NMR Studies of Field-Induced Phases and Phase Transitions

超导性可能是量子力学现象中最广为人知的在宏观层面上表现出来的例子。超导态的物理性质是惊人的，包括电流无损耗的流动和超导样品内部的磁通量的排出。这些相同的特性使得超导在许多应用中都非常有用。了解超导性如何在高磁场中存活下来，这在科学和技术上具有重要意义。许多年前，有人预测，这是可能的，通过一种在微观水平上产生物质不均匀状态的机制。在这个项目中，磁共振技术将用于研究超导系统，其中有证据表明在高场下形成新相，包括探索其物理性质，目的是识别非均匀相并表征其性质。这项工作将通过对磁绝缘体中新形式的磁场感应磁态的研究来补充。磁共振适用于这类问题，因为它对被探测核自旋附近的局部环境很敏感。研究生将在一系列极端条件下设计和执行实验程序，包括极低的温度和高磁场。本科生将协助设计和建造特定的部件。利用核磁共振技术，研究由大磁场稳定的超导体和量子自旋系统中磁场诱导相的性质。在超导体的情况下，材料是有机超导体，具有高度各向异性的超导特性，从而减弱了轨道对的断裂。有证据表明，每种情况下都存在不同的高场相，这使它们成为非均匀超导的绝佳候选者，这种形式早在40年前就被预测出来了，但尚未得到证实，其性质也尚未经过实验测试。磁共振很容易适应必要的极端条件，它是一个局部问题，对非均匀性很敏感。量子自旋二聚体系统是anS=1的系统，其中二聚体位于层状六边形晶格上，并且热力学测量给出了在温度为1k时场变化时的公式相变的证据。研究生将设计和执行实验程序，并由本科生协助，他们将学习如何设计特定的组件。人们将寻找非均匀态的证据，从中有望出现新的量子效应。

non-technical abstractSuperconductivity is probably the most widely known example of aquantum mechanical phenomenon manifested at a macroscopic level.The physical properties attributed to the superconducting stateare spectacular and include the flow of electrical current withoutloss and the expulsion of magnetic flux from the interior of asuperconducting sample. These same properties makesuperconductivity uniquely useful for many applications. Ofimportant scientific and technological importance is to understandhow superconductivity survives to high magnetic fields. Many yearsago, it was predicted that this is possible through a mechanismproducing a state of matter inhomogeneous on a microscopic level.In this project, magnetic resonance techniques will be used to study superconducting systems for which there is evidence for formation of a new phase at high fields, including an exploration of their physical properties, with the aim of identifying the inhomogeneous phase and characterizing its properties. This work will be complementedby investigations of new forms of field-induced magnetic states ina class of magnetic insulators. Magnetic resonance is suitable forthis type of problem because it is sensitive to the environmentlocally proximate to the nuclear spins being probed. Graduatestudents will engineer and carry out the experimental programunder a range of extreme conditions, including very lowtemperatures and high magnetic fields. Undergraduate students willassist in the design and construction of specific components.technical abstractThe nature of magnetic field-induced phases in superconductors andquantum spin systems, stabilized by large magnetic fields, will bestudied using nuclear magnetic resonance techniques. In the caseof the superconductors, the materials are organic superconductorswith highly anisotropic superconducting properties which weakensorbital pair-breaking. Evidence exists for distinct high-fieldphases in each case, making them excellent candidates forinhomogeneous superconductivity in a form predicted forty yearsago but not yet confirmed, and for which its properties remainuntested experimentally. Magnetic resonance accommodates easilythe necessary extreme conditions, and it is a local probesensitive to inhomogeneities. The quantum spin dimer system is anS=1 system in which the dimers sit on a layered hexagonal lattice,and for which thermodynamic measurements give evidence formultiple phase transitions as the field is varied at temperaturesT1K. Graduate students will design and carry out the experimentalprogram, and are assisted by undergraduates who will learn how toengineer specific components. Evidence for inhomogeneous stateswill be looked for, from which new quantum effects are expected toemerge.