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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.

虽然现在电子可以无耗散地流过许多固体（称为超导体）是普遍存在的，但固体原子可以通过其刚性晶格进行非耗散流动的想法仍然是非常违反直觉的。然而，这似乎是2004年在固体氦中观察到的，并且可以归因于由具有弱原子间吸引力的轻原子组成的固体（量子晶体）的量子性质。虽然在低于100 mK的温度下，观察到的扭转振荡中固体氦的惯性减少的机制仍然存在很大的争议，但逐渐形成的共识是，这种影响并不存在于整体完美晶体中，而是存在于晶体秩序的扩展缺陷中，例如位错和晶界。最近的量子蒙特卡罗模拟确实证实了固体hcp 4He中的位错核和晶界应该能够支持氦原子的持续流动，即超流动性。然而，迄今为止还没有实验将观察到的固体氦的AC质量流与测量到的密度和晶体缺陷类型联系起来；证明一个持续的直流电质量流（即超流动性）是可能通过固体氦；直接研究固体氦在外加应力作用下位错和晶界的迁移率。这些类型的实验对于理解新现象的进展是至关重要的，拟议的计划旨在在所有三个方向取得进展：1。我们将结合两种技术来表征相同的固体氦样品：一个扭转振荡器来监测其对加速度的响应，以及热导率的测量，在50 mK - 500 mK的温度下，由于晶体位错和晶界的散射，对声子的平均自由程很敏感。我们将制备各种质量的固体4He样品：从极度无序的样品（生长过程中在非均匀条件下生长，压力从70巴降至30巴）到在恒定压力下生长的完美单晶。这个实验将首次提供对质量流和缺陷密度之间的相关性（如果有的话）的必要观察。2. 我们将结合扭转交流振荡（监测(1)中固体氦的惯性）和连续直流旋转（尝试在旋转时进入超流体状态后，在充满固体氦的环形通道中产生持续的圆形质量流）。然后通过将低温恒温器的角速度与流动的角速度相匹配来检测持续流动的存在（因为当这些角速度相匹配时，扭振子的耗散预计会最小）。如果成功，这将是固体质量超流动性的突破性发现！同时，我们将尝试添加另一种补充的样品表征技术到这个扭转振荡器-测量超声波脉冲的传播。由于声速在hcp晶体中是非常各向异性的，这将有助于告诉单晶的方向（如果有的话）。超声衰减的频率依赖性是晶体中位错的另一个敏感探头。3. 我们将研究固体氦中位错和晶界的迁移性——通过注入离子给它们充电，然后观察这些缺陷在外加电场的外力作用下的位移和稳定运动。在大约100 mK的温度下，通常在固体氦样品中观察到新的状态，我们可以预期这些缺陷的迁移率会发生变化。此外，注入离子通过大块固体氦的迁移率也将首次在这个温度范围内进行研究，这可能有助于查明空位密度等方面的任何异常。