Exotic Ground States in New Materials

大多数磁性材料的基态，以及众所周知的所有磁性材料，都具有长程有序和传统的元素激发的特征，如自旋波。我们特别感兴趣的是一系列新的磁性材料，但情况并非如此；在这些材料中，由于存在非常强烈的自旋涨落，无序的低温相可以持续存在。这是当前凝聚态研究的一个非常热门的领域，因为人们普遍认为，如果传统的有序态在低温下能够被抑制，它就会打开一个窗口，让一种奇异的基态变得明显，可能具有质的新的和显著的物理性质。在磁性材料中，可以通过三种方式鼓励强烈的自旋涨落：磁矩可以装饰与简单有序态不相容的固体中的局部几何图形(通常基于三角形和四面体)-这被称为几何受挫；支持极端量子涨落的S=1/2和有效的S=1/2磁矩可以装饰晶格；最后，由于由磁性平面的堆叠或磁链的集束组成的三维晶体结构，磁性晶格可以有效地是二维或一维的。也就是说，我们可以利用几何受挫、量子涨落或降维来增强自旋涨落，从而破坏或削弱磁性材料在低温下表现出常规秩序的倾向。这项提议旨在探索能够做到这一点的新磁性材料。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几种这样的材料中随后出现的奇异基态的性质，以便能够充分理解它们所展示的新现象的基本原理。这项建议寻求对我们将以原始和单晶形式生长的几类奇异磁性材料进行中子散射测量计划。在过去六年中，可用于进行前沿非弹性中子散射的仪器和能力出现了戏剧性的复兴。这主要是通过散裂中子源和ISIS新的飞行时间中子技术的发展，也通过NIST、ILL和其他地方基于反应堆的新技术实现的。最终的结果是，与六年前相比，中子散射可以获得的与新材料的静态和动态行为相关的信息的性质在性质上是不同的，而且更全面。这项建议旨在将这一新的基础设施和这些新技术应用于与新材料中奇异的磁性基态有关的几个热点问题。这项建议主题的一个关键组成部分和起点是一个复杂的材料制备和晶体生长计划，该计划将提供新磁性材料的大而原始的单晶。由此产生的单晶样品将成为这些前沿中子和X射线散射研究的对象。我们处于非常有利的地位，无论是在我们生产感兴趣的单晶的能力方面，还是在获得与新的中子散射基础设施相关的专业知识方面，我们都处于非常有利的地位，可以在这一非常热门的凝聚态物理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 the appearance of 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 current 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, perhaps with qualitatively new and remarkable physical properties.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. That is, we can use geometrical frustration, quantum fluctuations or reduced dimensionality to enhance spin fluctuations and thereby destroy or weaken the tendency for a magnetic material to display conventional order at low temperatures. 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 several such materials, such that the basic 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 exoticmagnetic materials that we will grow in pristine and single crystal form. Over the last six years, there has been a very dramatic renaissance in the instrumentation and capabilities available to carry out forefront inelastic neutron scattering. This has occurred mostly through the development of new time-of-flight neutron techniques at the Spallation Neutron Source, and ISIS, but also through new reactor-based techniques at NIST, the ILL, and elsewhere. The net result is that the nature of the information which neutron scattering can access relevant to the static and dynamic behaviour of new materials is qualitatively different and much more comprehensive than what was possible six years ago. This proposal seeks to apply this new infrastructure and these new techniques 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 large and pristine single crystals of the new magnetic materials. The resulting single crystal samples will be the subject of these forefront neutron and x-ray scattering studies. We are exceptionally well positioned, both with respect to our capacity to produce single crystals 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.