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的超流体转变作为临界行为的范例。这项工作的目的是测试二阶相变的有限尺寸缩放理论。在这里，热力学行为应该由无约束系统的相关长度来衡量。除了氦外，对这个问题的实验研究很少。在早期的实验中，氦被限制在平面几何结构中，尺度问题一直存在。已经观察到，在超流体方面，预期的结垢失败了。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尺度。较低维度的作用也很重要，并将被追求。代表二维交叉的电影，其表现可能与附加维度变小的限制大不相同。由于二阶相变的普适性，本研究可能对其他接近二阶相变的系统的临界行为具有启示意义。用于定义约束的技术包括硅光刻和直接晶圆键合，以在cm2面积上实现纳米级约束的前所未有的均匀性。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，并接受了不同寻常的广泛的制作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受限流体的行为。通常材料的性质不随尺寸的变化而变化，因为最大的相关长度尺度是原子间间距的量级。然而，在相变附近出现了一个额外的长度尺度，即相关长度，它可以随着温度的变化而增长，变得相当大。在这种情况下，具有与相关长度相当的维度的样本的行为完全不同。本研究的重点是液氦的超流体转变，作为许多其他系统的一般行为的范例。这在当前的纳米技术研究中是很重要的，尤其是在磁性系统中。做这项工作需要大量的技术发展。已经开发了一种涉及硅片光刻和直接晶圆键合的工艺，以构建异常均匀的腔室，其中液体被限制。还开发了用于测量非常小样本的技术。目前的研究主要集中在维度的作用上。该程序将调查使1、2或所有3个维度变小的影响。在这一领域工作的学生，无论是研究生还是本科生，都要接受低温技术、硅光刻、晶圆键合以及使用计算机控制的高分辨率精密测量方面的培训。这为他们在学术界、工业界和政府的职业生涯或为高级学位的额外学习做好了准备。

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.