"Solid Helium: Supersolidity, Elasticity and Quantum Plasticity"

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Solid helium is the epitome of a "quantum solid" - a crystal whose properties are controlled by quantum statistics rather than by classical 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" - with absolutely no flow dissipation. By applying pressure, helium can be crystallized but the solid's properties are still dominated by quantum effects. One of the most surprising predictions was that crystalline order and superfluidity can coexist in solid helium - a new phase of matter known as a "supersolid". Despite decades of experimental effort, it was not until 2004 that convincing evidence of supersolidity was found in solid 4He at temperatures below 200 milliKelvin. Since then, laboratories and theorists around the world have intensively studied solid helium. Despite this effort, and clear evidence that dislocations and other crystal defects are crucially involved, the microscopic basis of supersolidity and even its very existence are still open questions.