"Solid Helium: Supersolidity, Elasticity and Quantum Plasticity"

固体氦是“量子固体”的缩影，这种晶体的性质是由量子统计而不是经典力学控制的。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，包括一种壮观的现象——“超流动性”——绝对没有流动耗散。通过施加压力，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秩序和超流动性可以在固体氦中共存——这是一种被称为“超固体”的物质的新阶段。尽管经过了几十年的实验努力，直到2004年，人们才在低于200毫开尔文的温度下，在固体4He中发现了令人信服的超固态证据。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。尽管有这样的努力，并且有明确的证据表明位错和其他晶体缺陷至关重要，但超固体的微观基础，甚至它的存在，仍然是一个悬而未决的问题。

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.