show that the infinite volume chiral condensate is non-zero in three and four dimensions. However, on a square lattice of size L we find ∑x〈ψ̄ψ(x)ψ̄ψ(0)〉∼ L2-η for large L where η=0.420(3)/N+0.078(4) /N2. These results differ from an earlier conclusion obtained using a different algorithm. Here we argue that the earlier calculations were misleading due to uncontrolled autocorrelation times encountered by the previous algorithm. © 2003 Elsevier Science B.V. All rights reserved.}, Doi = {10.1016/S0550-3213(03)00350-X}, Key = {fds245722} } @article{fds245723, Author = {Brower, R and Chandrasekharan, S and Negele, JW and Wiese, U-J}, Title = {LATTICE QCD AT FIXED TOPOLOGY}, Journal = {Phys. Lett. B}, Volume = {560}, Pages = {64-74}, Year = {2003}, Abstract = {Since present Monte Carlo algorithms for lattice QCD may become trapped in a fixed topological charge sector, it is important to understand the effect of calculating at fixed topology. In this work, we show that although the restriction to a fixed topological sector becomes irrelevant in the infinite volume limit, it gives rise to characteristic finite size effects due to contributions from all $\theta$-vacua. We calculate these effects and show how to extract physical results from numerical data obtained at fixed topology.}, Key = {fds245723} } @article{fds29982, Author = {S. Chandrasekharan}, Title = {CHIRAL LIMIT OF STAGGERED FERMIONS AT STRONG COUPLINGS: A LOOP REPRESENTATION}, Journal = {Nucl. Phys. B (Proceedings Suppl.)}, Volume = {119}, Pages = {929}, Editor = {Edwards, Negele and Richards}, Year = {2003}, Abstract = {The partition function of two dimensional massless staggered fermions interacting with U(N) gauge fields is rewritten in terms of loop variables in the strong coupling limit. We use this representation of the theory to devise a non-local Metropolis algorithm to calculate the chiral susceptibility. For small lattices our algorithm reproduces exact results quite accurately. Applying this algorithm to large volumes yields rather surprising results. In particular we find $m_\pi \neq 0$ for all $N$ and it increases with $N$. Since the talk was presented we have found reasons to believe that our algorithm breaks down for large volumes questioning the validity of our results.}, Key = {fds29982} } @article{Chandrasekharan:2002vk, Author = {Chandrasekharan, S. and Cox, J. and Osborn, J. C. and Wiese, U. J.}, Title = {Meron-Cluster Approach to Systems of Strongly Correlated Electrons}, Journal = {Nucl. Phys. B}, Volume = {673}, Pages = {405-436}, Year = {2003}, url = {http://arxiv.org/pdf/cond-mat/0201360}, Abstract = {http://arxiv.org/abs/cond-mat/0201360}, Key = {Chandrasekharan:2002vk} } @article{Chandrasekharan:2003im, Author = {Chandrasekharan, Shailesh and Jiang, Fu-Jiun}, Title = {Chiral limit of strongly coupled lattice QCD at finite temperatures}, Journal = {Phys. Rev. D}, Volume = {68}, Pages = {091501}, Year = {2003}, url = {http://arxiv.org/pdf/hep-lat/0309025}, Abstract = {http://arxiv.org/abs/hep-lat/0309025}, Key = {Chandrasekharan:2003im} } @article{Chandrasekharan:2003eu, Author = {Chandrasekharan, S.}, Title = {Connections between quantum chromodynamics and condensed matter physics}, Journal = {Pramana}, Volume = {61}, Pages = {901-910}, Year = {2003}, Key = {Chandrasekharan:2003eu} } @article{Adams:2003cc, Author = {Adams, David H. and Chandrasekharan, Shailesh}, Title = {Chiral limit of strongly coupled lattice gauge theories}, Journal = {Nucl. Phys. B}, Volume = {662}, Pages = {220-246}, Year = {2003}, url = {http://arxiv.org/pdf/hep-lat/0303003}, Abstract = {http://arxiv.org/abs/hep-lat/0303003}, Key = {Adams:2003cc} } @article{Chandrasekharan:2003wy, Author = {Chandrasekharan, S. and Pepe, M. and Steffen, F. D. and Wiese, U. J.}, Title = {Nonlinear realization of chiral symmetry on the lattice}, Journal = {JHEP}, Volume = {12}, Pages = {035}, Year = {2003}, url = {http://arxiv.org/pdf/hep-lat/0306020}, Abstract = {http://arxiv.org/abs/hep-lat/0306020}, Key = {Chandrasekharan:2003wy} } @article{Chandrasekharan:2003qv, Author = {Chandrasekharan, Shailesh and Strouthos, Costas G.}, Title = {Kosterlitz-Thouless universality in dimer models}, Journal = {Phys. Rev. D}, Volume = {68}, Pages = {091502}, Year = {2003}, url = {http://arxiv.org/pdf/hep-lat/0306034}, Abstract = {http://arxiv.org/abs/hep-lat/0306034}, Key = {Chandrasekharan:2003qv} } @article{Brower:2003yx, Author = {Brower, R and Chandrasekharan, S and Negele, JW and Wiese, U-J}, Title = {QCD at fixed topology}, Journal = {Physics Letters B}, Volume = {560}, Number = {1-2}, Pages = {64-74}, Year = {2003}, Month = {May}, url = {http://arxiv.org/pdf/hep-lat/0302005}, Abstract = {Since present Monte Carlo algorithms for lattice QCD may become trapped in a fixed topological charge sector, it is important to understand the effect of calculating at fixed topology. In this Letter, we show that although the restriction to a fixed topological sector becomes irrelevant in the infinite volume limit, it gives rise to characteristic finite-size effects due to contributions from all θ-vacua. We calculate these effects and show how to extract physical results from numerical data obtained at fixed topology. © 2003 Published by Elsevier Science B.V.}, Doi = {10.1016/S0370-2693(03)00369-1}, Key = {Brower:2003yx} } @article{fds303653, Author = {Chandrasekharan, S and Pepe, M and Steffen, FD and Wiese, U-J}, Title = {Nonlinear Realization of Chiral Symmetry on the Lattice}, Journal = {JHEP}, Volume = {0312}, Pages = {035}, Year = {2003}, Month = {June}, url = {http://arxiv.org/abs/hep-lat/0306020v2}, Abstract = {We formulate lattice theories in which chiral symmetry is realized nonlinearly on the fermion fields. In this framework the fermion mass term does not break chiral symmetry. This property allows us to use the Wilson term to remove the doubler fermions while maintaining exact chiral symmetry on the lattice. Our lattice formulation enables us to address non-perturbative questions in effective field theories of baryons interacting with pions and in models involving constituent quarks interacting with pions and gluons. We show that a system containing a non-zero density of static baryons interacting with pions can be studied on the lattice without encountering complex action problems. In our formulation one can also decide non-perturbatively if the chiral quark model of Georgi and Manohar provides an appropriate low-energy description of QCD. If so, one could understand why the non-relativistic quark model works.}, Doi = {10.1088/1126-6708/2003/12/035}, Key = {fds303653} } @article{Chandrasekharan:2004cn, Author = {Chandrasekharan, S and Wiese, U-J}, Title = {An introduction to chiral symmetry on the lattice}, Journal = {Progress in Particle and Nuclear Physics}, Volume = {53}, Number = {2}, Pages = {373-418}, Year = {2004}, url = {http://arxiv.org/pdf/hep-lat/0405024}, Abstract = {The SU(Nf)L⊗SU(Nf)R chiral symmetry of QCD is of central importance for the nonperturbative low-energy dynamics of light quarks and gluons. Lattice field theory provides a theoretical framework in which these dynamics can be studied from first principles. The implementation of chiral symmetry on the lattice is a nontrivial issue. In particular, local lattice fermion actions with the chiral symmetry of the continuum theory suffer from the fermion doubling problem. The Ginsparg-Wilson relation implies Lüscher's lattice variant of chiral symmetry which agrees with the usual one in the continuum limit. Local lattice fermion actions that obey the Ginsparg-Wilson relation have an exact chiral symmetry, the correct axial anomaly, they obey a lattice version of the Atiyah-Singer index theorem, and still they do not suffer from the notorious doubling problem. The Ginsparg-Wilson relation is satisfied exactly by Neuberger's overlap fermions which are a limit of Kaplan's domain wall fermions, as well as by Hasenfratz and Niedermayer's classically perfect lattice fermion actions. When chiral symmetry is nonlinearly realized in effective field theories on the lattice, the doubling problem again does not arise. This review provides an introduction to chiral symmetry on the lattice with an emphasis on the basic theoretical framework. © 2004 Published by Elsevier B.V.}, Doi = {10.1016/j.ppnp.2004.05.003}, Key = {Chandrasekharan:2004cn} } @article{Chandrasekharan:2003ug, Author = {Chandrasekharan, S}, Title = {Chiral and critical in strong coupling QCD}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {129-130}, Pages = {578-580}, Year = {2004}, url = {http://arxiv.org/pdf/hep-lat/0309098}, Abstract = {We use a cluster algorithm to study the critical behavior of strongly coupled lattice QCD in the chiral limit. We show that the finite temperature chiral phase transition belongs to the O(2) universality class as expected. When we compute the finite size effects of the chiral susceptibility in the low temperature phase close to the transition, we find clear evidence for chiral singularities predicted by chiral perturbation theory (ChPT). On the other hand it is difficult to reconcile the quark mass dependence of various quantities near the chiral limit with ChPT. © 2004 Published by Elsevier B.V.}, Doi = {10.1016/S0920-5632(03)02647-1}, Key = {Chandrasekharan:2003ug} } @article{Chandrasekharan:2003ub, Author = {Chandrasekharan, S and Pepe, M and Steffen, FD and Wiese, U-J}, Title = {Lattice theories with nonlinearly realized chiral symmetry}, Journal = {Nuclear Physics B - Proceedings Supplements}, Volume = {129-130}, Pages = {507-509}, Year = {2004}, url = {http://arxiv.org/pdf/hep-lat/0309093}, Abstract = {We present the lattice formulation of effective Lagrangians in which chiral symmetry is realized nonlinearly on the fermion fields. In this framework both the Wilson term removing unphysical doubler fermions and the fermion mass term do not break chiral symmetry. Our lattice formulation allows us to address non-perturbative questions in effective theories of baryons interacting with pions and in models involving constitutent quarks interacting with pions and gluons. With the presented methods, a system containing a non-zero density of static baryons interacting with pions can be studied on the lattice without encountering a complex action problem. This might lead to new insights into the phase diagram of strongly interacting matter at non-zero chemical potential. © 2004 Elsevier B.V. All rights reserved.