Weak Coupling Effects in helium-4

对于一个接近连续相变的系统，许多热力学性质的行为是由关联长度的增长引起的。这个长度在热力学极限上发散，并导致了奇异性，例如热容和磁化率。这种行为源于平均热力学性质的波动能够传播并在逐渐增大的距离上被感觉到。该奖项的研究将解决这种行为的一个特定方面：两个相邻但弱耦合的氦样品在从正常状态到超流状态的过渡附近如何表现？ 这将在盒子或通道中限制的氦阵列中进行研究，并通过非常薄的薄膜连接。这种几何形状是通过光刻和使用直接晶片键合在硅晶片上实现的。这项工作的初步结果非常令人惊讶，表明耦合延伸的距离比预期的要大得多。在某些方面，这表明超流氦和高温超导体的观测结果之间的相似性。该项目将支持两名攻读博士学位的研究生。学生将发展硅加工，真空技术，薄膜沉积和一系列低温技术和数据分析的专业知识。 非技术性摘要在相变时，系统将以这样一种方式改变其特性，即新的属性变得明显。冰的融化就是一个例子，冰转变成液体，后者具有容易“流动”的新能力。在某些类型的过渡中，变化更显着，而且往往伴随着非常不寻常的行为。液态氦在低温下会发生转变，从而从正常液体转变为超流体。后者能够在没有摩擦的情况下流动，就像超导体可以在没有电阻的情况下携带电流一样。拟议的研究将集中在这种行为的一个方面：两个“弱”连接的液氦区域在经历这种转变时如何相互影响？有人称之为邻近效应。这在超导体和磁系统中也很明显，并且可以是相当普遍的。这项工作将为这个问题提供一些独特的答案，因为氦的性质和弱连接的性质都将被研究。该项目将支持两名攻读博士学位的研究生。学生将发展硅加工，真空技术，薄膜沉积和一系列低温技术和数据分析的专业知识。预计这些结果将在会议上提出，并在档案期刊上发表，作为学生博士培训的一部分。

TECHNICALFor a system near a continuous phase transition, the behavior of many thermodynamic properties arises from the growth of the correlation length. This length diverges in the thermodynamic limit and is responsible for singularities, for instance, in the heat capacity and susceptibility. This behavior stems from the ability of fluctuations in the average thermodynamic properties to propagate and be felt over progressively larger distances. The research to be pursued as a result of this award will address a particular aspect of this behavior: How do two adjoining, but weakly coupled helium samples behave near the transition from the normal state to the superfluid state? This will be studied in arrays of helium confined in boxes or channels and connected through a very thin film. This geometry is achieved on silicon wafers through lithography and the use of direct wafer bonding. Preliminary results for this work have been very surprising, indicating that the coupling extends over much larger distances than expected. In some ways this indicates a similarity between superfluid helium and observations with high temperature superconductors. This project will support two graduate students working towards their doctorate. The students will develop expertise in silicon processing, vacuum techniques, thin film deposition and an array of cryogenic techniques and data analysis. NON-TECHNICAL ABSTRACTAt a phase transition a system will change its characteristics in such a way that new properties become manifest. The melting of ice is an example of this whereby ice transforms into a liquid and the latter has the new ability to easily "flow". At some types of transitions the changes are more dramatic, and they are often accompanied with very unusual behavior. Liquid helium has a transition at low temperatures whereby it transforms from a normal liquid into a superfluid. The latter is able to flow without friction in an analogous way that a superconductor can carry an electrical current without resistance. The proposed research will focus on one aspect of this behavior: How do two regions of liquid helium which are "weakly" connected influence each other as they undergo this transition? One calls this a proximity effect. This is also manifest in superconductors and in magnetic systems, and can be quite general. The proposed work will provide some unique answers to this question in the sense that both the properties of the helium will be studied and those of the weak link. This project will support two graduate students working towards their doctorate. The students will develop expertise in silicon processing, vacuum techniques, thin film deposition and an array of cryogenic techniques and data analysis. It is expected that these results will be presented at conferences and published in archival journals as part of the students' doctoral training.