Phase Transitions in the Presence of Disorder

9311918陈这个项目涉及进一步了解冻结无序(包括稀释极限)对经历连续相变的系统行为的影响的研究。这将通过将4He和3He-4He混合物放置在多孔金和二氧化硅气凝胶中来实现。由于制造工艺的不同，这两种衬底提供了在大范围内分别相关和不相关的无序--从而允许对当前理论思想进行关键测试。初步结果表明，当3He-4He混合物被置于轻质气凝胶内时，低温下的共存区域与超流转变线分离，形成了一种新的3He浓度下可混溶的超流体混合物。这种混合可能代表了稀玻色气体系统的实现，并暗示了在零温度极限下观察到互穿双超流体的可能性。在过去的二十年里，物理学家在理解经历连续相变的纯物质的行为方面取得了很大进展，例如从液体到蒸汽，从超导或超流状态到正常状态。这一知识并不总是适用于现实情况，因为大多数物质都含有缺陷和杂质。这个项目将通过将自然界最纯净的物质液氦放入无序的环境--多孔玻璃或多孔金--来研究杂质和无序对连续相变性质的影响。由于多孔玻璃的开放分数在0.98到0.3之间，这类似于杂质占样品的2%到20%的情况。初步结果表明，杂质对液-汽相变和超流体相变都有深远而意想不到的影响，并为流体混合物的混合和分相提供了新的机制。***

9311918 Chan This project involves studies undertaken to further the understanding of the effect of frozen-in disorder (including the dilute limit) on the behavior of systems undergoing continuous phase transitions. This will be accomplished by placing 4He and 3He-4He mixtures inside porous gold and silica aerogel. These two substrates, owing to the different manufacturing processes, provide disorder that are respectively correlated and uncorrelated at large length scales -- thus allowing for a critical test of current theoretical ideas. Preliminary results indicate that when a 3He- 4He mixture is placed inside light aerogel, the coexistence region at low temperature detaches from the superfluid transition line, giving rise to a novel miscible superfluid mixture at high 3He concentration. This mixture may represent the realization of a dilute Bose gas system and suggests the possibility of observing an interpenetrating double superfluid in the limit of zero temperature. %%% In the last twenty years, physicists have made great advances in understanding the behavior of pure substances undergoing continuous phase transformations, for example from liquid to vapor or from superconducting or superfluid states to the normal state. This knowledge is not always applicable to realistic situations as most substances contain defects and impurities. This project will investigate the effects of impurities and disorder on the nature of the continuous phase transitions by placing nature's purest substance, liquid helium, inside a disordered environment -- porous glass or porouss gold. Since the open fraction of the porous glasses ranges from 0.98 to 0.3, this is akin to the situation where the impurities constitute from two to twenty percent of a sample. Preliminary results indicate that impurities have a profound and unexpected effect on both the liquid-vapor and the superfluid transitions, and suggest new mechanisms as to how fluid mixtures undergo mixing and demixi ng. ***