Publications [#324418] of Dewey T. Lawson

Papers Published
  1. Lawson, DT; Fairbank, HA, Thermal conductivity and isotopic impurities in single crystals of helium, Journal of Low Temperature Physics, vol. 11 no. 3-4 (May, 1973), pp. 363-394 [doi] .

    We have utilized the enhancement of thermal conductivity by Poiseuille flow of the phonon gas to obtain highly sensitive measurements of phonon scattering by isotopic impurities in single crystals of helium. Crystal orientation, size, and quality may be inferred from the thermal conductivity data themselves. Our hcp 4 He crystals were grown at a constant pressure of 85.1 atm using apparatus and techniques that made possible some control over crystal orientation. An isotopic impurity concentration of 1.0×10 -5 decreases the peak conductivity in these crystals by a factor of 1.9 along a direction perpendicular to the c axis. In the Poiseuille region a relaxation time limit has been achieved experimentally that allows our data to be fitted as a function of temperature and concentration with only one parameter. The observed scattering strength is a factor of 2.7 greater than can be explained in terms of mass-defect scattering alone. A number of current theories are examined in the light of this result. Our pure 4 He data strongly support a T -3 dependence for the normal-process relaxation time. Measurements of the thermal conductivity parallel to the c axis reveal no anisotropy in either the normal-process relaxation time or the isotopic scattering strength. © 1973 Plenum Publishing Corporation.