}, Doi = {10.1016/S0920-5632(03)02624-0}, Key = {Chandrasekharan:2003ub} } @article{fds245717, Author = {Brower, R and Chandrasekharan, S and Riederer, S and Wiese, U-J}, Title = {D-theory: Field quantization by dimensional reduction of discrete variables}, Journal = {Nuclear Physics B}, Volume = {693}, Number = {1-3}, Pages = {149-175}, Year = {2004}, url = {http://dx.doi.org/10.1016/j.nuclphysb.2004.06.007}, Abstract = {D-theory is an alternative non-perturbative approach to quantum field theory formulated in terms of discrete quantized variables instead of classical fields. Classical scalar fields are replaced by generalized quantum spins and classical gauge fields are replaced by quantum links. The classical fields of a d-dimensional quantum field theory reappear as low-energy effective degrees of freedom of the discrete variables, provided the (d+1)-dimensional D-theory is massless. When the extent of the extra Euclidean dimension becomes small in units of the correlation length, an ordinary d-dimensional quantum field theory emerges by dimensional reduction. The D-theory formulation of scalar field theories with various global symmetries and of gauge theories with various gauge groups is constructed explicitly and the mechanism of dimensional reduction is investigated. © 2004 Elsevier B.V. All rights reserved.}, Doi = {10.1016/j.nuclphysb.2004.06.007}, Key = {fds245717} } @article{fds245720, Author = {Chandrasekharan, S and Strouthos, CG}, Title = {Connecting lattice QCD with chiral perturbation theory at strong coupling}, Journal = {Physical Review D}, Volume = {69}, Number = {9}, Pages = {091502}, Year = {2004}, ISSN = {0556-2821}, url = {http://dx.doi.org/10.1103/PhysRevD.69.091502}, Abstract = {We study the difficulties associated with detecting chiral singularities predicted by chiral perturbation theory (ChPT) in lattice QCD. We focus on the physics of the remnant O(2) chiral symmetry of staggered fermions in the strong coupling limit using the recently discovered directed path algorithm. Since it is easier to look for powerlike singularities as compared to logarithmic ones, our calculations are performed at a fixed finite temperature in the chirally broken phase. We show that the behavior of the chiral condensate, the pion mass, and the pion decay constant are all consistent with the predictions of ChPT for small masses. However, in order to demonstrate this we need quark masses that are much smaller (in lattice units) than those typically used in dynamical QCD simulations. We also need to use higher order terms in the chiral expansion to fit our data. © 2004 The American Physical Society.}, Doi = {10.1103/PhysRevD.69.091502}, Key = {fds245720} } @article{Chandrasekharan:2004kd, Author = {Chandrasekharan, Shailesh and Strouthos, Costas G.}, Title = {Connecting lattice QCD with chiral perturbation theory at strong coupling}, Journal = {Phys. Rev. D}, Volume = {69}, Pages = {091502}, Year = {2004}, url = {http://arxiv.org/pdf/hep-lat/0401002}, Abstract = {http://arxiv.org/abs/hep-lat/0401002}, Key = {Chandrasekharan:2004kd} } @article{Brower:2003vy, Author = {Brower, R. and Chandrasekharan, S. and Riederer, S. and Wiese, U. J.}, Title = {D-theory: Field quantization by dimensional reduction of discrete variables}, Journal = {Nucl. Phys. B}, Volume = {693}, Pages = {149-175}, Year = {2004}, url = {http://arxiv.org/pdf/hep-lat/0309182}, Abstract = {http://arxiv.org/abs/hep-lat/0309182}, Key = {Brower:2003vy} } @article{PhysRevB.72.024525, Author = {Lee, J-W and Chandrasekharan, S and Baranger, HU}, Title = {Quantum Monte Carlo study of disordered fermions}, Journal = {Physical Review B}, Volume = {72}, Number = {2}, Pages = {024525}, Year = {2005}, ISSN = {1098-0121}, url = {http://dx.doi.org/10.1103/PhysRevB.72.024525}, Abstract = {We study a strongly correlated fermionic model with attractive interactions in the presence of disorder in two spatial dimensions. Our model has been designed so that it can be solved using the recently discovered meron-cluster approach. Although the model is unconventional it has the same symmetries as the Hubbard model. Since the naive algorithm is inefficient, we develop an algorithm by combining the meron-cluster technique with the directed-loop update. This combination allows us to compute the pair susceptibility and the winding number susceptibility accurately. We find that the s -wave superconductivity, present in the clean model, does not disappear until the disorder reaches a temperature dependent critical strength. The critical behavior as a function of disorder close to the phase transition belongs to the Berezinky-Kosterlitz-Thouless universality class as expected. The fermionic degrees of freedom, although present, do not appear to play an important role near the phase transition. © 2005 The American Physical Society.}, Doi = {10.1103/PhysRevB.72.024525}, Key = {PhysRevB.72.024525} } @article{PhysRevB.71.201309, Author = {Yoo, J and Chandrasekharan, S and Kaul, RK and Ullmo, D and Baranger, HU}, Title = {Cluster algorithms for quantum impurity models and mesoscopic Kondo physics}, Journal = {Physical Review B - Condensed Matter and Materials Physics}, Volume = {71}, Number = {20}, Pages = {201309}, Year = {2005}, url = {http://dx.doi.org/10.1103/PhysRevB.71.201309}, Abstract = {Nanoscale physics and dynamical mean-field theory have both generated increased interest in complex quantum impurity problems and so have focused attention on the need for flexible quantum impurity solvers. Here we demonstrate that the mapping of single-quantum impurity problems onto spin chains can be exploited to yield a powerful and extremely flexible impurity solver. We implement this cluster algorithm explicitly for the Anderson and Kondo Hamiltonians, and illustrate its use in the "mesoscopic Kondo problem." To study universal Kondo physics, a large ratio between the effective bandwidth Deff and the temperature T is required; our cluster algorithm treats the mesoscopic fluctuations exactly while being able to approach the large Deff T limit with ease. We emphasize that the flexibility of our method allows it to tackle a wide variety of quantum impurity problems; thus, it may also be relevant to the dynamical mean-field theory of lattice problems. © 2005 The American Physical Society.}, Doi = {10.1103/PhysRevB.71.201309}, Key = {PhysRevB.71.201309} } @article{Yoo05_HFsign, Author = {Yoo, J and Chandrasekharan, S and Kaul, RK and Ullmo, D and Baranger, HU}, Title = {On the sign problem in the Hirsch-Fye algorithm for impurity problems}, Journal = {Journal of Physics A: Mathematical and General}, Volume = {38}, Number = {48}, Pages = {10307-10310}, Year = {2005}, url = {http://www.phy.duke.edu/research/cmtheory/bg/paper/yooCKUB05_hfsign.pdf}, Abstract = {We show that there is no fermion sign problem in the Hirsch and Fye algorithm for the single-impurity Anderson model. Beyond the particle-hole symmetric case for which a simple proof exists, this has been known only empirically. Here we prove the nonexistence of a sign problem for the general case by showing that each spin trace for a given Ising configuration is separately positive. We further use this insight to analyse under what conditions orbitally degenerate Anderson models or the two-impurity Anderson model develop a sign. © 2005 IOP Publishing Ltd.}, Doi = {10.1088/0305-4470/38/48/004}, Key = {Yoo05_HFsign} } @article{Chandrasekharan:2004uw, Author = {Chandrasekharan, Shailesh and Strouthos, Costas G.}, Title = {Failure of mean field theory at large N}, Journal = {Phys. Rev. Lett.}, Volume = {94}, Pages = {061601}, Year = {2005}, url = {http://arxiv.org/pdf/hep-lat/0410036}, Abstract = {http://arxiv.org/abs/hep-lat/0410036}, Key = {Chandrasekharan:2004uw} } @article{fds245715, Author = {Chandrasekharan, S and Strouthos, CG}, Title = {Failure of mean field theory at large N.}, Journal = {Physical Review Letters}, Volume = {94}, Number = {6}, Pages = {061601}, Year = {2005}, Month = {February}, ISSN = {0031-9007}, url = {http://www.ncbi.nlm.nih.gov/pubmed/15783719}, Abstract = {We study strongly coupled lattice QCD with N colors of staggered fermions in 3+1 dimensions. While mean field theory describes the low temperature behavior of this theory at large N, it fails in the scaling region close to the finite temperature second order chiral phase transition. The universal critical region close to the phase transition belongs to the 3D XY universality class even when N becomes large. This is in contrast to Gross-Neveu models where the critical region shrinks as N (the number of flavors) increases and mean field theory is expected to describe the phase transition exactly in the limit of infinite N. Our work demonstrates that infrared fluctuations can be important close to second order phase transitions even when N is strictly infinite.}, Doi = {10.1103/PhysRevLett.94.061601}, Key = {fds245715} } @article{Yoo:2004mh, Author = {Yoo, J and Chandrasekharan, S and Baranger, HU}, Title = {Multilevel algorithm for quantum-impurity models.}, Journal = {Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics}, Volume = {71}, Number = {3 Pt 2B}, Pages = {036708}, Year = {2005}, Month = {March}, ISSN = {1539-3755}, url = {http://www.ncbi.nlm.nih.gov/pubmed/15903634}, Abstract = {A continuous-time path integral quantum Monte Carlo method using the directed-loop algorithm is developed to simulate the Anderson single-impurity model in the occupation number basis. Although the method suffers from a sign problem at low temperatures, the new algorithm has many advantages over conventional algorithms. For example, the model can be easily simulated in the Kondo limit without time discretization errors. Furthermore, many observables including the impurity susceptibility and a variety of fermionic observables can be calculated efficiently. Finally the new approach allows us to explore a general technique, called the multilevel algorithm, to solve the sign problem. We find that the multilevel algorithm is able to generate an exponentially large number of configurations with an effort that grows as a polynomial in inverse temperature such that configurations with a positive sign dominate over those with negative signs. Our algorithm can be easily generalized to other multi-impurity problems.