Nanoscale Magnetism and Unconventional Quantum Phases and Transitions

该资助涉及理论凝聚态物理学的两个广泛的研究项目：纳米结构中自旋输运的建模和控制，以及强相关材料中量子物质的非常规集体态。这两个领域在实验上都非常活跃，并对现有理论提出了重要的难题和挑战。一个适当的自旋输运流体动力学理论将被开发，与微观的输运系数的导数。具体的问题是确定自旋电荷耦合项的结构，它们对各种散射过程的依赖性，以及测量这些效应的实验方法。另一个项目是铁磁稀磁半导体材料中的自旋弛豫，以及它如何进入适当的流体力学。最后，单量子自旋的非流体力学机制将被考虑，包括它们的相干效应和弛豫机制。这项研究受益于，并补充，现有的实验专业知识，在自旋电子学在UCSB。非常规阶段和过渡的项目是在最近的工作中，简单合理的模型可以表现出阶段和量子临界点完全超出通常的范式的实现的动机。因此，本项目的目的是将新兴的新概念框架应用于实验有趣材料中最近和长期存在的难题，并在较小的程度上扩展和系统化框架本身。要研究的一些实验现象是莫特电荷有序转变，尖晶石和稀土金属间化合物中的电荷和自旋挫折及其与重费米子行为的联系，以及有机和无机三角晶格磁体中的自旋液态。 这些问题将通过各种分析（场论，重整化群，规范理论，玻色化）和数值（精确对角化，变分波函数）技术来解决。 PI熟悉各种各样的方法，并在这些领域做出了实质性的贡献之前，这个grant.Intellectual优点：这些项目代表了凝聚态物理学的两个不同的前沿。 自旋的控制和测量正在经历一场技术上的实验革命（例如，时间分辨光学泵浦-探测光谱学、自旋-轨道相互作用的应变工程等）。和材料（稀磁半导体、数字磁结构）。集体量子秩序和涌现行为的新机制正在重振对强关联材料中旧的突出问题的理论研究，并提出新的问题。因此，这两个领域都已经成熟，可以将新的理论思想应用于实践，从而推进凝聚态物理学的基础知识。更广泛的影响：首先，这项工作旨在激励和解释实验和材料的性质。此外，在该提案的自旋电子学部分中开发的流体动力学模型实际上描述了宏观输运现象：它们是电荷传导的欧姆定律的自旋类似物。这种类型的详细模型显然对任何应用都至关重要。在教育方面，研究生和博士后将在凝聚态理论的这些前沿领域进行培训，并将开发新的课程材料，以向新生传达该领域的兴奋。将建立一个综合网站，以口语和研究生水平解释这项研究，以进一步与更广泛的受众沟通。

This grant involves two broad research projects in theoretical condensed matter physics: modeling and control of spin transport in nanostructures, and unconventional collective states of quantum matter in strongly correlated materials. Both areas are very active experimentally, and present important puzzles and challenges to existing theory.The spintronics project has several specific goals. A proper hydrodynamic theory of spin transport will be developed, with microscopic derivations of transport coefficients. Specific problems are to determine the structure of spin-charge coupling terms, their dependence upon various scattering processes, and experimental means of measuring these effects. Another project is spin relaxation in ferromagnetic diluted magnetic semiconductor materials, and how this enters the appropriate hydrodynamics. Finally, the non-hydrodynamic regime of single quantum spins will be considered,including their coherence effects and relaxation mechanisms. This research benefits from, and complements, existing experimental expertise in spintronics at UCSB.The project on unconventional phases and transitions is motivated by the realization in recent work that simple reasonable models can exhibit phases and quantum critical points completely outside the usual paradigms. Thus this project is aimed both at applying the emerging new conceptual framework to recent and long-standing puzzles in experimentally interesting materials, and, to a lessor extent, at extending and systematizing the framework itself. Some of the experimental phenomena to be studied are Mott charge ordering transitions, charge and spin frustration in spinels and rare earth intermetallics and their connection to heavy fermion behavior, and spin liquid states in organic and inorganic triangular lattice magnets. These problems will be addressed by a variety of analytical (field theory, renormalization group, gauge theory, bosonization) and numerical (exact diagonalization, variation wavefunction) techniques. The PI is familiar with a wide variety of such approaches, and has made substantial contributions in these fields prior to this grant.Intellectual Merit: These projects represent two different frontiers of condensed matter physics. Control and measurement of spin is undergoing an experimental revolution in techniques (e.g. time-resolved optical pump-probe spectroscopy, strain engineering of spin-orbit interactions, etc.) and materials (diluted magnetic semiconductors, digital magnetic structures). New mechanisms of collective quantum order and emergent behavior are reinvigorating the theoretical investigation of old outstanding problems - and suggesting new ones - in strongly correlated materials. Both fields are thus ripe for bringing new theoretical ideas into application, and thereby advancing the fundamental knowledge base of condensed matter physics.Broader Impacts: First and foremost, this work is designed to motivate and explain experiments and properties of materials. Moreover, the hydrodynamic models to be developed in the spintronics portion of the proposal in fact describe macroscopic transport phenomena: they are the spin analogs of Ohm's law for charge conduction. Detailed models of this type are clearly crucial for any applications. Educationally, graduate students and postdocs will be trained in these forefront areas of condensed matter theory, and new course materials will be developed to communicate theexcitement of the fields to new students. A comprehensive web site with colloquial and graduate student level explanations of the research will be developed to further communicate to a broader audience.