Mechanical Properties of Quantum Solids: Defects, Deformation and Flow

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Solid helium is the epitome of a “quantum solid” - a crystal whose properties are controlled by quantum mechanics. In the absence of external pressure, quantum motion prevents helium from freezing, even at absolute zero. At low temperatures, liquid helium exhibits unusual behavior, including a spectacular phenomenon – “superfluidity” – that allows it to flow with absolutely no dissipation. Helium can be crystallized by applying pressure but its properties are still dominated by quantum effects. One startling prediction is a new phase of matter known as a “supersolid”. In 2004, it appeared that mass decoupling from a torsional oscillator had finally been seen - the expected signature of supersolidity. However, after a decade of intensive effort it has become clear that the torsional oscillator “decoupling” was an artifact, the result of unexpected changes in solid helium’s elastic properties, not a sign of supersolidity. The elastic behavior, dubbed “giant plasticity”, reflects the extraordinary mobility of crystal defects. At very low temperatures dislocations glide freely through