}, Doi = {10.1103/PhysRevE.71.036708}, Key = {Yoo:2004mh} } @article{Kaul04_mesokondo, Author = {Kaul, RK and Ullmo, D and Chandrasekharan, S and Baranger, HU}, Title = {Mesoscopic Kondo problem}, Journal = {Epl (Europhysics Letters)}, Volume = {71}, Number = {6}, Pages = {973-979}, Year = {2005}, Month = {September}, url = {http://www.phy.duke.edu/research/cmtheory/bg/paper/kaulucb04_mesokondo.pdf}, Abstract = {We study the effect of mesoscopic fluctuations on a magnetic impurity coupled to a spatially confined electron gas with a temperature in the mesoscopic range (i.e. between the mean level spacing Δ and the Thouless energy ETh). Comparing "poor-man's scaling" with exact Quantum Monte Carlo, we find that for temperatures larger than the Kondo temperature, many aspects of the fluctuations can be captured by the perturbative technique. Using this technique in conjunction with semi-classical approximations, we are able to calculate the mesoscopic fluctuations for a wide variety of systems. For temperatures smaller than the Kondo temperature, we find large fluctuations and deviations from the universal behavior. © EDP Sciences.}, Doi = {10.1209/epl/i2005-10184-1}, Key = {Kaul04_mesokondo} } @article{Chandrasekharan:2005dn, Author = {Chandrasekharan, S and Jiang, F-J}, Title = {Chiral limit of 2-color QCD at strong couplings}, Journal = {PoS LAT}, Volume = {2005}, Pages = {198}, Year = {2005}, Month = {September}, url = {http://arxiv.org/abs/hep-lat/0509117v1}, Abstract = {We study two-color lattice QCD with massless staggered fermions in the strong coupling limit using a new and efficient cluster algorithm. We focus on the phase diagram of the model as a function of temperature $T$ and baryon chemical potential $\mu$ by working on $L_t \times L^d$ lattices in both $d=2,3$. In $d=3$ we find that at $\mu=0$ the ground state of the system breaks the global U(2) symmetry present in the model to U(1), while the finite temperature phase transition (with $L_t=4$) which restores the symmetry is a weak first order transition. In $d=2$ we find evidence for a novel phase transition similar to the Berezinky-Kosterlitz-Thouless phenomena. On the other hand the quantum (T=0) phase transition to a symmetric phase as a function of $\mu$ is second order in both $d=2,3$ and belongs to the mean field universality class.}, Key = {Chandrasekharan:2005dn} } @article{Chandrasekharan:2006iw, Author = {Chandrasekharan, S}, Title = {New approaches to strong coupling lattice QCD}, Journal = {Int. J. Mod. Phys.}, Volume = {B20}, Pages = {2714-2723}, Year = {2006}, Key = {Chandrasekharan:2006iw} } @article{lee-2006, Author = {Lee, Ji-Woo and Chandrasekharan, Shailesh and Baranger, Harold U.}, Title = {Disorder-Induced Superfluidity in Hardcore Bosons in Two Dimensions}, Year = {2006}, url = {http://arxiv.org/pdf/arXiv:0705.0617 [hep-lat]}, Abstract = {http://arxiv.org/abs/arXiv:0705.0617 [hep-lat]}, Key = {lee-2006} } @article{PhysRevLett.97.182001, Author = {Chandrasekharan, Shailesh}, Title = {Anomalous Superfluidity in $(2+1)$-Dimensional Two-Color Lattice QCD}, Journal = {Phys. Rev. Lett.}, Volume = {97}, Number = {18}, Pages = {182001}, Year = {2006}, Key = {PhysRevLett.97.182001} } @article{PhysRevLett.96.176802, Author = {Kaul, Ribhu K. and Zar\'and, Gergely and Chandrasekharan, Shailesh and Ullmo, Denis and Baranger, Harold U.}, Title = {Spectroscopy of the Kondo Problem in a Box}, Journal = {Phys. Rev. Lett.}, Volume = {96}, Number = {17}, Pages = {176802}, Year = {2006}, Key = {PhysRevLett.96.176802} } @article{Chandrasekharan:2006tz, Author = {Chandrasekharan, Shailesh and Jiang, Fu-Jiun}, Title = {Phase-diagram of two-color lattice QCD in the chiral limit}, Journal = {Phys. Rev.}, Volume = {D74}, Pages = {014506}, Year = {2006}, url = {http://arxiv.org/pdf/hep-lat/0602031}, Abstract = {http://arxiv.org/abs/hep-lat/0602031}, Key = {Chandrasekharan:2006tz} } @article{fds245711, Author = {Kaul, RK and Zaránd, G and Chandrasekharan, S and Ullmo, D and Baranger, HU}, Title = {Spectroscopy of the Kondo problem in a box.}, Journal = {Physical Review Letters}, Volume = {96}, Number = {17}, Pages = {176802}, Year = {2006}, Month = {May}, ISSN = {0031-9007}, url = {http://www.ncbi.nlm.nih.gov/pubmed/16712322}, Abstract = {Motivated by experiments on double quantum dots, we study the problem of a single magnetic impurity confined in a finite metallic host. We prove an exact theorem for the ground state spin, and use analytic and numerical arguments to map out the spin structure of the excitation spectrum of the many-body Kondo-correlated state, throughout the weak to strong coupling crossover. These excitations can be probed in a simple tunneling-spectroscopy transport experiment; for that situation we solve rate equations for the conductance.}, Doi = {10.1103/PhysRevLett.96.176802}, Key = {fds245711} } @article{fds245713, Author = {Chandrasekharan, S and Jiang, F-J}, Title = {Phase diagram of two-color lattice QCD in the chiral limit}, Journal = {Physical Review D}, Volume = {74}, Number = {1}, Pages = {014506}, Year = {2006}, Month = {July}, ISSN = {1550-7998}, url = {http://link.aps.org/doi/10.1103/PhysRevD.74.014506}, Abstract = {We study thermodynamics of strongly coupled lattice QCD with two colors of massless staggered fermions as a function of the baryon chemical potential μ in 3+1 dimensions using a new cluster algorithm. We find evidence that the model undergoes a weak first order phase transition at μ=0 which becomes second order at a finite μ. Symmetry considerations suggest that the universality class of these phase transitions should be governed by a O(N)×O(2) field theory with collinear order, with N=3 at μ=0 and N=2 at μ≠0. The universality class of the second order phase transition at μ≠0 appears to be governed by the decoupled XY fixed point present in the O(2)×O(2) field theory. Finally we show that the quantum (T=0) phase transition as a function of μ is a second order mean field transition. © 2006 The American Physical Society.}, Doi = {10.1103/PhysRevD.74.014506}, Key = {fds245713} } @article{PhysRevLett.97.115703, Author = {Priyadarshee, A and Chandrasekharan, S and Lee, J-W and Baranger, HU}, Title = {Quantum phase transitions of hard-core bosons in background potentials.}, Journal = {Physical Review Letters}, Volume = {97}, Number = {11}, Pages = {115703}, Year = {2006}, Month = {September}, ISSN = {0031-9007}, url = {http://www.ncbi.nlm.nih.gov/pubmed/17025902}, Abstract = {We study the zero temperature phase diagram of hard-core bosons in two dimensions subjected to three types of background potentials: staggered, uniform, and random. In all three cases there is a quantum phase transition from a superfluid (at small potential) to a normal phase (at large potential), but with different universality classes. As expected, the staggered case belongs to the XY universality, while the uniform potential induces a mean field transition. The disorder driven transition is clearly different from both; in particular, we find z approximately 1.4, nu approximately 1, and beta approximately 0.6.}, Doi = {10.1103/PhysRevLett.97.115703}, Key = {PhysRevLett.97.115703} } @article{fds245714, Author = {Chandrasekharan, S}, Title = {Anomalous superfluidity in (2+1)-dimensional two-color lattice QCD.}, Journal = {Physical Review Letters}, Volume = {97}, Number = {18}, Pages = {182001}, Year = {2006}, Month = {November}, ISSN = {0031-9007}, url = {http://www.ncbi.nlm.nih.gov/pubmed/17155536}, Abstract = {We study thermodynamics of strongly coupled lattice QCD with two colors of staggered fermions in 2+1 dimensions. The partition function of this model can be written elegantly as a statistical mechanics of dimers and baryon loops. The model is invariant under an SO(3) x U(1) symmetry. At low temperatures, we find evidence for superfluidity in the U(1) symmetry sector while the SO(3) symmetry remains unbroken. The finite temperature phase transition appears to belong to the Kosterlitz-Thouless universality class, but the superfluid density jump rho(s) (T(c)) at the critical temperature T(c) is anomalously higher than the normal value of 2T(c)/pi. We show that, by adding a small SO(3) symmetry breaking term to the model, the superfluid density jump returns to its normal value, implying that the extra symmetry causes anomalous superfluid behavior. Our results may be of interest to researchers studying superfluidity in spin-1 systems.}, Doi = {10.1103/PhysRevLett.97.182001}, Key = {fds245714} } @article{fds245710, Author = {Podolsky, D and Berkeley, UC and Chandrasekharan, S and Vishwanath, A and Berkeley, UC and Berkeley, LBL}, Title = {Novel transitions in S=1 spinor condensates and XY Ashkin-Teller universality}, Journal = {arXiv:0707.0695 [cond-mat.stat-mech]}, Year = {2007}, Abstract = {We study spin-1 polar spinor condensates with magnetic anisotropy, in two spatial dimensions at finite temperatures. The topological binding of vorticity to nematic disclinations leads to a rich phase diagram, which is captured by a U(1) version of the Ashkin-Teller model. In particular, a "cascaded" Kosterlitz-Thouless critical point, with two diverging scales, is predicted. Numerical simulations are performed to check our picture.}, Key = {fds245710} } @article{Chandrasekharan:2007up, Author = {Chandrasekharan, Shailesh and Mehta, Abhijit C.}, Title = {Effects of the anomaly on the two-flavor QCD chiral phase transition}, Journal = {Phys. Rev. Lett.}, Volume = {99}, Pages = {142004}, Year = {2007}, url = {http://arxiv.org/pdf/0705.0617}, Abstract = {http://arxiv.org/abs/0705.0617}, Key = {Chandrasekharan:2007up} } @article{fds303652, Author = {Chandrasekharan, S and Mehta, AC}, Title = {Effects of the anomaly on the two-flavor QCD chiral phase transition}, Journal = {Phys.Rev.Lett.