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.

虽然电子可以在没有任何耗散的情况下流经许多固体（称为超导体），这一点现在已经普遍存在，但固体原子可以非耗散流过其刚性晶格的想法仍然非常违反直觉。然而，这似乎是 2004 年在固体氦中观察到的情况，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 的温度下观察到参与扭转振荡的固态氦的惯性减少的机制仍然存在很大争议，但新出现的共识是，这种效应并不存在于块状完美晶体中，而是存在于扩展缺陷中。晶序 - 例如位错和晶界。最近的量子蒙特卡罗模拟确实证实，固体 hcp 4He 中的位错核心和晶界应该能够支持氦原子的持续流动，即超流性。然而，迄今为止还没有实验能够：将观察到的固体氦中的交流质量流量与测量的晶体缺陷的密度和类型联系起来；证明通过固体氦可以实现持续的直流质量流（即超流动性）；直接研究施加应力下固体氦中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至关重要，拟议的计划旨在实现所有三个方向的进展： 1. 我们将结合两种技术来表征同一固体氦样本：扭转振荡器监测其对加速度的响应和热导率测量，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：从极其无序的样品（在不均匀条件下生长，生长过程中压力从 70 巴降至 30 巴）到在恒压下生长的完美单晶。该实验将首次提供质量流量和缺陷密度的相关性（如果有的话）的必不可少的观察结果。 2. 我们将扭转交流振荡（如（1）中监测固体氦的惯性）与连续直流旋转（在旋转时进入超流态后尝试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匹配来检测持续流动的存在（因为当这些角速度匹配时，扭转振荡器的耗散预计具有最小值）。如果成功，这将是固体质量超流性的突破性发现！同时，我们将尝试向该扭转振荡器添加另一种样品表征的补充技术 - 超声波脉冲传播的测量。由于 hcp 晶体中的声速非常各向异性，这将有助于判断单晶体（如果有）的方向。超声波衰减的频率依赖性是晶体位错的另一个敏感探针。 3. 我们将研究固体氦中位错和晶界的迁移率 - 通过注入离子对它们进行充电，然后观察这些缺陷在施加电场的外力作用下的位移和稳定运动。在 100 mK 左右的温度下（低于该温度，通常会在固体氦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，还将首次在该温度范围内研究注入离子通过散装固体氦的迁移率，这可能有助于查明空位密度等方面的任何异常情况（如果有）。