Effects of Disorder on Phase Transitions of Quantum Fluids

这项研究将探索无序对相变的影响，特别是氦-4中的超流相变和氦-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混合物引入到多孔介质中来实现的，如二氧化硅气凝胶或多孔金，这两种衬底是由研究人员制作的。在这种随机的多孔介质环境中，发现受限纯氦-4的超流转变完全不同于大体积液体，尽管转变仍然非常尖锐，并表现出明确的简单幂规律行为。尽管硅基气凝胶只占总体积的几个百分点，但幂定律指数得到了显着的修正。当氦-3/氦-4混合物加入到气凝胶中时，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，还产生了一个新的富氦-3超流相。将研究气凝胶体积分数进一步降低时的相图和混合物的行为。在过去的二十年里，关于纯物质的一种相如何转变成另一种相，例如，液体变成气体，超导体变成普通金属，人们已经了解了很多。在现实世界中，这些系统中经常存在无序和杂质。这个项目将提供对这种杂质对特定系统性质的影响的理解，即液体氦-4从超流到正常状态的转变。这一系统可以仔细控制，以便将结论应用于具有重要技术意义的材料。通过向多孔气凝胶或多孔金中加入液氦来引入杂质，这些气凝胶或多孔金具有良好的孔隙率和拓扑结构。到目前为止，实验揭示了意想不到的结果，包括随机环境带来的异常急剧的转变，以及新的富3He超流相的产生。计划对这一异常行为以及这些性质随温度、压力和两种氦同位素浓度的变化进行研究。***

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. ***