}, Volume = {99}, Pages = {142004}, Year = {2007}, Month = {May}, url = {http://arxiv.org/abs/0705.0617v1}, Abstract = {We use strongly coupled lattice QED with two flavors of massless staggered fermions to model the physics of pions in two-flavor massless QCD. Our model has the right chiral symmetries and can be studied efficiently with cluster algorithms. In particular we can tune the strength of the QCD anomaly and thus study its effects on the chiral phase transition. Our study confirms the widely accepted view point that the chiral phase transition is first order in the absence of the anomaly. Turning on the anomaly weakens the transition and turns it second order at a critical anomaly strength. The anomaly strength at the tricritical point is characterized using $r = (M_{\eta'}-M_{\pi})/\rho_{\eta'}$ where $M_{\eta'}, M_{\pi}$ are the screening masses of the anomalous and regular pions and $\rho_{\eta'}$ is the mass-scale that governs the low energy fluctuations of the anomalous symmetry. We estimate that $r \sim 7 $ in our model. This suggests that a strong anomaly at the two-flavor QCD chiral phase transition is necessary to wash out the first order transition.}, Doi = {10.1103/PhysRevLett.99.142004}, Key = {fds303652} } @article{fds245712, Author = {Chandrasekharan, S and Mehta, AC}, Title = {Effects of the anomaly on the two-flavor QCD chiral phase transition.}, Journal = {Physical Review Letters}, Volume = {99}, Number = {14}, Pages = {142004}, Year = {2007}, Month = {October}, ISSN = {0031-9007}, url = {http://www.ncbi.nlm.nih.gov/pubmed/17930663}, Abstract = {We use strongly coupled lattice QED with two flavors of massless staggered fermions to model the chiral phase transition in two-flavor massless QCD. Our model allows us to vary the QCD anomaly and thus study its effects on the transition. Our study confirms the widely accepted viewpoint that the chiral phase transition is first order in the absence of the anomaly. Turning on the anomaly weakens the transition and turns it second order at a critical anomaly strength. The anomaly strength at the tricritical point is characterized using r=(M(eta')-M(pi))/rho(eta'), where M(eta'), M(pi) are the screening masses of the anomalous and regular pions and rho(eta') is the mass scale that governs the low energy fluctuations of the anomalous symmetry. We estimate that r ~ 7 in our model. This suggests that a strong anomaly at the two-flavor QCD chiral phase transition is necessary to wash out the first order transition.}, Doi = {10.1103/physrevlett.99.142004}, Key = {fds245712} } @article{fds245705, Author = {Cecile, DJ and Chandrasekharan, S}, Title = {Role of the σ resonance in determining the convergence of chiral perturbation theory}, Journal = {Physical Review D}, Volume = {77}, Number = {9}, Pages = {091501}, Year = {2008}, ISSN = {1550-7998}, url = {http://link.aps.org/doi/10.1103/PhysRevD.77.091501}, Abstract = {The dimensionless parameter ξ=Mπ2/(16π2Fπ2), where Fπ is the pion decay constant and Mπ is the pion mass, is expected to control the convergence of chiral perturbation theory applicable to QCD. Here we demonstrate that a strongly coupled lattice gauge theory model with the same symmetries as two-flavor QCD but with a much lighter σ-resonance is different. We first confirm that the leading low-energy constants appearing in the chiral Lagrangian are the same when calculated from the p-regime and the -regime as expected. However, ξ 0.002 is necessary before 1-loop chiral perturbation theory predicts the data within 1%. For ξ>0.0035 the data begin to deviate dramatically from 1-loop chiral perturbation theory predictions. We argue that this qualitative change is due to the presence of a light σ-resonance in our model. Our findings may be useful for lattice QCD studies. © 2008 The American Physical Society.}, Doi = {10.1103/PhysRevD.77.091501}, Key = {fds245705} } @article{Chandrasekharan:2006wn, Author = {Chandrasekharan, S and Jiang, F-J and Pepe, M and Wiese, U-J}, Title = {Rotor spectra, berry phases, and monopole fields: From antiferromagnets to QCD}, Journal = {Physical Review D}, Volume = {78}, Number = {7}, Pages = {077901}, Year = {2008}, ISSN = {1550-7998}, url = {http://arxiv.org/pdf/cond-mat/0612252}, Abstract = {The order parameter of a finite system with a spontaneously broken continuous global symmetry acts as a quantum mechanical rotor. Both antiferromagnets with a spontaneously broken SU(2)s spin symmetry and massless QCD with a broken SU(2)L×SU(2)R chiral symmetry have rotor spectra when considered in a finite volume. When an electron or hole is doped into an antiferromagnet or when a nucleon is propagating through the QCD vacuum, a Berry phase arises from a monopole field and the angular momentum of the rotor is quantized in half-integer units. © 2008 The American Physical Society.}, Doi = {10.1103/PhysRevD.78.077901}, Key = {Chandrasekharan:2006wn} } @article{fds245707, Author = {Jiang, F-J and Nyfeler, M and Chandrasekharan, S and Wiese, U-J}, Title = {From an antiferromagnet to a valence bond solid: Evidence for a first-order phase transition}, Journal = {Journal of Statistical Mechanics: Theory and Experiment}, Volume = {2008}, Number = {2}, Pages = {P02009}, Year = {2008}, ISSN = {1742-5468}, url = {http://iopscience.iop.org/1742-5468/2008/02/P02009/}, Abstract = {Using a loop-cluster algorithm we investigate the spin- Heisenberg antiferromagnet on a square lattice with exchange coupling J and an additional four-spin interaction of strength Q. We confirm the existence of a phase transition separating antiferromagnetism at J/Q>Jc/Q from a valence bond solid (VBS) state at J/Q<Jc/Q. Although our Monte Carlo data are consistent with those of previous studies, we do not confirm the existence of a deconfined quantum critical point. Instead, using a flowgram method on lattices as large as 802, we find evidence for a weak first-order phase transition. We also present a detailed study of the antiferromagnetic phase. For J/Q>Jc/Q the staggered magnetization, the spin stiffness and the spinwave velocity of the antiferromagnet are determined by fitting Monte Carlo data to analytic results from the systematic low-energy effective field theory for magnons. Finally, we also investigate the physics of the VBS state at J/Q<Jc/Q and we show that long but finite antiferromagnetic correlations are still present. © 2008 IOP Publishing Ltd.}, Doi = {10.1088/1742-5468/2008/02/P02009}, Key = {fds245707} } @article{fds245708, Author = {Cecile, DJ and Chandrasekharan, S}, Title = {Absence of vortex condensation in a two dimensional fermionic XY model}, Journal = {Physical Review D}, Volume = {77}, Number = {5}, Pages = {054502}, Year = {2008}, ISSN = {1550-7998}, url = {http://link.aps.org/abstract/PRD/v77/e054502}, Abstract = {Motivated by a puzzle in the study of two-dimensional lattice quantum electrodynamics with staggered fermions, we construct a two-dimensional fermionic model with a global U(1) symmetry. Our model can be mapped into a model of closed packed dimers and plaquettes. Although the model has the same symmetries as the XY model, we show numerically that the model lacks the well-known Kosterlitz-Thouless phase transition. The model is always in the gapless phase showing the absence of a phase with vortex condensation. In other words the low energy physics is described by a noncompact U(1) field theory. We show that by introducing an even number of layers one can introduce vortex condensation within the model and thus also induce a Kosterlitz-Thouless transition. © 2008 The American Physical Society.}, Doi = {10.1103/PhysRevD.77.054502}, Key = {fds245708} } @article{fds245709, Author = {Cecile, DJ and Chandrasekharan, S}, Title = {Modeling pion physics in the -regime of two-flavor QCD using strong coupling lattice QED}, Journal = {Physical Review D}, Volume = {77}, Number = {1}, Pages = {014506}, Year = {2008}, ISSN = {1550-7998}, url = {http://link.aps.org/abstract/PRD/v77/e014506}, Abstract = {In order to model pions of two-flavor QCD we consider a lattice field theory involving two flavors of staggered quarks interacting strongly with U(1) gauge fields. For massless quarks, this theory has an SUL(2)×SUR(2) ×UA(1) symmetry. By adding a four-fermion term we can break the UA(1) symmetry and thus incorporate the physics of the QCD anomaly. We can also tune the pion decay constant F, to be small compared to the lattice cutoff by starting with an extra fictitious dimension, thus allowing us to model low energy pion physics in a setting similar to lattice QCD from first principles. However, unlike lattice QCD, a major advantage of our model is that we can easily design efficient algorithms to compute a variety of quantities in the chiral limit. Here we show that the model reproduces the predictions of chiral perturbation theory in the -regime. © 2008 The American Physical Society.}, Doi = {10.1103/PhysRevD.77.014506}, Key = {fds245709} } @article{Cecile:2007dv, Author = {Cecile, D. J. and Chandrasekharan, Shailesh}, Title = {Modeling pion physics in the $\epsilon$-regime of two- flavor QCD using strong coupling lattice QED}, Journal = {Phys. Rev.}, Volume = {D77}, Pages = {014506}, Year = {2008}, url = {http://arxiv.org/pdf/arXiv:0708.0558 [hep-lat]}, Abstract = {http://arxiv.org/abs/arXiv:0708.0558 [hep-lat]}, Key = {Cecile:2007dv} } @article{Chandrasekharan:2008gp, Author = {Chandrasekharan, Shailesh}, Title = {A new computational approach to lattice quantum field theories}, Journal = {PoS}, Volume = {LATTICE2008}, Pages = {003}, Year = {2008}, url = {http://arxiv.org/pdf/0810.2419}, Abstract = {http://arxiv.org/abs/0810.2419}, Key = {Chandrasekharan:2008gp} } @article{Chandrasekharan:2007sch, Author = {Jiang, F.-J. and Nyfeler, M. and Chandrasekharan, S. and Wiese, U. -J.}, Title = {From an Antiferromagnet to a Valence Bond Solid: Evidence for a First Order Phase Transition}, Journal = {J. of Stat. Mech}, Volume = {P02009}, Year = {2008}, Key = {Chandrasekharan:2007sch} } @article{chandrasekharan:077901, Author = {S. Chandrasekharan and F.-J. Jiang and M. Pepe and U.-J. Wiese}, Title = {Rotor spectra, berry phases, and monopole fields: From antiferromagnets to QCD}, Journal = {Physical Review D (Particles and Fields)}, Volume = {78}, Number = {7}, Pages = {077901}, Publisher = {APS}, Year = {2008}, url = {http://link.aps.org/abstract/PRD/v78/e077901}, Key = {chandrasekharan:077901} } @article{Cecile:2008kp, Author = {Cecile, D. J. and Chandrasekharan, Shailesh}, Title = {Role of the $\sigma$-resonance in determining the convergence of chiral perturbation theory}, Journal = {Phys. Rev.}, Volume = {D77}, Pages = {091501}, Year = {2008}, url = {http://arxiv.org/pdf/0801.3823}, Abstract = {http://arxiv.org/abs/0801.3823}, Key = {Cecile:2008kp} } @article{Cecile:2008nb, Author = {Cecile, D. J. and Chandrasekharan, Shailesh}, Title = {Absence of vortex condensation in a two dimensional fermionic XY model}, Journal = {Phys. Rev.