Elastic Properties of Solid 4He
Elastic Properties of Solid 4He
复制标题
固体 4He 的弹性性质
DOI：
10.1007/s10909-012-0621-8
复制
发表时间：
2012
影响因子：
2
通讯作者：
J. Beamish
中科院分区：
文献类型：
--
作者：
J. Beamish
固体 4He 的剪切模量增加到 200 mK 以下，与最近在扭转振荡器 (TO) 实验中看到的频率变化同样依赖于温度、振幅和 3He 浓度。这些被解释为超固体中的质量解耦，但剪切模量行为有位错的自然解释。本文总结了早期的超声波和弹性实验，这些实验确立了固体氦中位错的基本性质。然后描述了我们对固体氦的低温剪切模量的实验结果。模量变化可以用位错来解释，位错在 200 mK 以上可移动，但在低温下被 3He 杂质钉扎。当我们对晶体进行退火或施加应力时，我们观察到的变化证实了其中存在缺陷。他们还明确指出，在最低温度下测量的剪切模量是内在值——它是因缺陷而降低的高温模量。通过测量不同频率下的剪切模量，我们表明振幅依赖性取决于晶体中的应力，而不是反映类似超流体的临界速度。随着频率的降低，剪切模量的变化会转移到较低的温度，这表明它们是由热激活松弛过程中的交叉产生的，而不是由真正的相变产生的。该过程的活化能约为 0.7 K，但需要广泛的能量分布才能适应广泛的交叉。尽管剪切模量行为可以用位错来解释，但它显然与 TO 行为相关。然而，我们对 hcp 3He 进行了测量，显示出基本相同的模量硬化，但没有相应的 TO 异常。这意味着 TO 频率的变化不仅仅是由于振荡器的机械刚性所致——它们只发生在 Bose 固体中。最后，我们指出了一些涉及固体氦的弹性和 TO 行为的悬而未决的问题。
The shear modulus of solid 4He increases below 200 mK, with the same dependence on temperature, amplitude and 3He concentration as the frequency changes recently seen in torsional oscillator (TO) experiments. These have been interpreted as mass decoupling in a supersolid but the shear modulus behavior has a natural explanation in terms of dislocations. This paper summarizes early ultrasonic and elastic experiments which established the basic properties of dislocations in solid helium. It then describes the results of our experiments on the low temperature shear modulus of solid helium. The modulus changes can be explained in terms of dislocations which are mobile above 200 mK but are pinned by 3He impurities at low temperature. The changes we observe when we anneal or stress our crystals confirm that defects are involved. They also make it clear that the shear modulus measured at the lowest temperatures is the intrinsic value—it is the high temperature modulus which is reduced by defects. By measuring the shear modulus at different frequencies, we show that the amplitude dependence depends on stress in the crystal, rather than reflecting a superfluid-like critical velocity. The shear modulus changes shift to lower temperatures as the frequency decreases, showing that they arise from a crossover in a thermally activated relaxation process rather than from a true phase transition. The activation energy for this process is about 0.7 K but a wide distribution of energies is needed to fit the broad crossover. Although the shear modulus behavior can be explained in terms of dislocations, it is clearly related to the TO behavior. However, we made measurements on hcp 3He which show essentially the same modulus stiffening but there is no corresponding TO anomaly. This implies that the TO frequency changes are not simply due to mechanical stiffening of the oscillator—they only occur in the Bose solid. We conclude by pointing out some of the open questions involving the elastic and TO behavior of solid helium.