This review addresses experiments on elasticity, plasticity, and flow of solid ${}^4\mathrm{He}$ and ${}^3\mathrm{He}$, focusing on dislocations and other defects that are responsible for the unusual mechanical behavior of such quantum crystals. Helium’s zero point motion prevents it from freezing unless pressure is applied and makes the solid extremely compressible, with elastic constants orders of magnitude smaller than those of conventional solids. Tunneling allows defects to remain mobile at low temperatures, so dislocations have much larger effects on mechanical properties than in conventional solids. At temperatures below 400 mK, dislocations in hexagonal-close-packed (hcp) ${}^4\mathrm{He}$ are essentially undamped and, in the absence of pinning by ${}^3\mathrm{He}$ impurities, glide freely in the basal plane. In this regime, dislocation motion reduces the shear modulus by as much as 90%, an effect that has been referred to as “giant plasticity” although it is reversible and so might be better described as “softening.” In this low temperature regime, macroscopic plastic deformation occurs via sudden dislocation avalanches with a wide range of time and length scales. At higher temperatures, dislocation motion is damped, introducing dissipation in elastic measurements, and thermally activated defect motion makes helium crystals extremely ductile, flowing under millibar stresses near melting. During the last decade, most of the properties of the dislocations that are responsible for the elastic effects described in this review have been accurately measured: their orientation, density, and length distributions, the nature of their networks, and their binding to isotopic impurities. Despite this detailed understanding of mobile dislocations, there remain open questions. Much less is known about defects’ roles in the elastic and plastic behavior of hcp and bcc ${}^3\mathrm{He}$ crystals and even in hcp ${}^4\mathrm{He}$, and almost nothing is known about other types of dislocations that are immobile and thus do not affect elastic properties. These might be responsible for recently observed superfluidlike mass flow in ${}^4\mathrm{He}$ at low temperatures, although it is now clear that the apparent mass decoupling seen in torsional oscillator experiments with solid ${}^4\mathrm{He}$ was due to the elastic effects described in this review, not to supersolidity.

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固体氦的机械行为：弹性，可塑性和缺陷

这篇评论讨论了关于弹性，可塑性和固体流动的实验 ${}^4他$ 和 ${}^3他$重点研究了导致这种量子晶体异常机械行为的位错和其他缺陷。氦的零点运动可以防止冻结，除非施加压力，并且使固体具有极高的可压缩性，其弹性常数比常规固体小几个数量级。隧穿可使缺陷在低温下保持可移动状态，因此，位错对机械性能的影响要比常规固体大得多。在低于400 mK的温度下，位错以六方密堆积（hcp）${}^4他$ 基本上没有阻尼，并且在没有钉扎的情况下 ${}^3他$杂质，在基面上自由滑动。在这种情况下，位错运动将剪切模量降低了90％，尽管这种作用是可逆的，但这种作用已被称为“巨大的可塑性”，因此可以更好地描述为“软化”。在这种低温条件下，宏观塑性变形是通过突然错位的雪崩发生的，时间和长度范围很广。在较高的温度下，位错运动受到抑制，从而在弹性测量中引入了耗散，并且热激活的缺陷运动使氦晶体极易延展，在接近熔化的毫巴应力下流动。在过去的十年中，已经准确测量了导致该评价中所述的弹性效应的位错的大多数特性：其方向，密度和长度分布，网络的性质以及它们与同位素杂质的结合。尽管对移动性脱位有详细的了解，但仍然存在未解决的问题。关于缺陷在hcp和bcc的弹性和塑性行为中的作用知之甚少${}^3他$ 晶体甚至在hcp中 ${}^4他$，关于固定的其他类型的位错几乎一无所知，因此不会影响弹性。这些可能是造成最近观察到的超流体质量流的原因${}^4他$ 在低温下，尽管现在很明显在扭转振动器实验中固体存在明显的质量解耦 ${}^4他$ 这是由于本评价中描述的弹性效应，而不是由于超坚固性。