Effects of Disorder on Phase Transitions of Quantum Fluids

本研究将探讨无序对相变的影响，特别是氦-4中的超流体相变和氦-3/氦-4混合物中的相分离。这是通过将液态氦-4和氦-3/氦-4混合物引入多孔介质来实现的，例如二氧化硅气凝胶或多孔金，这是研究者制作的两种基质。在这种多孔介质的随机环境中，发现封闭纯氦-4的超流体转变与散装液体完全不同，尽管转变仍然非常明显，并表现出明确的简单幂律行为。幂律指数被显著地修改，即使二氧化硅基气凝胶只占总体积的百分之几。当氦-3/氦-4混合物加入气凝胶时，氦-3/氦-4混合物的相图完全改变，除了通常的富氦-3超流体外，还产生了新的富氦-3超流体。研究了气凝胶体积分数进一步降低时混合物的相图和行为。在过去的二十年里，人们对纯物质的一相如何转变为另一相，例如，液体如何转变为气体，超导体如何转变为普通金属，已经了解了很多。在现实世界中，这些系统中经常存在无序和杂质。该项目将提供对这种杂质对特定系统性质的影响的理解，即液态氦-4从超流体到正常状态的转变。可以仔细控制这一系统，以便将结论应用于技术上重要的材料。通过在孔隙度和拓扑结构表征良好的多孔气凝胶或多孔金中加入液氦来引入杂质。迄今为止的实验揭示了意想不到的结果，包括在随机环境下的异常急剧转变和新的富3He超流体相的产生。计划研究这种不寻常的行为以及这些性质随两种氦同位素的温度、压力和浓度的变化。＊＊＊

w:\awards\awards96\*.doc 9630736 Chan This research will explore the effects of disorder on phase transitions, particularly the superfluid transition in Helium-4 and phase separation in Helium-3/Helium-4 mixtures. This is accomplished by introducing liquid Helium-4 and Helium-3/Helium-4 mixtures into porous media, such as silica aerogel or porous gold, two substrates made by the investigator. In this random environment of the porous media, the superfluid transition of confined pure Helium-4 was found to be completely altered from the bulk liquid although the transition remains remarkably sharp and exhibits well defined simple power law behavior. The power law exponents are significantly modified even though the silica based aerogel constitutes only a few percent of the total volume. When Helium-3/Helium-4 mixtures are added to the aerogel the phase diagram of Helium-3/Helium-4 mixture is completely altered giving rise to a new Helium-3 rich superfluid phase in addition to the usual Helium-3 rich superfluid. The phase diagram and behavior of mixtures when the volume fraction of the aerogel is further reduced will be studied. %%% In the last two decades, a great deal has been learned about how one phase of pure matter transforms into another, e.g., liquid into gas and superconductor into normal metal. In the real world, disorder and impurities are very often present in these systems. This project will provide an understanding of the effect of such impurities on the nature of a specific system, namely the transformation of liquid Helium-4 from the superfluid to the normal state. This system can be carefully controlled so that conclusions can be applied to technologically important materials. Impurities are introduced by adding liquid helium into porous aerogel or porous gold of well characterized porosity and topology. Experiments to date reveal unexpected results including an extraordinary sharp transition inspite of the random environment and the creation of a new 3He rich superfluid phase. Study is planned of this unusual behavior and the variation of these properties with temperature, pressure and concentration of the two helium isotopes. ***