"Solid Helium: Supersolidity, Elasticity and Quantum Plasticity"

固体氦是“量子固体”的缩影--一种由量子统计而不是经典力学控制其性质的晶体。在没有外部压力的情况下，量子运动可以防止氦冻结，即使在绝对零度下也是如此。在低温下，液氦表现出不同寻常的行为，包括一种壮观的现象--“超流”--绝对没有流动耗散。通过加压，氦可以结晶，但固体的性质仍然受量子效应的支配。最令人惊讶的预测之一是，晶体有序和超流可以在固体氦中共存--这是一种被称为“超固体”的新物质相。尽管经过了几十年的实验努力，直到2004年，才在温度低于200毫克开尔文的固体4He中发现了令人信服的超固体证据。从那时起，世界各地的实验室和理论家都在深入研究固体氦。尽管做出了这样的努力，而且有明确的证据表明，位错和其他晶体缺陷是至关重要的，但超固体的微观基础，甚至其本身的存在，仍然是悬而未决的问题。 2007年，我们发现氦的弹性刚度在低温下也表现出不寻常的行为，这使得我们能够研究量子固体中的位错行为。我们建议将这些实验扩展到极低的应力(氦晶体非常脆弱--它们在自身重量下变形)和广泛的频率范围。这将使我们能够理解这种不同寻常的固体中缺陷的基本性质，并寻找被称为位错的结构缺陷的预测新的量子运动。我们将生长尽可能高质量和纯度的氦晶体，以便与理想量子晶体的本征行为进行比较。我们还将研究氦晶体在如此低的温度下如何变形，以至于通常的塑性流动机制是不可能的。通过将冶金学家的工具应用于这种独特的量子材料，我们将推进一个被称为“量子塑性”的新领域。

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".