"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. In 2007, we discovered that the elastic stiffness of helium also showed unusual behavior at low temperatures, which allowed us to study the behavior of dislocations in a quantum solid. We propose to extend these experiments to extremely low stresses (helium crystals are extraordinarily fragile - they deform under their own weight) and to a wide range of frequencies. This will allow us to understand the fundamental nature of defects in this unusual solid and search for predicted new quantum motion of structural defects known as dislocations. We will grow helium crystals of the highest possible quality and purity in order to compare to the intrinsic behavior of an ideal quantum crystal. We will also study how helium crystals deform at temperatures so low that the usual plastic flow mechanisms are impossible. By applying the metallurgist's tools to this unique quantum material we will be advancing a new field which has been referred to as "quantum plasticity".