Low frequency shear modulus of bcc 3He below 1 K

Abstract

Recent measurements on hcp 4He crystals showed an anomalous stiffening of the shear modulus below 200 mK. This is the temperature range in which torsional oscillator experiments saw evidence of supersolidity. The temperature, frequency, strain amplitude and 3He impurity dependence of the shear modulus mirrored that of the torsional oscillator period, indicating that they are closely connected, although the exact relationship is not clear. The elastic behavior appears to be related to the mobility and impurity pinning of dislocations. Since the behavior of dislocations depends on crystal structure, while supersolidity is not expected in solid 3He, we have extended our shear modulus measurements to the bcc phase of high isotopic purity 3He (1 ppm 4He). We do not see a modulus anomaly like that in hcp 4He, nor is there significant amplitude dependence. We compare the behavior of hcp 4He to that of bcc 3He. Measurements on high purity hcp 3He will provide the most valuable comparison to the supersolid 4He system.