Nuclear Magnetic Resonance Studies of Quantum Criticality in Correlated Electron Materials

摘要：固体中的电子可以表现出从超导性到磁性等广泛的重要现象。通常可以通过控制一些外部参数（如温度、压力或磁场）来调整这种行为。本项目通过一组受控核磁共振（NMR）实验研究了几种关键材料，目的是了解如何调整和控制它们的性质。这种调谐可以导致物质行为发生戏剧性和意想不到的变化，据信这是一种被称为“量子临界”的现象。这项工作通过在一组精心挑选的材料中探测奇异电子状态的性质，推进了对量子临界性的基本理解，并为下一代科学家和工程师的研究生和本科生提供现代物理培训和指导。技术摘要：本提案的目的是深入了解强相关电子材料在量子相变附近的行为。强相关电子材料在新技术方面具有巨大的潜力，但对其相关多体物理的基本理解仍然是凝聚态物理中最大的挑战之一。通常，最有趣的科学出现在量子临界点，在那里非常规的超导性出现，非费米液体的行为主导了正常状态的行为。核磁共振（NMR）是一种强大的工具，可以研究驱动这些效应的波动，但需要控制实验来解开自旋和电荷自由度的影响。本项目利用新的核磁共振技术来探测互补系统在静水压力和单轴应变下的这些动力学，包括在量子临界点处表现出费米表面跳跃的模型重费米子系统，以及具有铁四极序局域f电子的金属。本项目对具有多组分手性序参量的自旋三重态超导体涡旋核心激发的性质，以及正常状态下自旋波动对单轴应变的响应进行了若干理论预测。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical Abstract: Electrons in solids may exhibit a wide range of important phenomena, ranging from superconductivity to magnetism. Often this behavior can be tuned by controlling some external parameter such as temperature, pressure, or magnetic field. This project investigates several key materials through a set of controlled nuclear magnetic resonance (NMR) experiments with the aim of understanding how to tune and control their properties. Such tuning can lead to dramatic and unexpected changes in the material behavior, believed to emerge from a phenomenon called "quantum criticality". This work advances fundamental understanding of quantum criticality by probing the nature of exotic electronic states within a set of carefully selected materials, and provides modern physics training and mentoring to graduate and undergraduate students for the next generation of scientists and engineers.Technical Abstract: The objective of this proposal is to develop a deeper understanding of the behavior of strongly correlated electron materials near quantum phase transitions. Strongly correlated electron materials hold tremendous potential for new technologies, but a fundamental understanding of their relevant many-body physics remains one of the greatest challenges in condensed matter physics. Often the most interesting science arises at a quantum critical point, where unconventional superconductivity emerges and non-Fermi liquid behavior dominates the normal state behavior. Nuclear magnetic resonance (NMR) is a powerful tool to investigate the fluctuations that drive much of these effects, but requires controlled experiments to disentangle the effects of spin and charge degrees of freedom. This project utilizes new NMR techniques to probe these dynamics under both hydrostatic pressure and uniaxial strain in complementary systems, including a model heavy fermion system exhibiting a Fermi surface jump at a quantum critical point, and a metal with ferro-quadrupolar order of localized f-electrons. Several theoretical predictions are being tested in this project about the nature of the excitations in the vortex cores of a spin-triplet superconductor with a multicomponent chiral order parameter, and how the spin fluctuations in the normal state respond to uniaxial strain.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.