Anomalous plasticity and underlying dislocation behavior in quantum solid 4He

The plastic behavior of solids is crucial to their mechanical properties, and solid helium offers a unique perspective due to its quantum nature. Unlike typical materials, the defects in solid helium-such as dislocations and grain boundaries-are profoundly affected by atomic zero-point motion. This paper explores the mechanisms of helium's unusual plasticity by examining the dislocation dynamics during deformation. Measurements of shear moduli reveal that helium undergoes strain hardening due to dislocation interactions below 400 mK, and an unexpected softening driven by dislocation alignment and grain boundary evolutions during subsequent warm-up. Notably, this softening is linked to dislocation "superclimb," a quantum phenomenon involving superfluidity along specific dislocation cores. These findings shed light on the distinctive mechanical properties of solid helium and hint at a connection to macroscopic superfluidlike mass flow.