AMPLITUDE-DEPENDENT SOUND ATTENUATION IN bcc ³He CRYSTALS WITH ⁴He IMPURITIES

The attenuation of longitudinal sound wave at 10 MHz was investigated in bcc 3He crystals with 4He impurities at a molar volume of 24.4 cm3/mole. The attenuation became amplitude-dependent after the crystals were cooled below the phase separation temperature. Namely, the amplitude dependence appeared at 86 mK and 98 mK for 4He concentrations x4 = 480 ppm and 1600 ppm, respectively, and disappeared at 350 mK irrespective of x4. The critical amplitude above which the attenuation increased was independent of x4 and T. It is proposed that screw dislocations which were multiplied in the course of the phase separation were responsible for the effect. The critical resolved shear stress is estimated to be τc = 5x10-5µ from the critical amplitude, where µ is the shear modulus. The value of τc is considerably lower than the Peierls stress so that the dislocations possibly tunnel the Peierls barrier.