}, Volume = {D77}, Pages = {054502}, Year = {2008}, url = {http://arxiv.org/pdf/arXiv:0801.1857 [hep-lat]}, Abstract = {http://arxiv.org/abs/arXiv:0801.1857 [hep-lat]}, Key = {Cecile:2008nb} } @article{fds245669, Author = {Jiang, F-J and Nyfeler, M and Chandrasekharan, S and Wiese, U-J}, Title = {From an antiferromagnet to a valence bond solid: evidence for a first-order phase transition}, Journal = {Journal of Statistical Mechanics: Theory and Experiment}, Year = {2008}, Month = {February}, ISSN = {1742-5468}, url = {http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000254376400010&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=47d3190e77e5a3a53558812f597b0b92}, Doi = {10.1088/1742-5468/2008/02/P02009}, Key = {fds245669} } @article{Cecile:2008gs, Author = {Cecile, DJ and Chandrasekharan, S}, Title = {Sigma-resonance and convergence of chiral perturbation theory}, Journal = {PoS LATTICE}, Volume = {2008}, Pages = {071}, Year = {2008}, Month = {October}, url = {http://arxiv.org/abs/0810.2423v1}, Abstract = {The dimensionless parameter $\xi' = M^2/(16 \pi^2 F^2)$, where $F$ is the pion decay constant in the chiral limit and $M$ is the pion mass at leading order in the quark mass, is expected to control the convergence of chiral perturbation theory applicable to QCD. Here we demonstrate that a strongly coupled lattice gauge theory model with the same symmetries as two-flavor QCD but with a much lighter $\sigma$-resonance is different. Our model allows us to study efficiently the convergence of chiral perturbation theory as a function of $\xi'$. We first confirm that the leading low energy constants appearing in the chiral Lagrangian are the same when calculated from the $\epsilon$-regime and the $p$-regime. However, $\xi' \lesssim 0.002$ is necessary before 1-loop chiral perturbation theory predicts the data within 1%. However, for $\xi' > 0.0035$ the data begin to deviate qualitatively from 1-loop chiral perturbation theory predictions. We argue that this qualitative change is due to the presence of a light $\sigma$-resonance in our model. Our findings may be useful for lattice QCD studies.}, Key = {Cecile:2008gs} } @article{Chandrasekharan:2008gp, Author = {Chandrasekharan, S}, Title = {A new computational approach to lattice quantum field theories}, Journal = {PoS LATTICE}, Volume = {2008}, Pages = {003}, Year = {2008}, Month = {October}, url = {http://arxiv.org/abs/0810.2419v1}, Abstract = {Developments in algorithms over the past decade suggest that there is a new computational approach to a class of quantum field theories. This approach is based on rewriting the partition function in a representation similar to the world-line representation and hence we shall call it the "WL-approach". This approach is likely to be more powerful than the conventional approach in some regions of parameter space, especially in the presence of chemical potentials or massless fermions. While world-line representations are natural in the Hamiltonian formulation, they can also be constructed directly in Euclidean space. We first describe the approach and its advantages by considering the classical XY model in the presence of a chemical potential. We then argue that, $CP^{N-1}$ models, models of pions on the lattice and the lattice massless Thirring model, can all be formulated and solved using the WL-approach. In particular, we discover that the WL-approach to the Thirring model leads to a novel determinantal Monte-Carlo algorithm which we call the "dynamical-bag" algorithm. Finally, we argue that a simple extension of the WL-approach to gauge theories leads to a world-sheet, "WS-approach", in Abelian Lattice Gauge theory.}, Key = {Chandrasekharan:2008gp} } @article{fds245704, Author = {Kaul, RK and Ullmo, D and Zaránd, G and Chandrasekharan, S and Baranger, HU}, Title = {Ground state and excitations of quantum dots with magnetic impurities}, Journal = {Physical Review B}, Volume = {80}, Number = {3}, Pages = {035318}, Year = {2009}, ISSN = {1098-0121}, url = {http://link.aps.org/abstract/PRB/v80/e035318}, Abstract = {We consider an "impurity" with a spin degree of freedom coupled to a finite reservoir of noninteracting electrons, a system which may be realized by either a true impurity in a metallic nanoparticle or a small quantum dot coupled to a large one. We show how the physics of such a spin impurity is revealed in the many-body spectrum of the entire finite-size system; in particular, the evolution of the spectrum with the strength of the impurity-reservoir coupling reflects the fundamental many-body correlations present. Explicit calculation in the strong- and the weak-coupling limits shows that the spectrum and its evolution are sensitive to the nature of the impurity and the parity of electrons in the reservoir. The effect of the finite-size spectrum on two experimental observables is considered. First, we propose an experimental setup in which the spectrum may be conveniently measured using tunneling spectroscopy. A rate equation calculation of the differential conductance suggests how the many-body spectral features may be observed. Second, the finite-temperature magnetic susceptibility is presented, both the impurity and the local susceptibilities. Extensive quantum Monte Carlo calculations show that the local susceptibility deviates from its bulk scaling form. Nevertheless, for special assumptions about the reservoir-the "clean Kondo box" model-we demonstrate that finite-size scaling is recovered. Explicit numerical evaluations of these scaling functions are given, both for even and odd parities and for the canonical and the grand-canonical ensembles. © 2009 The American Physical Society.}, Doi = {10.1103/PhysRevB.80.035318}, Key = {fds245704} } @article{fds184647, Author = {D. Podolski and S. Chandrasekharan and A.Vishwanath}, Title = {Phase Transitions of S=1 Spinor Condensates in an optical lattice}, Journal = {Physical Review B}, Volume = {80}, Pages = {214513}, Year = {2009}, url = {http://prb.aps.org/abstract/PRB/v80/i21/e214513}, Abstract = {We study the phase diagram of spin-one polar condensates in a two dimensional optical lattice with magnetic anisotropy. We show that the topological binding of vorticity to nematic disclinations allows for a rich variety of phase transitions. These include Kosterlitz-Thouless-like transitions with a superfluid stiffness jump that can be experimentally tuned to take a continuous set of values, and a new cascaded Kosterlitz-Thouless" transition, characterized by two divergent length scales. For higher boson spin S, the thermal phase transitions are strongly affected by the parity of S.}, Key = {fds184647} } @article{Podolsky:2007x, Author = {Podolsky, D and Chandrasekharan, S and Vishwanath, A}, Title = {Phase transitions of spinor condensates in an optical lattice}, Journal = {Physical Review B}, Volume = {80}, Number = {21}, Year = {2009}, Month = {December}, ISSN = {1098-0121}, url = {http://hdl.handle.net/10161/3298 Duke open access}, Abstract = {We study the phase diagram of spin-one polar condensates in a two-dimensional optical lattice with magnetic anisotropy. We show that the topological binding of vorticity to nematic disclinations allows for a rich variety of phase transitions. These include Kosterlitz-Thouless-like transitions with a superfluid stiffness jump that can be experimentally tuned to take a continuous set of values, and a cascaded Kosterlitz-Thouless transition, characterized by two divergent length scales. For higher integer spin bosons S, the thermal phase transition out of the planar polar phase is strongly affected by the parity of S. © 2009 The American Physical Society.}, Doi = {10.1103/PhysRevB.80.214513}, Key = {Podolsky:2007x} } @article{fds245684, Author = {Liu, DE and Chandrasekharan, S and Baranger, HU}, Title = {Conductance of quantum impurity models from quantum monte carlo}, Journal = {Physical Review B}, Volume = {82}, Number = {16}, Pages = {165447}, Year = {2010}, ISSN = {1098-0121}, url = {http://hdl.handle.net/10161/4258 Duke open access}, Abstract = {The conductance of two Anderson impurity models, one with twofold and another with fourfold degeneracy, representing two types of quantum dots, is calculated using a world-line quantum Monte Carlo (QMC) method. Extrapolation of the imaginary time QMC data to zero frequency yields the linear conductance, which is then compared to numerical renormalization-group results in order to assess its accuracy. We find that the method gives excellent results at low temperature (T TK) throughout the mixed-valence and Kondo regimes but it is unreliable for higher temperature. © 2010 The American Physical Society.}, Doi = {10.1103/PhysRevB.82.165447}, Key = {fds245684} } @article{fds245685, Author = {Chandrasekharan, S}, Title = {Fermion bag approach to lattice field theories}, Journal = {Physical Review D}, Volume = {82}, Number = {2}, Pages = {025007}, Year = {2010}, ISSN = {1550-7998}, url = {http://hdl.handle.net/10161/4276 Duke open access}, Abstract = {We propose a new approach to the fermion sign problem in systems where there is a coupling U such that when it is infinite the fermions are paired into bosons, and there is no fermion permutation sign to worry about. We argue that as U becomes finite, fermions are liberated but are naturally confined to regions which we refer to as fermion bags. The fermion sign problem is then confined to these bags and may be solved using the determinantal trick. In the parameter regime where the fermion bags are small and their typical size does not grow with the system size, construction of Monte Carlo methods that are far more efficient than conventional algorithms should be possible. In the region where the fermion bags grow with system size, the fermion bag approach continues to provide an alternative approach to the problem but may lose its main advantage in terms of efficiency. The fermion bag approach also provides new insights and solutions to sign problems. A natural solution to the "silver blaze problem" also emerges. Using the three-dimensional massless lattice Thirring model as an example, we introduce the fermion bag approach and demonstrate some of these features. We compute the critical exponents at the quantum phase transition and find ν=0.87(2) and η=0.62(2). © 2010 The American Physical Society.