Grants and Contributions:

Grant or Award spanning more than one fiscal year. (2017-2018 to 2022-2023)

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 4 He crystals.

The unusual behavior of crystal defects (impurities, vacancies, and structural defects known as dislocations) leads to surprising effects. Helium crystals are extraordinarily fragile - they deform under their own weight. The giant plasticity we discovered in 2012 involves ten-fold reductions in their already tiny rigidity. The dislocations responsible for this enormous softening may also be the origin of non-classical mass flow recently discovered in 4 He crystals. The nature of this flow is an open question but it may be due to dislocations with superfluid cores, as suggested by theorists, allowing plastic flow via a new mechanism – “superclimb”.

The experiments in this proposal include elastic, plastic and flow measurements on crystals of both 4 He and the rare isotope, 3 He (which is a “fermion” and cannot have the superfluid properties ascribed to solid 4 He). Our measurements will extend to very low temperatures (below 15 mK, i.e. within 0.015 degrees of absolute zero) where quantum effects dominate. They will use piezoelectrics to deform crystals and “listen” for sound waves emitted by moving dislocations. These sensitive devices will also detect the tiny pressure changes that occur when atomic planes of atoms are injected into solid helium (the "syringe effect”).

Our experiments will reveal the fundamental nature of defects in this unusual solid and will search for the predicted superfluidity in dislocations. We will grow helium crystals of the highest possible quality and purity, to compare to the intrinsic behavior of an ideal quantum crystal. We will learn how helium crystals deform at temperatures so low that the normal plastic flow mechanisms are impossible. Applying a metallurgist's tools to this unique quantum material is a an example of a topic sometimes referred to as “quantum plasticity”.