Phase Transitions and Phase Stability in Superfluid Helium Three

* 非技术摘要 *：在非常低的温度下，氦变成液体，在更低的温度下，它变成超流体。 超流体将无摩擦地流动。 在这一点上，它类似于超导体，超导体没有电阻地传输电流。 因此，了解超流氦可以提供对超导体的洞察。该研究项目将研究杂质如何改变包括高温超导体在内的有序状态，使用超流3 He（一种氦）作为模型系统。 先前对包含在非常低密度的二氧化硅气凝胶（一种非常低密度、高度多孔的材料）中的液体3 He中的超流性的研究发现，超流在其转变温度附近的有序状态被杂质改变，其方式与杂质的密度无关。 目前的研究项目将扩大气凝胶（杂质）的密度范围，看看密度可以有多低，仍然修改3 He有序状态;并看看其他有序状态是否可以在不同的杂质密度范围内稳定。 将对有序状态进行纵向磁共振研究，以确定气凝胶稳定状态的微观性质。这些研究也将在不同程度的应变作用于气凝胶的情况下进行，以了解气凝胶“杂质”的影响如何取决于各向异性。 该项目将涉及本科生和研究生的培训，PI作为诺贝尔奖获得者提供了许多技术和公开讲座。技术摘要 *：该奖项支持一项研究，即非常规BCS状态下的杂质散射如何影响有序状态的特性，使用3 He中的超流性作为模型系统，低密度二氧化硅气凝胶作为模型杂质。 先前的工作已经表明，具有低至0.5%的密度（由气凝胶占据的分数体积）的气凝胶可以稳定在本体中不稳定的接近Tc的状态，并且该新状态在气凝胶密度的变化中在2倍以上不变。 目前的研究将把气凝胶密度的范围扩大到0.2%，并将包括新状态下的纵向共振研究，以帮助确定其微观特性。 可变纵向应变的应用将有助于确定气凝胶的影响是否取决于其各向异性。 另外，3 He A到B的转变是一个有价值的模型，为一级相变的研究。 A到B过渡的研究将进行，以更好地了解表面粗糙度在成核中的作用，并区分两种拟议的成核机制，涉及电离辐射成核。 这项工作将包括培训本科生和研究生。

*****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.