}, Doi = {10.1103/PhysRevD.82.025007}, Key = {fds245685} } @article{fds245686, Author = {Banerjee, D and Chandrasekharan, S}, Title = {Finite size effects in the presence of a chemical potential: A study in the classical nonlinear O(2) sigma model}, Journal = {Physical Review D}, Volume = {81}, Number = {12}, Pages = {125007}, Year = {2010}, ISSN = {1550-7998}, url = {http://hdl.handle.net/10161/4275 Duke open access}, Abstract = {In the presence of a chemical potential, the physics of level crossings leads to singularities at zero temperature, even when the spatial volume is finite. These singularities are smoothed out at a finite temperature but leave behind nontrivial finite size effects which must be understood in order to extract thermodynamic quantities using Monte Carlo methods, particularly close to critical points. We illustrate some of these issues using the classical nonlinear O(2) sigma model with a coupling β and chemical potential μ on a 2+1-dimensional Euclidean lattice. In the conventional formulation this model suffers from a sign problem at nonzero chemical potential and hence cannot be studied with the Wolff cluster algorithm. However, when formulated in terms of the worldline of particles, the sign problem is absent, and the model can be studied efficiently with the "worm algorithm." Using this method we study the finite size effects that arise due to the chemical potential and develop an effective quantum mechanical approach to capture the effects. As a side result we obtain energy levels of up to four particles as a function of the box size and uncover a part of the phase diagram in the (β,μ) plane. © 2010 The American Physical Society.}, Doi = {10.1103/PhysRevD.81.125007}, Key = {fds245686} } @article{fds245682, Author = {Chandrasekharan, S and Li, A}, Title = {Anomaly and a QCD-like phase diagram with massive bosonic baryons}, Journal = {Journal of High Energy Physics}, Volume = {2010}, Number = {12}, Year = {2010}, Month = {December}, ISSN = {1126-6708}, url = {http://www.springerlink.com/content/f44x269565536614/}, Abstract = {We study a strongly coupled Z 2 lattice gauge theory with two flavors of quarks, invariant under an exact SU(2)×SU(2)×U A(1)×U B(1) symmetry which is the same as in QCD with two flavors of quarks without an anomaly. The model also contains a coupling that can be used to break the U A(1) symmetry and thus mimic the QCD anomaly. At low temperatures T and small baryon chemical potential μ B the model contains massless pions and massive bosonic baryons similar to QCD with an even number of colors. In this work we study the T - μ B phase diagram of the model and show that it contains three phases: (1) A chirally broken phase at low T and μ B, (2) a chirally symmetric baryon superfluid phase at low T and high μ B, and (3) a symmetric phase at high T. We find that the nature of the finite temperature chiral phase transition and in particular the location of the tricritical point that seperates the first order line from the second order line is affected significantly by the anomaly. © 2010 SISSA.}, Doi = {10.1007/JHEP12(2010)021}, Key = {fds245682} } @article{fds245683, Author = {Liu, DE and Chandrasekharan, S and Baranger, HU}, Title = {Quantum phase transition and emergent symmetry in a quadruple quantum dot system.}, Journal = {Phys Rev Lett}, Volume = {105}, Number = {25}, Pages = {256801}, Year = {2010}, Month = {December}, url = {http://www.ncbi.nlm.nih.gov/pubmed/21231607}, Abstract = {We propose a system of four quantum dots designed to study the competition between three types of interactions: Heisenberg, Kondo, and Ising. We find a rich phase diagram containing two sharp features: a quantum phase transition (QPT) between charge-ordered and charge-liquid phases and a dramatic resonance in the charge liquid visible in the conductance. The QPT is of the Kosterlitz-Thouless type with a discontinuous jump in the conductance at the transition. We connect the resonance phenomenon with the degeneracy of three levels in the isolated quadruple dot and argue that this leads to a Kondo-like emergent symmetry from left-right Z2 to U(1).}, Doi = {10.1103/PhysRevLett.105.256801}, Key = {fds245683} } @article{fds245681, Author = {Chandrasekharan, S and Li, A}, Title = {Fermion bag approach to the sign problem in strongly coupled lattice QED with Wilson fermions}, Journal = {Journal of High Energy Physics}, Volume = {2011}, Number = {1}, Year = {2011}, ISSN = {1126-6708}, url = {http://www.springerlink.com/content/e051x42j2xn1x087/}, Abstract = {We explore the sign problem in strongly coupled lattice QED with one flavor of Wilson fermions in four dimensions using the fermion bag formulation. We construct rules to compute the weight of a fermion bag and show that even though the fermions are confined into bosons, fermion bags with negative weights do exist. By classifying fermion bags as either simple or complex, we find numerical evidence that large complex bags with positive and negative weights come with equal probabilities. On the other hand simple bags have a large probability of having a positive weight. In analogy with the meron cluster approach, we suggest that eliminating the complex bags from the partition function should alleviate the sign problem while capturing the important physics. We also find a modified model containing only simple bags which does not suffer from any sign problem and argue that it contains a parity breaking phase transition similar to the original model. We also prove that when the hopping parameter is strictly infinite all fermion bags are non-negative. © SISSA 2011.}, Doi = {10.1007/JHEP01(2011)018}, Key = {fds245681} } @article{fds303646, Author = {Chandrasekharan, S and Li, A}, Title = {The generalized fermion-bag approach}, Journal = {Proceedings of Science}, Volume = {139}, Pages = {058}, Year = {2011}, Month = {January}, url = {http://arxiv.org/abs/1111.5276v1}, Abstract = {© Copyright owned by the author(s). We present a new approach to some four-fermion lattice field theories which we call the generalized fermion bag approach. The basic idea is to identify unpaired fermionic degrees of freedom that cause sign problems and collect them in a bag. Paired fermions usually act like bosons and do not lead to sign problems. A resummation of all unpaired fermion degrees of freedom inside the bag is sufficient to solve the fermion sign problem in a variety of interesting cases. Using a concept of duality we then argue that the size of the fermion bags is small both at strong and weak couplings. This allows us to construct efficient algorithms in both these limits. Using the fermion bag approach, we study the quantum phase transition of the 3D massless lattice Thirrring model which is of interest in the context of Graphene. Using our method we are able to solve the model on lattices as large as 403with moderate computational resources. We obtain the precise location of the quantum critical point and the values of the critical exponents through this study.}, Key = {fds303646} } @article{fds303643, Author = {Chandrasekharan, S and Wiese, U-J}, Title = {Partition Functions of Strongly Correlated Electron Systems as "Fermionants"}, Year = {2011}, Month = {August}, url = {http://arxiv.org/abs/1108.2461v1}, Abstract = {We introduce a new mathematical object, the "fermionant" ${\mathrm{Ferm}}_N(G)$, of type $N$ of an $n \times n$ matrix $G$. It represents certain $n$-point functions involving $N$ species of free fermions. When N=1, the fermionant reduces to the determinant. The partition function of the repulsive Hubbard model, of geometrically frustrated quantum antiferromagnets, and of Kondo lattice models can be expressed as fermionants of type N=2, which naturally incorporates infinite on-site repulsion. A computation of the fermionant in polynomial time would solve many interesting fermion sign problems.}, Key = {fds303643} } @article{PhysRevD.85.091502, Author = {Chandrasekharan, S and Li, A}, Title = {Fermion bag solutions to some sign problems in four-fermion field theories}, Journal = {Physical Review D}, Volume = {85}, Number = {9}, Pages = {091502}, Publisher = {American Physical Society}, Year = {2012}, ISSN = {1550-7998}, url = {http://link.aps.org/doi/10.1103/PhysRevD.85.091502}, Abstract = {Lattice four-fermion models containing N flavors of staggered fermions, which are invariant under Z 2 and U(1) chiral symmetries, are known to suffer from sign problems when formulated using the auxiliary field approach. Although these problems have been ignored in previous studies, they can be severe. Here we show that the sign problems disappear when the models are formulated in the fermion bag approach, allowing us to solve them rigorously for the first time. © 2012 American Physical Society.}, Doi = {10.1103/PhysRevD.85.091502}, Key = {PhysRevD.85.091502} } @article{PhysRevD.86.021701, Author = {Chandrasekharan, S}, Title = {Solutions to sign problems in lattice Yukawa models}, Journal = {Physical Review D}, Volume = {86}, Number = {2}, Pages = {021701}, Publisher = {American Physical Society}, Year = {2012}, ISSN = {1550-7998}, url = {http://link.aps.org/doi/10.1103/PhysRevD.86.021701}, Abstract = {We prove that sign problems in the traditional approach to some lattice Yukawa models can be completely solved when fermions are formulated using fermion bags and bosons are formulated in the worldline representation. We prove this within the context of two examples of three-dimensional models, symmetric under U L(1)×U R(1)×Z 2(parity) transformations, one involving staggered fermions and the other involving Wilson fermions. We argue that these models have interesting quantum phase transitions that can now be studied using Monte Carlo methods. © 2012 American Physical Society.}, Doi = {10.1103/PhysRevD.86.021701}, Key = {PhysRevD.86.021701} } @article{PhysRevLett.108.140404, Author = {Chandrasekharan, S and Li, A}, Title = {Fermion bags, duality, and the three dimensional massless lattice thirring model.