Phase Transitions and Phase Stability in Superfluid Helium Three

非技术摘要：氦在很低的温度下变成液体，在更低的温度下变成超流体。超流体将在没有摩擦的情况下流动。在这种情况下，它类似于超导体，它携带的电流没有电阻。因此，了解超流氦可以提供对超导体的洞察。这个研究项目将以超流3He(一种氦)为模型系统，研究杂质如何改变包括高温超导体在内的有序态。以前对作为杂质的极低密度二氧化硅气凝胶(一种非常低密度、高度多孔的材料)中所含液体3He的超流动性的研究发现，杂质改变了超流体在其转变温度附近的有序状态，而改变的方式与杂质的密度无关。本研究项目将扩大气凝胶(杂质)的密度范围，看看密度可以低到什么程度，并仍然修改3He有序态；以及看看其他有序态是否可以在不同的杂质密度范围内稳定下来。将进行有序状态的纵向磁共振研究，以确定气凝胶稳定状态的微观性质。这些研究还将在气凝胶上施加不同程度的应变，以了解气凝胶“杂质”的影响如何取决于各向异性。这个项目将包括本科生和研究生的培训，PI作为诺贝尔奖获得者进行了大量的技术和公开演讲。*技术摘要*：这个奖项支持一项研究，以3He中的超流性为模型系统，以低密度二氧化硅气凝胶为模型杂质，研究非传统BCS态中的杂质散射如何影响有序态的身份。先前的工作已经表明，密度(气凝胶所占的体积分数)低至0.5%的气凝胶可以稳定在体相中不稳定的T_c附近的状态，并且这种新状态在气凝胶密度变化两倍的情况下保持不变。目前的研究将把气凝胶密度范围扩大到0.2%，并将包括新状态下的纵向共振研究，以帮助确定其微观身份。应用可变的纵向应变将有助于确定气凝胶的影响是否取决于其各向异性。另外，3HeA-B相变是研究一级相成核的一个有价值的模型。将对A到B转变进行研究，以更好地了解表面粗糙度在成核中的作用，并区分所提出的两种涉及电离辐射成核的成核机制。这项工作将涉及本科生和研究生的培养。

*****NON-TECHNICAL ABSTRACT*****:At very low temperatures helium becomes a liquid, at still lower temperatures it becomes a superfluid. A superfluid will flow with no friction. In that way it is similar to a superconductor, which carried an electric current with no resistance. Therefore understanding superfluid helium can provide insight into superconductors. This research project will study how impurities modify the class of ordered states that include the high temperature superconductors, using superfluid 3He (one type of helium) as a model system. Prior studies of superfluidity in liquid 3He contained within very low density silica aerogels (a very low density, highly porous material), which act as the impurities, found that the ordered state of the superfluid near its transition temperature was changed by the impurities, in a manner that was independent of the density of the impurities. The present research project will extend the range of density of the aerogel (impurities) to see how low the density can be and still modify the 3He ordered state; and to see if other ordered states can be stabilized over different ranges of impurity densities. Longitudinal magnetic resonance studies of the ordered state will be carried out to identify the microscopic nature of the state stabilized by the aerogel. The studies will also be carried out with different degrees of strain applied to the aerogel, to see how the affect of the aerogel 'impurities' depends upon anisotropy. This project will involve the training of undergraduate and graduate students, and the PI gives numerous technical and public lectures as a Nobel Laureate.*****TECHNICAL ABSTRACT*****:This award supports a study of how impurity scattering in unconventional BCS states can affect the identity of the ordered state, using superfluidity in 3He as a model system, and low density silica aerogels as model impurities. Prior work has shown that aerogels with densities (fractional volume occupied by the aerogel) as low as 0.5% can stabilize a state near Tc that is not stable in the bulk, and that this new state is unchanged over a factor of two in variation in the aerogel density. The present research will extend the range of aerogel densities to 0.2%, and will include longitudinal resonance studies in the new state to help determine its microscopic identity. The application of variable longitudinal strain will help determine if the affects of the aerogel depend upon its anisotropy. Separately, the 3He A to B transition is a valuable model for studies of first order phase nucleation. A study of the A to B transition will be carried out to better understand the role of surface roughness in nucleation and to differentiate between the two proposed nucleation mechanisms involving nucleation by ionizing radiation. This work will involve the training of both undergraduate and graduate research students.