Phase Transitions and Dynamical Processes in Quantum Crystals: from Spinodal Decomposition to Supersolidity

量子晶体（固体4He和3He是最明显的例子）可能是自然界中最引人注目的固体，因为原子实际上可以在晶格点之间移动。这是通过量子隧穿过程发生的，由原子的大零点运动引起。固体氦氦的一个相关特征是，稀杂质，如空位或氦氦原子，表现为准粒子激发，在晶体中表现出远程弹道扩散。这一建议涉及纯4He晶体和3He在4He中的溶液的研究。量子晶体的独特性质导致了这样一种理论建议，即玻色-爱因斯坦凝聚可能在这种固体中被观察到，这种凝聚在超流体4He和最近的稀碱气体中已经确立了重要性，这可能是由于所谓的零点空位的存在。对于固体来说，能够无阻力地流动将是一种不寻常而迷人的现象。最近Kim和Chan做了一些观察，他们用这种超固体转变来解释这些观察。我们建议研究这种现象以及3He杂质（在低于100 ppm的极低浓度下）对它的影响。我们将采用高精度扭振技术和核磁共振方法对高质量单晶样品进行系统的研究。为了从如此低浓度的样品中解析极小的核磁共振信号，我们将使用我们实验室开发的最先进的SQUID核磁共振光谱仪。扩展缺陷和点缺陷的作用以及它们与3He杂质的相互作用将是本研究的一个关键方面。高浓度的同位素氦混合物为研究新相成核问题提供了一个独特的实验室。这是相变研究中的一个关键的一般性问题；子阶段是如何从父阶段成长起来的？云和雾的形成就是一个熟悉的例子。氦混合物中相关的相变是相分离。我们将通过相分离温度对混合物进行淬火，以研究spinodal分解和均相成核。由于上面讨论的量子隧穿过程，这些不同的机制可以实时观察到。这将使用（传统的）脉冲核磁共振技术来完成。我们还将探讨利用中子散射实时成像相分离混合物显影结构的可行性。

Quantum crystals (solid 4He and 3He being the clearest examples) are probably the most remarkable solids in nature, since the atoms can actually move between lattice sites. This occurs by a process of quantum tunnelling, arising from the large zero-point motion of the atoms. A related feature of solid 4He is that dilute impurities, such as vacancies or 3He atoms, behave as quasi-particle excitations, exhibiting long-range ballistic diffusion through the crystal.This proposal concerns the study of crystals of pure 4He, and solutions of 3He in 4He.The unique properties of quantum crystals led to the theoretical proposal that Bose Einstein Condensation, of established importance in superfluid 4He and more recently in dilute alkali gases, might be observed in such solids, perhaps due to the existence of so-called zero point vacancies. For a solid to be able to flow without resistance would be an unusual and fascinating phenomenon. Recently Kim and Chan have made observations, which they interpret in terms of this supersolid transition.We propose to investigate this phenomenon and the influence on it of 3He impurities (at extremely low concentrations below 100 ppm). We will make a systematic study of high quality single crystal samples by high precision torsional oscillator techniques and NMR methods. To resolve the extremely small NMR signals from such low concentration samples we will use state-of-the-art SQUID NMR spectrometers developed in our laboratory. The role of extended defects and point defects and their interaction with 3He impuritons will be one key aspect of this investigation.Higher concentration isotopic helium mixtures provide a unique laboratory for the study of the problem of new-phase nucleation. This is a key generic problem in the study of phase transitions; how does the daughter phase grow from the parent phase? The formation of clouds and fog is a familiar example. The relevant phase transition in helium mixtures is phase separation. We will quench mixtures through the phase separation temperature, to investigate spinodal decomposition and homogeneous phase nucleation. Because of the quantum tunnelling processes discussed above these distinct mechanisms can be observed in real time. This will be done using (conventional) pulsed NMR techniques. We will also investigate the feasibility of using neutron scattering to image the developing structure of the phase-separated mixture in real time.