NMR Studies of Organic and Inorganic Frustrated Quantum Magnets

非技术摘要本项目由材料研究部凝聚态物理项目资助，属于磁性材料领域。无序基态假设出现在磁性系统中，其中自旋排列在特定的受挫几何形状中。长期以来，这些所谓的自旋液体相具有在量子计算技术中使用的潜力，并被提议作为高温超导的一种途径。然而，这些候选材料直到最近才在实验中实现。由几何学决定的相互竞争作用被理解为其稳定性的重要组成部分，但真正的系统如何依赖于细节仍不清楚。这项研究项目旨在通过利用与几何有关的相互作用的微妙平衡来探索受挫量子磁体无序和有序基态的稳定性条件。调谐是通过改变应变、压力和磁场强度来完成的。被访问的相的性质和相关的激发由测量的组合来确定，并且相当依赖于固态磁共振技术。该项目的目标是通过对研究生和本科生进行国家重要技术领域的科学教育和培训来实现的，这些领域包括高频和磁共振技术，以及高压和低温仪器。技术摘要该项目需要使用固态磁共振技术来探测受挫量子磁体和场致非均匀超导的性质。对于量子磁铁，长程有序系统和量子无序系统之间的对比尤其令人感兴趣，基态的性质、激发光谱以及磁场或应变调谐的影响也是特别感兴趣的。所选择的研究材料可以分为两类。第一类包括两种结构相似的无机化合物，它们表现出反铁磁性(AF)和无序基态。第二类包括两个有机的各向异性三角体系。通过施加单轴应变或压力，两者的基态都是连续可调的。重点放在与磁有序或自旋间隙相的抑制相关的物理性质的演变上。相变的信号，以及激发的性质，将从核磁共振光谱和弛豫测量中推断出来。清洁和层状超导体是场致超导相的理想候选者。选择用于研究的系统对于弱耦合层是已知的，并且很容易获得适当的场范围。磁共振通常是解决这些问题的理想探测器，因为它与所需的极端条件兼容，而且因为它是通过超精细相互作用对电子环境敏感的局部探测器。这项研究将在加州大学洛杉矶分校的P.I.实验室和国家高磁场实验室(NHMFL)进行，研究的磁场比当地可用的磁场更大。

Non-technical abstractThis project, supported by the Condensed Matter Physics Program in the Division of Materials Research, is in the area of magnetic materials. Disordered ground states are postulated to arise in magnetic systems where the spins are arranged in specific frustrated geometries. Long sought-after, these so-called spin liquid phases have the potential for use in quantum computation technology and are proposed as a route to high temperature superconductivity. However, candidate materials for these were experimentally realized only recently. Competing interactions, determined by geometry, are understood as an important component for their stability, but how the real systems are dependent on the details remains unclear. This research project aims to explore the circumstances for stability of disordered and ordered ground states of frustrated quantum magnets by exploiting the delicate balance of interactions linked to the geometry. Tuning is accomplished by means of varying strain, pressure, and magnetic field strength. The nature of the accessed phases and associated excitations are determined by a combination of measurements, with considerable reliance on solid state magnetic resonance techniques. The project goals are achieved in tandem with the science education and training of graduate students and undergraduate students in nationally important technical fields, including high frequency and magnetic resonance techniques, as well as high pressure and cryogenic instrumentation.Technical abstractThis project entails the use of solid state magnetic resonance techniques to probe the properties of frustrated quantum magnets and field-induced inhomogeneous superconductivity. With respect to the quantum magnets, the contrast between long-range ordered and quantum disordered systems is of particular interest, as are the nature of the ground states, the spectrum of excitations, and the effects of tuning by magnetic field or strain. The materials selected for study can be classified into two groups. The first includes two structurally similar inorganic compounds, which exhibit antiferromagnetic (AF) and disordered ground states. The second group includes two organic anisotropic triangular systems. The ground states of both are continuously tunable by the application of uniaxial strain or pressure. The focus is on the evolution of physical properties associated with the suppression of magnetically ordered or spin-gapped phases. Signatures for phase transitions, and the nature of excitations, will be inferred from NMR spectroscopy and relaxation measurements. Clean and layered superconductors are ideal candidates for field-induced superconducting phases. The system chosen for study is known for weakly-coupled layers, and the appropriate field range is easily accessed. Magnetic resonance is often an ideal probe for these problems, because of its compatibility to the required extreme conditions, and because it is a local probe sensitive to the electronic environment through the hyperfine interaction. The research will be conducted in the laboratory of the P.I. at UCLA, and at the National High Magnetic Field Laboratory (NHMFL) for fields larger than what is available locally.