Exotic Ground States in New Materials

大多数磁性材料的基态，以及所有被很好地理解的磁性材料，其特征是出现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，如果传统的有序态可以在低温下被抑制，它就为一种奇异的基态的出现打开了一扇窗，这种基态可能具有新的、显著的物理性质。**在磁性材料中，可以通过三种方式鼓励强自旋涨落：磁矩可以装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。也就是说，我们可以使用几何挫折、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显示常规秩序的趋势。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，主要是用新的中子和x射线散射技术，我们的目标是阐明在一些这样的材料中随之而来的奇异基态的性质，这样，它们所显示的新现象的基本原理就可以完全理解。**本提案旨在对几种我们将以原始和单晶形式生长的奇异磁性材料家族进行中子散射测量。在过去的六年中，用于前沿非弹性中子散射的仪器和能力有了非常戏剧性的复兴。这主要是通过在散裂中子源和ISIS开发新的飞行时间中子技术，以及在NIST、ILL和其他地方开发的基于反应堆的新技术实现的。最终的结果是，中子散射能够获得的与新材料的静态和动态行为有关的信息的性质与六年前相比有了质的不同，而且更加全面。本提案旨在将这种新的基础设施和这些新技术应用于与新材料中的奇异磁基态相关的几个主题问题。**本提案主题的关键组成部分和出发点是复杂材料制备和晶体生长计划，该计划将提供新的磁性材料的大而原始的单晶。所得的单晶样品将是这些前沿中子和x射线散射研究的主题。我们处于非常有利的位置，无论是在我们生产感兴趣的单晶的能力方面，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 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 exotic*magnetic 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.