}, Journal = {Phys Rev Lett}, Volume = {108}, Number = {14}, Pages = {140404}, Publisher = {American Physical Society}, Year = {2012}, Month = {April}, url = {http://www.ncbi.nlm.nih.gov/pubmed/22540775}, Abstract = {The recently proposed fermion-bag approach is a powerful technique to solve some four-fermion lattice field theories. Because of the existence of a duality between strong and weak couplings, the approach leads to efficient Monte Carlo algorithms in both these limits. The new method allows us for the first time to accurately compute quantities close to the quantum critical point in the three dimensional lattice Thirring model with massless fermions on large lattices. The critical exponents at the quantum critical point are found to be ν=0.85(1), η=0.65(1), and η(ψ)=0.37(1).}, Doi = {10.1103/PhysRevLett.108.140404}, Key = {PhysRevLett.108.140404} } @article{PosLattice2012, Author = {Chandrasekharan Shailesh}, Title = {Fermion Bag Solutions to Sign Problems}, Journal = {Proceedings of Science}, Volume = {Lattice2012}, Pages = {224}, Year = {2012}, Month = {December}, Key = {PosLattice2012} } @article{fds323137, Author = {Chandrasekharan, S}, Title = {Quantum critical behavior with massless staggered fermions in three dimensions}, Journal = {Proceedings of Science}, Volume = {29-July-2013}, Pages = {049}, Year = {2013}, Month = {January}, url = {http://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=187}, Abstract = {We report on studies of quantum critical behavior in three dimensional lattice Gross-Neveu models with one flavor of staggered fermions. We focus on two models, one with SU(2)×Z2 symmetry and the other with an SU(2)×U(1) symmetry. Both these models could not be studied earlier with conventional Monte Carlo methods due to sign problems. However, the fermion bag approach is free of sign problems for these models and allows us to compute the critical exponents at the quantum phase transition that separates the massless fermion phase at small couplings and the massive fermion phase at large couplings. Our results help resolve some old puzzles in the field.}, Key = {fds323137} } @article{fds303641, Author = {Chandrasekharan, S}, Title = {Fermion Bag Approach to Fermion Sign Problems}, Volume = {49}, Pages = {90}, Year = {2013}, Month = {April}, url = {http://arxiv.org/abs/1304.4900v1}, Abstract = {The fermion bag approach is a new method to tackle fermion sign problems in lattice field theories. Using this approach it is possible to solve a class of sign problems that seem unsolvable by traditional methods. The new solutions emerge when partition functions are written in terms of fermion bags and bosonic worldlines. In these new variables it is possible to identify hidden pairing mechanisms which lead to the solutions. The new solutions allow us for the first time to use Monte Carlo methods to solve a variety of interesting lattice field theories, thus creating new opportunities for understanding strongly correlated fermion systems.}, Doi = {10.1140/epja/i2013-13090-y}, Key = {fds303641} } @article{fds303640, Author = {Chandrasekharan, S and Li, A}, Title = {Quantum critical behavior in three dimensional lattice Gross-Neveu models}, Volume = {88}, Pages = {021701}, Year = {2013}, Month = {April}, url = {http://arxiv.org/abs/1304.7761v1}, Abstract = {We study quantum critical behavior in three dimensional lattice Gross-Neveu models containing two massless Dirac fermions. We focus on two models with SU(2) flavor symmetry and either a $Z_2$ or a U(1) chiral symmetry. Both models could not be studied earlier due to sign problems. We use the fermion bag approach which is free of sign problems and compute critical exponents at the phase transitions. We estimate $\nu = 0.83(1)$, $\eta = 0.62(1)$, $\eta_\psi = 0.38(1)$ in the $Z_2$ and $\nu = 0.849(8)$, $\eta = 0.633(8)$, $\eta_\psi = 0.373(3)$ in the U(1) model.}, Doi = {10.1103/PhysRevD.88.021701}, Key = {fds303640} } @article{fds303639, Author = {Huffman, EF and Chandrasekharan, S}, Title = {Solution to sign problems in half-filled spin-polarized electronic systems}, Journal = {Phys. Rev. B}, Volume = {89}, Pages = {111101}, Year = {2013}, Month = {October}, url = {http://arxiv.org/abs/1311.0034v1}, Abstract = {We solve the sign problem in a particle-hole symmetric spin-polarized fermion model on bipartite lattices using the idea of fermion bags. The solution can be extended to a class of models at half filling but without particle-hole symmetry. Attractive Hubbard models with an odd number of fermion species can also be solved. The new solutions should allow us to study quantum phase transitions that have remained unexplored so far due to sign problems.}, Doi = {10.1103/PhysRevB.89.111101}, Key = {fds303639} } @article{fds225570, Author = {E.F. Huffman and S. Chandrasekharan}, Title = {Solution to sign problems in half-filled spin-polarized electronic systems}, Journal = {Phys. Rev. (Rapid Communications)}, Volume = {B89}, Pages = {111101}, Year = {2014}, Month = {February}, url = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.111101}, Abstract = {http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.111101}, Doi = {10.1103/PhysRevB.89.111101}, Key = {fds225570} } @article{fds303638, Author = {Huffman, E and Chandrasekharan, S}, Title = {Solution to new sign problems with Hamiltonian Lattice Fermions}, Journal = {PoS (LATTICE 2014) 058}, Year = {2014}, Month = {November}, url = {http://arxiv.org/abs/1411.7147v2}, Abstract = {We present a solution to the sign problem in a class of particle-hole symmetric Hamiltonian lattice fermion models on bipartite lattices using the idea of fermion bags. The solution remains valid when the particle-hole symmetry is broken through a staggered chemical potential term. This solution allows, for the first time, simulations of some massless four-fermion models with minimal fermion doubling and with an odd number of fermion flavors using ultra-local actions. One can thus study a variety of quantum phase transitions that have remained unexplored so far due to sign problems.}, Key = {fds303638} } @article{fds245668, Author = {Zou, H and Liu, Y and Lai, C-Y and Unmuth-Yockey, J and Yang, L-P and Bazavov, A and Xie, ZY and Xiang, T and Chandrasekharan, S and Tsai, S-W and Meurice, Y}, Title = {Progress towards quantum simulating the classical model}, Journal = {Physical Review A}, Volume = {90}, Number = {6}, Year = {2014}, Month = {December}, ISSN = {1050-2947}, url = {http://dx.doi.org/10.1103/PhysRevA.90.063603}, Doi = {10.1103/PhysRevA.90.063603}, Key = {fds245668} } @article{fds245667, Author = {Ayyar, V and Chandrasekharan, S}, Title = {Massive fermions without fermion bilinear condensates}, Journal = {Physical Review D}, Volume = {91}, Number = {6}, Year = {2015}, Month = {March}, ISSN = {1550-7998}, url = {http://dx.doi.org/10.1103/PhysRevD.91.065035}, Doi = {10.1103/PhysRevD.91.065035}, Key = {fds245667} } @article{fds327863, Author = {Chandrasekharan, S}, Title = {Fermion bags, topology and index theorems}, Journal = {Proceedings of Science}, Volume = {Part F128557}, Year = {2016}, Month = {January}, Abstract = {© Copyright owned by the author(s). The fermion bag formulation helps to extend the concepts of topology and index theorem associated with non-Abelian gauge theories to simple lattice fermion field theories. Using this extension we can argue that fermion masses can arise either through the traditional mechanism where some lattice symmetry of the action that forbids fermion mass terms is explicitly, anomalously, or spontaneously broken, or through a non-traditional mechanism where all lattice symmetries continue to be preserved. We provide examples of simple fermion lattice field theories for each of these scenarios of fermion mass generation.}, Key = {fds327863} } @article{fds323136, Author = {Ayyar, V and Chandrasekharan, S}, Title = {Origin of fermion masses without spontaneous symmetry breaking}, Journal = {Physical Review D}, Volume = {93}, Number = {8}, Year = {2016}, Month = {April}, url = {http://dx.doi.org/10.1103/PhysRevD.93.081701}, Doi = {10.1103/PhysRevD.93.081701}, Key = {fds323136} } @article{fds323135, Author = {Huffman, E and Banerjee, D and Chandrasekharan, S and Wiese, U-J}, Title = {Real-time evolution of strongly coupled fermions driven by dissipation}, Journal = {Annals of Physics}, Volume = {372}, Pages = {309-319}, Year = {2016}, Month = {September}, url = {http://dx.doi.org/10.1016/j.aop.2016.05.019}, Doi = {10.1016/j.aop.2016.05.019}, Key = {fds323135} } @article{fds323134, Author = {Ayyar, V and Chandrasekharan, S}, Title = {Fermion masses through four-fermion condensates}, Journal = {Journal of High Energy Physics}, Volume = {2016}, Number = {10}, Year = {2016}, Month = {October}, url = {http://dx.doi.org/10.1007/JHEP10(2016)058}, Doi = {10.1007/JHEP10(2016)058}, Key = {fds323134} } @article{fds323133, Author = {Huffman, E and Chandrasekharan, S}, Title = {Solution to sign problems in models of interacting fermions and quantum spins.}, Journal = {Physical Review. E}, Volume = {94}, Number = {4-1}, Pages = {043311}, Year = {2016}, Month = {October}, url = {http://dx.doi.org/10.1103/physreve.94.043311}, Abstract = {We show that solutions to fermion sign problems that are found in the formulation where the path integral is expanded in powers of the interaction in continuous time can be extended to systems involving fermions interacting with dynamical quantum spins. While these sign problems seem unsolvable in the auxiliary field approach, solutions emerge in the world-line representation of quantum spins. Combining this idea with meron-cluster methods, we are able to further extend the class of models that are solvable. We demonstrate these solutions to sign problems by considering several examples of strongly correlated systems that contain the physics of semimetals, insulators, superfluidity, and antiferromagnetism.