Dimensionality and Weak-links Effects at the Superfluid Transition of 4He

这个低温物理研究项目将研究受限4He的超流转变，作为临界行为的范例。这项工作旨在检验二阶相变上的有限尺寸标度理论。在这里，热力学行为应该通过非受限系统的关联长度来衡量。除了氦，关于这个问题的实验研究很少。从早先用氦限制在平面几何中的实验中，比例成了一个问题。已经观察到，在超流体一侧，预期的比例是失败的。这也得到了超流体分数的理论计算的支持。其他理论模型也指出可能缺乏可伸缩性。较低维度的作用也很重要，并将继续发挥作用。代表二维交叉的薄膜，其行为可能与附加维度较小的限制条件截然不同。由于二阶相变的普遍性质，这项研究可能对其他系统在二阶相变附近的临界行为有一定的指导意义。在定义限制时使用的技术包括硅光刻和直接晶片键合，以在平方厘米面积上实现前所未有的纳米级别限制的一致性。这与实验技术相结合，可以对非常小的样本进行高精度的研究。研究生和本科生都参与了这项工作，并接受了异常广泛的制造和实验方法方面的培训。这为他们在工业、学术界或政府的职业生涯做好了准备。这个低温物理项目涉及研究相变附近的受限流体的行为。通常情况下，材料的性质不会随着尺寸的变化而改变，因为最大的相关长度尺度是原子间距的量级。然而，在相变附近出现了一个额外的长度标度，关联长度可以随着温度的函数增长而变得相当大。在这种情况下，维度与关联长度相当的样本表现得非常不同。这项研究的重点是液氦在其超流体转变时的情况，作为许多其他系统的一般行为的范例。这在当前对纳米技术的追求中很重要，特别是对磁性系统。要完成这项工作，需要大量的技术发展。已经开发出一种工艺，它涉及硅片的光刻和直接晶片键合，以构建非常均匀的腔室，其中液体被限制在其中。还开发了测量非常小样本的技术。本研究侧重于维度的作用。该程序将调查将1维、2维或全部3维缩小的效果。在这一领域工作的学生，包括研究生和本科生，都接受过低温技术、硅光刻、晶片键合以及使用计算机控制的高分辨率、高精度测量方面的培训。这为他们在学术界、工业界和政府部门的职业生涯做好了准备，或者为攻读更高学位做好了准备。

This low-temperature physics research project will investigate the superfluid transition of confined 4He as a paradigm of critical behavior. The work is intended to test theories of finite-size scaling at a second order phase transition. Here, the thermodynamic behavior should be scaled by the correlation length of the unconfined system. Excepting helium, there are very few experimental studies of this problem. Scaling has come into question from earlier experiments with helium confined in a planar geometry. It has been observed that on the superfluid side the expected scaling fails. This is also supported by theoretical calculations of the superfluid fraction. Other theoretical models also point out possible lack of scaling. The role of the lower dimension is also important and will be pursued. Films, representing 2-dimensional crossover, might behave quite differently from confinements where additional dimensions are made small. This research, because of the universal character of second order transitions, may have implications for critical behavior of other systems near a second order phase transition. The technique used in defining the confinement involves silicon lithography, and direct wafer bonding to achieve unprecedented uniformity in confinement at the nanometer level over cm2 areas. This is coupled with experimental techniques that allow study of very small samples with high accuracy. Both graduate and undergraduates, are involved in this work, and receive training in an unusually broad range of fabrication and experimental methods. This prepares them for careers in industry, academia, or government.This low-temperature physics project involves studies of the behavior of a confined fluid near a phase transition. Usually materials do not change their properties with size because the largest relevant length scale is of the order of the interatomic spacing. However, near a phase transition an additional length scale emerges, the correlation length, which can grow as function of temperature to become quite large. In this case, samples which have dimensions comparable to the correlation length behave quite differently. This study focuses on liquid helium at its superfluid transition as a paradigm of what is expected to be a general behavior of many other systems. This is important in the current pursuit of nanotechnology, especially for magnetic systems. Substantial technological development is needed to do this work. A process has been developed which involves lithography of silicon wafers, and direct wafer bonding to construct exceptionally uniform chambers in which the liquid is confined. Techniques have also been developed for measuring very small samples. The present research focuses on the role of dimensionality. The program will investigate the effects of making 1, 2, or all 3 dimensions small. Students working in this area, both graduate and undergraduates are trained in cryogenic techniques, silicon lithography, wafer bonding, and high-resolution, precision measurements using computer controls. This prepares them for careers in academe, industry, and government or for additional study for advanced degrees.