Research on Superfluid 3He Weak Links

本项目致力于了解超流3He中弱连接阵列(WLA)的静态和动态性质。WLA由直径在超流体相干长度范围内的量子相干孔径阵列组成：通常为几十纳米。过去的研究已经揭示了几个意想不到的特征，包括：双稳定、p态和新的超流耗散来源。过去的研究仅在零环境压力下进行。本项目将探索3He的整个相图，直到熔化压力。将回答的具体问题包括：对于所有相关的温度和压力，电流-相关系i(F)的形式是什么？在约瑟夫森区，临界电流密度是如何标度的？在弱链接阵列中保持相干的光圈数量的限制(如果有的话)是多少？在压力升高时，耗散过程是如何变化的？弱连接阵列总是有两个稳定状态吗？约瑟夫森振荡可以由温度梯度和压力梯度驱动吗？在从A到B的转变过程中，I(F)函数是如何变化的？当阵列上存在A-B偏置时，I(F)的性质是什么？该项目培训的研究生将学习最先进的微制造、超低温、鱿鱼磁学、电子仪器和计算机控制与分析技术。从中子星内部到超冷玻色-爱因斯坦凝聚气体，都有“宏观量子”系统。相关的体系包括超导体，包括常规超导体和高T_c超导体，超流4He和超流3He，A相和B相。物理学的一个主要目标是理解为什么这种不同的系统显示出对整个班级都是通用的现象。这个项目将致力于理解超流3He中弱连接的物理。弱链接包括将两个宏观量子系统连接成一个单一的、弱连接的整体的装置。在3He的情况下，薄弱环节是一组纳米大小的孔，在50 nm厚的“窗口”中进行微机械加工。这些装置显示了各种意想不到的现象，这些现象已经使人们对超流、高T_c超导和量子计算有了更好的理解。由于过去的研究只涵盖了零环境压力下的实验，本项目将扩大勘探范围，覆盖很大的压力范围，并将包括两个不同的3He超流体相。为了为未来的科学生涯做好准备，参与该项目的学生将接受各种最先进的物理技术方面的培训，包括超低温、微机械加工、超灵敏磁力测量、电子学和计算机控制。

This project is focused on understanding the static and dynamic properties of weak link arrays (WLA) in superfluid 3He. The WLA consists of a quantum coherent array of apertures with diameter on the scale of the superfluid coherence length: typically a few tens of nanometers. Past research has already exposed several unanticipated features including: bi-stability, p-states, and new sources of superfluid dissipation. The past research was performed only at zero ambient pressure. The present project will explore the entire phase diagram of 3He, up to the melting pressure. The specific questions which will be answered include: What is the form of the current-phase relation, I(f), for all relevant temperatures and pressures? In the Josephson region, how does the critical current density scale? What is the limit, if any, to the number of apertures that will remain coherent in a weak link array? How do the dissipation processes change at elevated pressure? Are there always two stable states of the weak link array? Can a Josephson oscillation be driven by a temperature gradient as well as a pressure gradient? How does the I(f) function change in the transition from A to B phase? What is the nature of I(f)when there is an A-B bias across the array? Graduate students trained in the project will learn state-of-the-art techniques in microfabrication, ultra-cryogenics, SQUID magnetometry, electronic instrumentation and computer control and analysis."Macroscopic quantum" systems span the range from the interior of neutron stars to ultra-cold Bose-Einstein condensed gases. Relevant systems include superconductors, both conventional and high Tc, superfluid 4He and superfluid 3He, both A and B phases. It is a major goal of physics to understand why such diverse systems display phenomena which are generic to the entire class. This project will be directed toward understanding the physics of weak links in superfluid 3He. Weak links consist of devices which join two macroscopic quantum systems into a single, weakly connected whole. In the case of 3He the weak link is an array of nanometer-sized apertures, micro-machined in a 50nm thick "window". These devices have been shown to display a variety of unexpected phenomena, which have already led to a better understanding of superfluidity, high Tc superconductivity and quantum computing. Since past research has only covered experiments at zero ambient pressure, the present project will extend the range of exploration over a wide range of pressure and will include two different superfluid phases of 3He. To prepare them for future scientific careers, students participating in the project will receive state-of-the-art training in a variety of physics techniques including ultra-cryogenics, micro-machining, ultrasensitive magnetometry, electronics and computer control.