Mechanical Properties of Quantum Solids: Defects, Deformation and Flow

固体氦是“量子固体”的缩影——一种性质由量子力学控制的晶体。在没有外部压力的情况下，量子运动可以防止氦冻结，即使是在绝对零度。在低温下，液态氦表现出不寻常的行为，包括一种壮观的“超流动性”现象，它允许它在绝对没有耗散的情况下流动。氦可以通过施加压力而结晶，但它的性质仍然受量子效应的支配。一个令人吃惊的预测是物质的新阶段被称为“超固体”。2004年，似乎终于看到了扭转振子的质量解耦——预期的超固体特征。然而，经过十年的密集努力，人们已经清楚地认识到，扭转振子的“解耦”是人造的，是固体氦弹性特性意外变化的结果，而不是超固体的迹象。这种被称为“巨大塑性”的弹性行为反映了晶体缺陷的非凡迁移性。在极低的温度下，位错自由地通过氦晶体。******晶体缺陷（杂质、空位和称为位错的结构缺陷）的不寻常行为导致了令人惊讶的效果。氦晶体非常脆弱——它们在自身重量的作用下会变形。我们在2012年发现了巨大的可塑性，这使得它们本来就很小的刚度降低了10倍。造成这种巨大软化的位错也可能是最近在4He晶体中发现的非经典质量流的起源。这种流动的本质是一个悬而未决的问题，但它可能是由于超流体核心的位错，正如理论家所建议的那样，允许塑性流动通过一种新的机制“超攀”。******本提案中的实验包括对4He和稀有同位素3He晶体的弹性、塑性和流动测量（3He是一种“费米子”，不可能具有固体4He的超流体性质）。我们的测量将扩展到非常低的温度（低于15 mK，即在绝对零度0.015度以内），在那里量子效应占主导地位。他们将利用压电使晶体变形，并“聆听”移动错位发出的声波。这些灵敏的装置还能探测到原子平面被注入固体氦时所发生的微小压力变化（“注射器效应”）。******我们的实验将揭示这种不寻常固体中缺陷的基本性质，并将在位错中寻找预测的超流动性。我们将培养尽可能高质量和纯度的氦晶体，以与理想量子晶体的内在行为进行比较。我们将学习氦晶体如何在如此低的温度下变形，以至于正常的塑性流动机制是不可能的。将冶金学家的工具应用于这种独特的量子材料是有时被称为“量子可塑性”的主题的一个例子

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 4He 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 4He 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 4He and the rare isotope, 3He (which is a “fermion” and cannot have the superfluid properties ascribed to solid 4He). 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”.**