Crystalline Defects and Possible Superfluidity in Solid Helium

固体氦中的晶体缺陷和可能的超流动性

基本信息

批准号：
EP/H014691/1
负责人：
Andrei Golov
金额：
$ 64.43万
依托单位：
University of Manchester
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2010
资助国家：
英国
起止时间：
2010 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FH014691%2F1
关键词：
Crystalline Defects Possible Superfluidity Solid

项目摘要

While it is nowadays ubiquitous that electrons can flow through many solids (called superconductors ) without any dissipation, the idea that atoms of a solid can be engaged in a non-dissipative flow past its rigid lattice is very counterintuitive still. Yet, this is what seemed to be observed in solid helium in 2004 and can be attributed to the quantum nature of solids made of light atoms with weak interatomic attraction ( quantum crystalls ). While the mechanism responsible for the observed reduction, at temperatures below 100 mK, of the inertia of solid helium engaged in torsional oscillations is still highly controversial, the emerging concensus is that the effect is located not in the bulk perfect crystal but in the extended defects of the crystalline order - such as dislocations and grain boundaries. Recent quantum Monte Carlo simulations indeed confirmed that cores of dislocations and grain boundaries in solid hcp 4He should be able to support persistent flow of helium atoms, i.e. superfluidity . However, there were no experiments so far that: relate the observed AC mass flow in solid helium with the measured density and type of crystalline defects; demonstrate that a persistent DC mass flow (i.e. superfluidity) is possible through solid helium; directly investigate the mobility of dislocations and grain boundaries in solid helium under applied stress. These types of experiments will be vital for the progress in the understanding of the new phenomena, and the proposed programme is aimed at advances in all three directions: 1. We will combine two techniques to characterize the same sample of solid helium: a torsional oscillator to monitor its response to acceleration and measurements of thermal conductivity which, at temperatures 50 mK - 500 mK, is sensitive to the mean free path of phonons due to scattering off crystal dislocations and grain boundaries. We will prepare samples of solid 4He of various quality: from extremely disordered ones (grown under non-uniform conditions at pressure decreasing from 70 to 30 bar during growth) to perfect monocrystals grown at constand pressure. This experiment will provide, for the first time, indispensible observations of correlations (if any) of the mass flow and density of defects. 2. We will combine torsional AC oscillations (to monitor the inertia of solid helium as in (1)) with continuous DC rotation (to attempt to generate persistent circular mass flow in an annular channel filled with solid helium after entering the superfluid state while rotating). The presence of the persistent flow will be then detected by matching the angular velocity of the cryostat to that of the flow (as the dissipation of the torsional oscillator is expected to have the minimum when these angular velocities match). If succesfull, this will be a ground-breaking discovery of mass superfluidity in solids! Simultaneously, we will attempt to add another complementary technique of sample characterization to this torsional oscillator - measurements of the propagation of ultrasound pulses. As the sound velocity is very anisotropic in hcp crystals, this will help tell the orientation of a monocrystal (if any). And the frequency dependence of the attenuation of ultrasound is another sensitive probe of dislocations in crystals. 3. We will investigate the mobility of dislocations and grain boundaries in solid helium - by charging them by injected ions and then observing the displacement and steady motion of these defects under an external force due to the applied electric field. At temperatures around 100 mK, below which the new state is usually observed in samples of solild helium, we would expect changes in the mobility of these defects. Furthemore, the mobility of injected ions through bulk solid helium will also be investigated in this temperature range for the first time, that might help to pinpoint any anomalies, if any, in the density of vacancies, etc.

尽管现在无处不在的是，电子可以在没有任何耗散的情况下流过许多固体（称为超导体），但固体的原子可以通过其刚性晶格经过的非疾病流动的想法仍然非常违反直觉。然而，这似乎是2004年在固体氦气中观察到的，可以归因于具有弱原子间吸引（量子晶体）的光原子制成的固体的量子性质。尽管在低于100 mk的温度下，导致观察到的降低的机制仍然是高度争议的，但在扭转振荡的固体惯性仍然是高度争议的，但新出现的共和力是，这种效果不是位于散装的完美晶体中，而是在晶体秩序的扩展缺陷中 - 诸如晶体秩序的扩展 - 诸如晶体界限和晶粒和晶粒和晶粒和晶粒和晶粒和晶粒。最近的量子蒙特卡洛模拟确实证实了固体HCP 4HE中的位错和晶界的核心应该能够支持氦原子的持续流动，即超流体。但是，到目前为止，没有实验：将观察到的AC质量流与固体氦气中的AC质量流与测量的密度和晶体缺陷类型相关联；证明持续的直流质量流（即超流量）是通过固体氦进行的。在施加应力下，直接研究固体固体中位错和晶界的迁移率。这些类型的实验对于对新现象的理解进展至关重要，并且提出的程序旨在在所有三个方向上进步：1。我们将结合两种技术来表征相同的固体氦气样本：旋转振荡器：一种对加速度的响应，并在频繁的perty频率上进行ptherive式的旋转率50 mk -500 mk -500 mk，是在频繁的范围内，是-500 mk -500 mk，是500 mk -500 mk，是-500 mk，是500 mk -500 mk，是500 mk -500 mk，是500 mk -500 mk，是频繁的旋转率。晶体位错和晶界。我们将准备各种质量的固体4HE样品：从极度无序的样品（在生长期间的压力下从70 bar下降低到30 bar）到在稳定压力下生长的完美单晶。该实验将首次提供对缺陷的质量流量和密度的相关性不可或缺的观察结果。 2。我们将与连续的直流旋转（在（1）中的固体氦的惯性（如（1）中）结合使用（在旋转时进入超级流体状态后，在充满固体氦气的环形通道中产生持续的圆形质量流动）（在（1）中）。然后，将通过将低温恒温器的角速度与流动速度匹配（因为扭转振荡器的耗散时，这些角速度匹配时，这些扭曲速度的存在将具有最小值）。如果成功，这将是固体中质量超流量的开创性发现！同时，我们将尝试在此扭转振荡器中添加另一种互补的样品表征技术 - 超声脉冲传播的测量。由于声速在HCP晶体中是非常各向异性的，因此这将有助于分辨单晶的方向（如果有）。超声衰减的频率依赖性是晶体中位错的另一种敏感探针。 3。我们将通过注射离子为固体氦气中的脱位和晶界的迁移率，然后观察由于施加的电场而在外力下观察这些缺陷的位移和稳定运动。在约100 mk的温度下，通常在Solild氦气样品中观察到新状态，我们预计这些缺陷的迁移率会发生变化。此外，还将首次在此温度范围内研究注射离子通过大量固体氦气的迁移率，这可能有助于确定空位密度的任何异常（如果有的话）。