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位置之间移动。这是由原子的大零点运动引起的量子隧穿过程发生的。固体4 He的一个相关特征是，空位或3 He原子等稀释杂质表现为准粒子激发，表现出通过晶体的远程弹道扩散。该提议涉及纯4 He晶体和溶液的研究。3 He在4 He中。量子晶体的独特性质导致了玻色爱因斯坦凝聚，在超流4He和最近在稀碱气体中已经确定的重要性，可能在这样的固体中观察到，也许是由于所谓的零点空位的存在。固体能够无阻力地流动将是一个不寻常的和迷人的现象。最近，Kim和Chan进行了观察，他们解释了这种超固体转变。我们建议研究这种现象以及3He杂质（低于100 ppm的极低浓度）对它的影响。我们将利用高精度扭振技术和核磁共振方法对高质量的单晶样品进行系统的研究。为了从如此低浓度的样品中分辨出极小的NMR信号，我们将使用我们实验室开发的最先进的SQUID NMR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问题：子相是如何从母相生长出来的？云和雾的形成是一个熟悉的例子。氦混合物中的相关相变是相分离。我们将借由相分离温度来淬灭混合物，以研究失稳分解与均相形核。由于上面讨论的量子隧穿过程，这些不同的机制可以在真实的时间内观察到。这将使用（常规）脉冲NMR技术来完成。我们也将探讨利用中子散射成像的相分离的混合物在真实的时间的发展结构的可行性。

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.