}, Doi = {10.1103/physreve.94.043311}, Key = {fds323133} } @article{fds323235, Author = {Hann, CT and Huffman, E and Chandrasekharan, S}, Title = {Solution to the sign problem in a frustrated quantum impurity model}, Journal = {Annals of Physics}, Volume = {376}, Pages = {63-75}, Year = {2017}, Month = {January}, url = {http://dx.doi.org/10.1016/j.aop.2016.11.006}, Doi = {10.1016/j.aop.2016.11.006}, Key = {fds323235} } @article{fds331600, Author = {Ayyar, V and Chandrasekharan, S}, Title = {Generating a nonperturbative mass gap using Feynman diagrams in an asymptotically free theory}, Journal = {Physical Review D}, Volume = {96}, Number = {11}, Year = {2017}, Month = {December}, url = {http://dx.doi.org/10.1103/PhysRevD.96.114506}, Abstract = {© 2017 American Physical Society. Using the example of a two-dimensional four-fermion lattice field theory we demonstrate that Feynman diagrams can generate a mass gap when massless fermions interact via a marginally relevant coupling. We introduce an infrared cutoff through the finite system size so that the perturbation series for the partition function and observables become convergent. We then use the Monte Carlo approach to sample sufficiently high orders of diagrams to expose the presence of a mass gap in the lattice model.}, Doi = {10.1103/PhysRevD.96.114506}, Key = {fds331600} } @article{fds331601, Author = {Huffman, E and Chandrasekharan, S}, Title = {Fermion bag approach to Hamiltonian lattice field theories in continuous time}, Journal = {Physical Review D}, Volume = {96}, Number = {11}, Year = {2017}, Month = {December}, url = {http://dx.doi.org/10.1103/PhysRevD.96.114502}, Abstract = {© 2017 American Physical Society. We extend the idea of fermion bags to Hamiltonian lattice field theories in the continuous time formulation. Using a class of models we argue that the temperature is a parameter that splits the fermion dynamics into small spatial regions that can be used to identify fermion bags. Using this idea we construct a continuous time quantum Monte Carlo algorithm and compute critical exponents in the 3d Ising Gross-Neveu universality class using a single flavor of massless Hamiltonian staggered fermions. We find η=0.54(6) and ν=0.88(2) using lattices up to N=2304 sites. We argue that even sizes up to N=10,000 sites should be accessible with supercomputers available today.}, Doi = {10.1103/PhysRevD.96.114502}, Key = {fds331601} } @article{fds336441, Author = {Ayyar, V and Chandrasekharan, S}, Title = {Generating a mass gap using Feynman diagrams in an asymptotically free theory}, Journal = {Epj Web of Conferences}, Volume = {175}, Pages = {11010-11010}, Editor = {Della Morte and M and Fritzsch, P and Gámiz Sánchez and E and Pena Ruano, C}, Year = {2018}, url = {http://dx.doi.org/10.1051/epjconf/201817511010}, Abstract = {© The Authors, published by EDP Sciences, 2018. Using the example of a two dimensional four-fermion lattice field theory, we show that Feynman diagrams can generate a mass gap in a theory with massless fermions that interact via a marginally relevant coupling. We show this by introducing an infrared cutoff that makes the perturbation series for the partition function convergent. We use a Monte Carlo approach to sample sufficiently high orders of diagrams and thus expose the presence of the mass gap.}, Doi = {10.1051/epjconf/201817511010}, Key = {fds336441} } @article{fds332870, Author = {Banerjee, D and Chandrasekharan, S and Orlando, D}, Title = {Conformal Dimensions via Large Charge Expansion.}, Journal = {Physical Review Letters}, Volume = {120}, Number = {6}, Pages = {061603}, Year = {2018}, Month = {February}, url = {http://dx.doi.org/10.1103/physrevlett.120.061603}, Abstract = {We construct an efficient Monte Carlo algorithm that overcomes the severe signal-to-noise ratio problems and helps us to accurately compute the conformal dimensions of large-Q fields at the Wilson-Fisher fixed point in the O(2) universality class. Using it, we verify a recent proposal that conformal dimensions of strongly coupled conformal field theories with a global U(1) charge can be obtained via a series expansion in the inverse charge 1/Q. We find that the conformal dimensions of the lowest operator with a fixed charge Q are almost entirely determined by the first few terms in the series.}, Doi = {10.1103/physrevlett.120.061603}, Key = {fds332870} } @article{fds333670, Author = {Ayyar, V and Chandrasekharan, S and Rantaharju, J}, Title = {Benchmark results in the 2D lattice Thirring model with a chemical potential}, Journal = {Physical Review D}, Volume = {97}, Number = {5}, Year = {2018}, Month = {March}, url = {http://dx.doi.org/10.1103/PhysRevD.97.054501}, Abstract = {© 2018 authors. We study the two-dimensional lattice Thirring model in the presence of a fermion chemical potential. Our model is asymptotically free and contains massive fermions that mimic a baryon and light bosons that mimic pions. Hence, it is a useful toy model for QCD, especially since it, too, suffers from a sign problem in the auxiliary field formulation in the presence of a fermion chemical potential. In this work, we formulate the model in both the world line and fermion-bag representations and show that the sign problem can be completely eliminated with open boundary conditions when the fermions are massless. Hence, we are able accurately compute a variety of interesting quantities in the model, and these results could provide benchmarks for other methods that are being developed to solve the sign problem in QCD.}, Doi = {10.1103/PhysRevD.97.054501}, Key = {fds333670} } %% Papers Accepted @article{fds225572, Author = {Haiyuan Zou and Yuzhi Liu and Chen-Yen Lai and J. Unmuth-Yockey and A. Bazavov, Z.Y. Xie and T. Xiang and S. Chandrasekharan and S. -W. Tsai, Y. Meurice}, Title = {Towards quantum computing for the classical O(2) model}, Journal = {Phys. Rev. A}, Year = {2014}, url = {http://arxiv.org/abs/arXiv:1403.5238}, Abstract = {http://arxiv.org/abs/arXiv:1403.5238}, Key = {fds225572} } %% Papers Submitted @article{fds303651, Author = {Lee, J-W and Chandrasekharan, S and Baranger, HU}, Title = {Disorder-Induced Superfluidity in Hardcore Bosons in Two Dimensions}, Year = {2006}, Month = {November}, url = {http://arxiv.org/abs/cond-mat/0611109v1}, Abstract = {We study the effect of disorder on hardcore bosons in two dimensions at the SU(2) symmetric ``Heisenberg point''. We obtain our results with quantum Monte Carlo simulations using the directed loop algorithm. In the absence of disorder, the system has no long-range order at finite temperature due to the enhanced symmetry. However, the introduction of a disordered potential, uniformly distributed from -D to D, induces a finite-temperature superfluid phase. In particular the diagonal correlation length \xi decreases but the superfluid order-parameter correlation function becomes a power-law. A non-monotonic finite-size behavior is noted and explained as arising due to \xi. We provide evidence that at long distances the effects of a weak disordered potential can be mimicked by an effective uniform potential with a root-mean-square value: mu_eff = D/sqrt{3}. For strong disorder, the system becomes a Bose glass insulator.}, Key = {fds303651} } @article{fds303637, Author = {Chandrasekharan, S}, Title = {Fermion bags and a new origin for a fermion mass}, Journal = {Proceedings of Science}, Volume = {Part F130500}, Year = {2014}, Month = {January}, url = {http://arxiv.org/abs/1412.3532v1}, Abstract = {© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. The fermion bag is a powerful idea that helps to solve fermion lattice field theories using Monte Carlo methods. Some sign problems that had remained unsolved earlier can be solved within this framework. In this work we argue that the fermion bag also gives insight into a new mechanism of fermion mass generation, especially at strong couplings where fermion masses are related to the fermion bag size. On the other hand, chiral condensates arise due to zero modes in the Dirac operator within a fermion bag. Although in traditional four-fermion models the two quantities seem to be related, we show that they can be decoupled. While fermion bags become small at strong couplings, the ability of zero modes of the Dirac operator within fermion bags to produce a chiral condensate, can be suppressed by the presence of additional zero modes from other fermions. Thus, fermions can become massive even without a chiral condensate. This new mechanism of mass generation was discovered long ago in lattice field theory, but has remained unappreciated. Recent work suggests that it may be of interest even in continuum quantum field theory.}, Key = {fds303637} } @article{fds225571, Author = {V. Ayyar and S. Chandrasekharan}, Title = {Massive fermions without fermion bilinear condensates}, Journal = {arXiv:1410.6474 (submitted to Phys. Rev. D)}, Year = {2014}, Month = {October}, url = {http://arxiv.org/abs/arXiv:1410.6474}, Abstract = {http://arxiv.org/abs/arXiv:1410.6474}, Key = {fds225571} } %% Preprints @article{fds4139, Author = {S. Chandrasekharan and B. Scarlet and U.-J. Wiese}, Title = {MERON CLUSTER SIMULATION OF QUANTUM SPIN LADDERS IN A MAGNETIC FIELD}, Year = {1999}, Key = {fds4139} } @article{fds4134, Author = {S. Chandrasekharan and U.-J. Wiese}, Title = {SO(10) UNIFICATION OF COLOR SUPERCONDUCTIVITY AND CHIRAL SYMMETRY BREAKING?}, Year = {2000}, Abstract = {Motivated by the SO(5) theory of high-temperature superconductivity and antiferromagnetism, we ask if an SO(10) theory unifies color superconductivity and chiral symmetry breaking in QCD. The transition to the color superconducting phase would then be analogous to a spin flop transition. While the spin flop transition generically has a unified SO(3) description, the SO(5) and SO(10) symmetric fixed points are unstable, at least in (4 - epsilon) dimensions, and require the fine-tuning of one additional relevant parameter. If QCD is near the SO(10) fixed point, it has interesting consequences for heavy ion collisions and neutron stars.}, Key = {fds4134} } @article{fds212507, Author = {S. Chandrasekharan and U.-J. Wiese}, Title = {Partition Functions of Strongly Correlated Electron Systems as 'Fermionants'.}, Journal = {arXiv:1108.2461}, Year = {2011}, Month = {July}, url = {http://arxiv.org/abs/arXiv:1108.2461}, Abstract = {We introduce a new mathematical object, the "fermionant" ${\mathrm{Ferm}}_N(G)$, of type $N$ of an $n \times n$ matrix $G$. It represents certain $n$-point functions involving $N$ species of free fermions. When N=1, the fermionant reduces to the determinant. The partition function of the repulsive Hubbard model, of geometrically frustrated quantum antiferromagnets, and of Kondo lattice models can be expressed as fermionants of type N=2, which naturally incorporates infinite on-site repulsion. A computation of the fermionant in polynomial time would solve many interesting fermion sign problems.}, Key = {fds212507} }