Quantum Phase Transitions in Mott Insulator Systems and Itinerant-Electron Magnets: MuSR Studies of Magnetic Order, Volume Evolution and Spin Fluctuations

莫特绝缘体系统和流动电子磁体中的量子相变：磁序、体积演化和自旋涨落的 MuSR 研究

• 批准号: 1610633
• 负责人: Yasutomo Uemura
• 金额: $ 35.6万
• 依托单位: Columbia University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-09-15 至 2020-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1610633&HistoricalAwards=false
• 关键词: Quantum; Phase; Transitions; Mott; Insulator

Non-technical Abstract:One of the central and unresolved issues in modern condensed matter physics is the evolution of phases in materials with strong interactions among constituent electrons. Quantum phase transitions (QPT), achieved at nearly zero temperature by doping charges or application of pressure, are of particular interest, since they involve many research themes yet to be thoroughly explored, such as the roles of disorder, and short-time and short-ranged dynamic magnetic fluctuations which may be viewed as a precursor phenomenon to the magnetic order. Using a beam of radioactive muon particles generated at particle accelerator facilities and implanting muons into solids, Muon Spin Relaxation (MuSR) provides a novel method to measure magnetic properties of solids. In studies of magnetic phase transitions, MuSR measurements can determine the volume fraction of the magnetically ordered regions independently from the size of the individual ordered magnetic moments within the ordered regions. Thanks to this feature, MuSR has a unique advantage over conventional magnetization or elastic neutron scattering studies which measure volume-integrated responses. In this project, PI Uemura performs MuSR studies on magnetic order in selected systems which exhibit Mott metal-insulator transitions. Mott transition was originally proposed by Sir Nevil Mott of the UK who was awarded a Nobel Prize for his theoretical prediction. Interplays among metal-insulator transition, magnetic order, and structural phase transitions, however, have not yet been fully clarified. In addition, the present project also examines magnetic phase transitions of particular weakly magnetic metals near disappearance of magnetic order. These studies may lead to better understandings of subtle phase transition phenomena caused by strong interactions among constituent electrons. Technical Abstract:The planned MuSR studies focus on (A) RENiO3 [RE=Rare Earth], V2O3 and several other Mott transition systems where strong correlations drive localization and magnetic order of otherwise metallic charges; and (B) (Mn,Fe)Si and a few other itinerant-electron magnets (IEM) where competing interactions and spin fluctuations lead to variety of novel spin phenomena. In Mott systems (A), the PI aims to disentangle the roles of spin, charge and lattice and to develop a comprehensive understanding of generic and system-specific features of Mott transitions controlled by band width, charge filling and/or electric current. The goals of IEM studies (B) are to elucidate magnetic volume evolution, to clarify the roles of disorder in prototypical IEM systems and to study novel topological phase transitions in MnGe due to magnetic monopoles and the 3-dimensional Skyrmion lattice. Inelastic neutron scattering studies of V2O3 and (La,Sr)VO3 will also be performed to search for dynamic spin-charge soft mode of the imminent Mott transition. To date, most theoretical studies on QPT were limited to second order QPT, due to lack of experimental results demonstrating first-order QPT / phase separation. The PI's recent results showed clear evidence for first-order QPT in MnSi, RENiO3 and V2O3, and a recovery of second order QPT due presumably to disorder in (Mn,Fe)Si. Present MuSR studies combined with other methods will contribute to new views and conjectures for first-order QPT, which may have far-reaching implications for studies of other relevant systems such as high-Tc and heavy fermion superconductors.

非技术摘要：现代冷凝物质物理学中的中心和未解决的问题之一是构成电子之间具有较强相互作用的材料的演变。量子相变（QPT）在几乎零温度下通过掺杂电荷或施加压力而达到的量子相变（QPT）特别感兴趣，因为它们涉及许多尚未详尽探索的研究主题，例如无序的作用，短时和短时的动态磁波动的作用，可以将其视为前体现象到磁性效果。 使用在颗粒加速器设施中产生的放射性μ子颗粒并将兆植入固体中的光束，muon自旋松弛（MOSR）提供了一种测量固体磁性特性的新方法。 在对磁相跃迁的研究中，MUSR测量值可以独立于有序区域内的单个有序磁矩的大小来确定磁有序区域的体积分数。借助此功能，MUSR比常规磁化或弹性中子散射研究具有独特的优势，该研究测量了体积积分响应。在这个项目中，Pi Uemura在表现出莫特金属绝缘体过渡的选定系统中对磁性进行了MUSR研究。 莫特过渡最初是由英国尼维尔·莫特爵士提出的，他因其理论预测而获得诺贝尔奖。 但是，金属 - 绝缘体过渡，磁性和结构相变的相互作用尚未完全阐明。 此外，本项目还检查了在磁性消失附近的特定弱磁金属的磁相变。这些研究可能导致对由组成电子之间的强烈相互作用引起的细微相变现象的更好理解。 技术摘要：计划的MUSR研究重点关注（a）Renio3 [Re =稀土]，V2O3和其他几个Mott Transition Systems，其中强相关性促进了定位和原本金属电荷的磁顺序； （b）（Mn，Fe）SI和其他一些巡回电子磁铁（IEM），其中竞争相互作用和自旋波动导致各种新型自旋现象。 在Mott Systems（A）中，PI旨在解散旋转，电荷和晶格的角色，并对由带宽，充电填充和/或电流控制的Mott Transitions的通用和系统特异性特征进行全面了解。 IEM研究的目标（B）是为了阐明磁体积的演变，以阐明疾病在典型IEM系统中的作用，并研究由于磁单波测和3维天际晶状体而导致的MNGE的新型拓扑相变。还将执行V2O3和（LA，SR）VO3的非弹性中子散射研究，以搜索即将到来的Mott Mott Transition的动态自旋荷利软模式。 迄今为止，由于缺乏实验结果证明了一阶QPT /相分离，因此大多数关于QPT的理论研究仅限于二阶QPT。 PI的最新结果显示了MNSI，Renio3和V2O3中一阶QPT的明确证据，以及恢复二阶QPT大概是由于（MN，FE）SI的疾病。目前的MUSR研究与其他方法相结合将有助于一阶QPT的新观点和猜想，这可能对其他相关系统（例如高-TC和重型费米昂超导体）的研究具有深远的影响。