Exotic Magnetic Ground States and Ground State Selection in Quantum Materials

大多数磁性材料的基态，以及所有已知的磁性材料，都具有长程有序和常规的基本激发，如自旋波。我们对新磁性材料的家族特别感兴趣，在那里情况并非如此；在那里，由于存在非常强的自旋涨落，无序的低温相可以持续存在。这是当代凝聚态研究的一个非常热门的领域，因为人们普遍认为，如果传统的有序态可以在低温下被抑制，它就为奇异的基态的出现打开了一扇窗。适当的量子无序态被认为具有新的和显著的物理性质，如离域量子自旋激发，这可能为量子计算提供一个平台。在磁性材料中，可以通过三种方式鼓励强自旋涨落：磁矩可以修饰固体中与简单有序状态不相容的局部几何形状（通常基于三角形和四面体）——这被称为几何挫折；利用支持极端量子涨落的S=1/2和有效S=1/2磁矩来修饰晶格；最后，磁性晶格可以是二维或一维的，因为三维晶体结构是由磁性平面的堆叠，或磁链的聚集组成的。这项提议旨在探索新的磁性材料，正是这样做的。通过进行先进的表征，主要是利用新的中子和x射线散射技术，我们的目标是阐明在这些材料中随之而来的奇异基态的性质，以便充分理解它们所显示的新现象的基本原理。本提案旨在对我们将以单晶、多晶和外延薄膜形式生长的几种奇异磁性材料进行中子散射测量。我们正处于进行前沿非弹性中子散射的能力复兴之中。最终的结果是，与六年前相比，新的中子散射仪器提供了质量上不同的、更全面的材料信息。本提案旨在将这种新的基础设施应用于与新材料中的奇异磁基态相关的几个主题问题。这个提案主题的一个关键组成部分和出发点是一个复杂材料制备和晶体生长的项目，它将提供适当形式的新磁性材料。我们处于非常有利的位置，无论是在我们生产感兴趣的材料的能力方面，还是在与新的中子散射基础设施相关的访问和专业知识方面，都可以在凝聚态物理这个非常热门的领域取得真正的进展。

The ground states of most magnetic materials, and all magnetic materials which are well understood, are characterized by long range order and conventional elementary excitations, such as spin waves. We are specifically interested in families of new magnetic materials, where this isn't the case; that is where a disordered low temperature phase can persist due to the presence of very strong spin fluctuations. This is a very topical area of contemporary condensed matter research as it is widely believed that if a conventional ordered state can be suppressed at low temperatures, it opens a window for an exotic ground state to be manifest.  Appropriate quantum disordered states are thought to possess qualitatively new and remarkable physical properties, such as delocalized quantum spin excitations, that may provide a platform for quantum computation. Strong spin fluctuations can be encouraged in three ways in magnetic materials: magnetic moments can decorate local geometries (typically based on triangles and tetrahedra) in solids which are incompatible with a simple ordered state - this is referred to as geometrical frustration; S=1/2 and effective S=1/2 magnetic moments, which support extreme quantum fluctuations, can be made to decorate the lattice; and finally the magnetic lattice can be effectively two or one dimensional due to a three dimensional crystal structure which is made of a stacking of magnetic planes, or a bunching together of magnetic chains.  This proposal seeks to explore new magnetic materials which do precisely this. By carrying out advanced characterization, primarily with new neutron and x-ray scattering techniques we aim to elucidate the nature of the exotic ground states which ensue in such materials, such that the principles that underlie the novel phenomena they display can be fully understood. This proposal seeks to carry out programs of neutron scattering measurements on several families of exotic magnetic materials that we will grow in single crystalline, polycrystalline and epitaxial thin film form. We are in the midst of a renaissance in the capabilities available to carry out forefront inelastic neutron scattering.  The net result is that new neutron scattering instrumentation provides qualitatively different, and much more comprehensive, information on materials than was possible six years ago. This proposal seeks to apply this new infrastructure to several topical problems related to exotic magnetic ground states in new materials. A crucial component and starting point for the themes of this proposal is a program of sophisticated materials preparation and crystal growth which will provide new magnetic materials in appropriate forms. We are exceptionally well positioned, both with respect to our capacity to produce materials of interest, and with respect to access and expertise associated with the new neutron scattering infrastructure, to make real progress in this very topical area of condensed matter physics.