Symmetry Tests and Vortex Imaging in Unconventional Superconductors

该个人研究者奖将为一位教授的项目提供支持，该项目将解决相和相相干在超导体系统中的作用。该项目的一个目标是确定一系列奇异超导材料中序参量的对称性。约瑟夫森干涉测量实验，有助于建立高温铜酸盐的对称性，将应用于其他几个被怀疑表现出非常规对称性的系统，包括表现出两种不同超导相变的重费米子超导体，被认为是d波的有机超导体，以及某些钌酸盐超导体。被认为是具有复序参数的p波，对应于时间反转对称性破缺的状态。此外，还测量了这些材料的低温穿透深度，这是对超导有序参数中节点最敏感的测试。这些实验对于确定配对对称性至关重要，从而可以表征复杂超导体的热力学、电磁和输运性质，并为发现超导性的微观机制指明方向。第二组实验将应用扫描SQUID显微镜来研究超导薄膜和晶体中磁涡流的分布和运动。需要解决的关键问题是非常规超导体中涡旋晶格的几何形状和涡旋核心的结构，涡旋在低温下的钉住和通量蠕变，以及涡旋在图像化薄膜中的运动，包括阶梯和棘轮结构中涡旋流动的不对称性。与此项目相关，将开发扫描SQUID显微镜技术的几项重大进展：将空间分辨率提高到亚微米级别，在稀释冰箱中实现超低温成像，以及开发超导体系统中单个漩涡的移动和定位方案，从而能够直接测试漩涡相互作用和固定。与此项目相关的是，将介绍一门已建立的研究生水平的超导体器件物理学课程，并开发一门新的纳米器件和探针课程。该个人研究者奖将为一位教授的项目提供支持，该项目将解决相和相相干在超导体系统中的作用。超导材料的特征是一个量，即序参量，它在很大程度上决定了它们的电子特性。与任何单个量相比，这个序参量的相位对超导体的迷人特性负责，并导致超导体电子设备的独特潜力和能力。两个最重要的进展是在铜酸盐超导体中发现了非常规配对，其特点是相各向异性强（与在普通超导体中观察到的各向同性顺序参数相反），以及开发了新型磁场探测仪器，使磁畴和漩涡的直接成像成为可能。本项目将在两个方向上建立这些进展：(1)将进行实验，以确定几种被怀疑是非常规的奇异超导材料的序参数对称性，包括重费米子超导体、有机超导体和钌酸盐超导体。将采用两种互补的方法：相敏干涉实验，直接探测顺序参数的相位各向异性，最确定的配对对称性测试，以及低温磁穿透深度的测量，这可能是对顺序参数大小最敏感的探针。(2)扫描SQUID显微镜将用于研究超导薄膜和晶体中磁涡流的分布和运动。该仪器利用灵敏的直流SQUID探测器对样品表面进行扫描，绘制出高磁场和空间分辨率的磁场分布图。该项目将为超导相电子学领域的一些研究生提供培训，超导相电子学是量子信息处理和计算领域当前感兴趣的课题。与此项目相关的是，将介绍一门已建立的研究生水平的超导体器件物理学课程，并开发一门新的纳米器件和探针课程。＊＊＊

This individual investigator award will provide support to a professor for a project that will address the role of the phase and phase coherence in superconductor systems. One objective of the project will be to determine the symmetry of the order parameter in a series of exotic superconducting materials. Josephson interferometry experiments, instrumental in establishing the symmetry of the high temperature cuprates, will be applied to several other systems suspected to exhibit unconventional symmetry, including heavy fermion superconductors, which exhibit two distinct superconducting phases transitions, organic superconductors, believed to be d-wave, and certain ruthenate superconductors, thought to be p-wave with a complex order parameter that corresponds to a state with broken time-reversal symmetry. Supplementing these are measurements of the low temperature penetration depth in these materials, the most sensitive test of nodes in the superconducting order parameter. These experiments are crucial to identifying the pairing symmetry, thereby allowing characterization of the thermodynamic, electromagnetic, and transport properties of complex superconductors, and pointing the way to discovery of the microscopic mechanism responsible for the superconductivity. A second set of experiments will apply Scanning SQUID Microscopy to study the distribution and motion of magnetic vortices in superconducting films and crystals. Key problems to be addressed are the geometry of the vortex lattice and structure of the vortex core in unconventional superconductors, the pinning of vortices and flux creep at low temperatures, and the motion of vortices in patterned films including asymmetries in vortex flow in step and ratchet structures. In connection with this project, several significant advances of the Scanning SQUID Microscopy technique will be developed: enhancement of the spatial resolution into the submicron regime, implementation of imaging at ultralow temperatures in a dilution refrigerator, and development of schemes for moving and positioning individual vortices in superconductor systems that will enable direct tests of vortex interactions and pinning. Associated with this project will be the presentation of an established graduate level course in Superconductor Device Physics and the development of a new course in Nanoscale Devices and Probes. %%%This individual investigator award will provide support to a professor for a project that will address the role of the phase and phase coherence in superconductor systems. Superconducting materials are characterized by a quantity, the order parameter, which largely determines their electronic properties. More than any single quantity, it is the phase of this order parameter that is responsible for the fascinating properties of superconductors and leads to the unique potential and capabilities of superconductor electronic devices. Two of the most significant advances have been the identification of unconventional pairing in the cuprates superconductors that is characterized by a strong phase anisotropy (in contrast to the isotropic order parameter observed in ordinary superconductors), and the development of novel magnetic field detection instruments that make possible direct imaging of magnetic domains and vortices. This project will build on these advances in two directions: (1) Experiments will be carried out to determine the order parameter symmetry of several exotic superconducting materials that are suspected to be unconventional, including heavy fermion superconductors, organic superconductors, and ruthenate superconductors. Two complementary approaches will be used: phase-sensitive interferometry experiments which directly probe the anisotropy of the phase of the order parameter, the most definitive test of the pairing symmetry, and measurements of the low-temperature magnetic penetration depth, which is perhaps the most sensitive of the probe of the magnitude of the order parameter. (2) Scanning SQUID Microscopy will be used to study the distribution and motion of magnetic vortices in superconducting films and crystals. This instrument scans a sensitive dc SQUID detector over the surface of a sample to map out the magnetic field distribution with high magnetic field and spatial resolution. This project will provide training for a number of graduate research students in the field of superconducting phase electronics, an topic of current interest for quantum information processing and computing. Associated with this project will be the presentation of an established graduate level course in Superconductor Device Physics and the development of a new course in Nanoscale Devices and Probes. ***