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Thursday, September 22, 2011
2:00 PM - 3:00 PM
CNLS Conference Room (TA-3, Bldg 1690)

Postdoc Seminar

Dislocation induced anomalous softening of solid helium

Caizhi Zhou
T-3 and CNLS

The observation of softening of the low-temperature shear modulus in solid 4He with increasing temperature around 100 mK has been taken as evidence for anomalous elastic properties tied to supersolidity. Measurements of the shear modulus and the change of the resonant period of torsional oscillator of solid 3He and 4He showed softening of the shear modulus in the same temperature range. This result suggested the importance of the role of quantum statistics and the role of defects and moving dislocations on supersolidity. The classical motion of gliding dislocation lines in slip planes of crystalline solid helium leads to plastic deformation even at low temperatures far below the Debye temperature and can affect the determination of elastic properties. We present a dislocation motion model that describes the stress-strain curves and work hardening rate, dτ/dε, of a shear experiment performed on solid helium. The calculated dτ/dε exhibits strong softening with increasing temperature due to the motion of dislocations, which mimics anomalous softening of the elastic shear modulus. In the same temperature region where dτ/dε changes most significantly, the energy dissipation caused by plastic deformations also shows a peak. This study indicated that the gliding of dislocations and plasticity may be the origin of many observed elastic anomalies in solid 4He.

Host: Peter Loxley,