Phonon-Phonon and Phonon-Defect Scattering in 4He Crystals
The thermal conductivity measurements were made in pure 4He and 4He+3He liquid and solid mixtures under pressure up to 84 atm.The data cover the temperature range down to 0,4 K and include the results of investigations of recovery processes in plastically deformed crystals.The importance of different phonon relaxation mechanisms is estimated from the behavior of the phonon mean free path calculated from the thermal conductivity.The main results are:(1) the frequency of phonon-phonon scattering in volume is increasing nearly an order on solidifying the superfluid liquid; (2) in solid helium with rise the pressure from 25 to 100 atm the frequency of normal phonon-phonon scattering is decreasing an order at the same reduced temperature T/Q;(3) the value of additional thermal resistivity in plastically deformed crystals is determined by a resonant phonon-dislocation scattering; (4) the activation energy of recovery processes in bent samples containing up to 0,5% of 3He atoms is close to the energy of vacancy creation.So the motion of freshly introduced dislocations is vacancy assisted,and concentration of vacancies in samples,grown at 31 or 26 atm. is less than 2*10−10% at temperatures T<0,45 K.(5) the activation energy of recovery processes falls more than twice on increasing the impurity content up to 5%,probably due to self-localization of 3He atoms.
KeywordsThermal Conductivity Activation Energy Thermal Resistivity Recovery Process Liquid Helium
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