Plastic Flow of Solid 4He and 3He at Low Temperatures (Review Article)

A review of experimental and theoretical studies of the plastic flow of solid helium in the temperature range of 0.1-1.0 K is presented. The most reliable and interesting results were obtained in the two scientific groups where a plastic flow was observed between interconnected chambers containing solid helium that occurred after an overpressure in one of the chambers, and in which the direct contact of solid helium with the superfluid phase was excluded in the measuring cell. A significantly different nature of plastic flow was found in solid ⁴He and ³He: if in solid ⁴He the plastic flow rate decreases with decreasing temperature, which corresponds to a thermally activated process, then in solid ³He at temperatures below ≈0.2 K, quantum creep was first recorded when the flow rate ceased to depend on temperature. Only in one group in the experiments conducted near the melting curve, an increase in the flow rate of ⁴He was observed with decreasing temperature, which allowed the authors to conclude that some form of superfluidity manifested itself. In the thermally activated region of the plastic flow of ⁴He and ³He, the values of the activation energy and the activation volume, where the activation volume exceeds the atomic volume by a factor from 30 to 70. The analysis of the possible physical mechanisms of plastic flow of solid helium was carried out.