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：现代凝聚态物理学的核心和未解决的问题之一是组成电子之间具有强相互作用的材料中的相演化。通过掺杂电荷或施加压力在接近零的温度下实现的量子相变（QPT）特别令人感兴趣，因为它们涉及许多尚未彻底探索的研究主题，例如无序的作用以及短时和短时间的作用。 -范围动态磁涨落，可以被视为磁序的前兆现象。 使用粒子加速器设施产生的放射性 μ 子粒子束并将 μ 子注入固体中，μ 子自旋弛豫 (MuSR) 提供了一种测量固体磁性的新方法。 在磁相变研究中，MuSR 测量可以独立于有序区域内各个有序磁矩的大小来确定磁有序区域的体积分数。由于这一特性，MuSR 比测量体积积分响应的传统磁化或弹性中子散射研究具有独特的优势。在这个项目中，PI Uemura 对表现出莫特金属-绝缘体转变的选定系统中的磁序进行 MuSR 研究。 莫特跃迁最初是由英国内维尔·莫特爵士提出的，他因其理论预测而获得了诺贝尔奖。 然而，金属-绝缘体转变、磁序和结构相变之间的相互作用尚未完全阐明。 此外，本项目还研究了特定弱磁性金属在磁序消失附近的磁相变。这些研究可能有助于更好地理解由组成电子之间的强相互作用引起的微妙相变现象。 技术摘要：计划中的 MuSR 研究重点是 (A) RENiO3 [RE=稀土]、V2O3 和其他几种莫特跃迁系统，其中强相关性驱动其他金属电荷的局域化和磁序； (B) (Mn,Fe)Si 和其他一些巡回电子磁体 (IEM)，其中竞争相互作用和自旋涨落导致各种新颖的自旋现象。 在莫特系统 (A) 中，PI 旨在理清自旋、电荷和晶格的作用，并全面了解由带宽、电荷填充和/或电流控制的莫特跃迁的一般特征和系统特定特征。 IEM 研究 (B) 的目标是阐明磁体积演化，阐明原型 IEM 系统中无序的作用，并研究由于磁单极子和 3 维斯格明子晶格而导致的 MnGe 中新颖的拓扑相变。还将进行 V2O3 和 (La,Sr)VO3 的非弹性中子散射研究，以寻找即将发生的莫特跃迁的动态自旋电荷软模式。 迄今为止，由于缺乏证明一阶QPT/相分离的实验结果，大多数关于QPT的理论研究仅限于二阶QPT。 PI 最近的结果显示了 MnSi、RENiO3 和 V2O3 中一级 QPT 的明确证据，以及可能由于 (Mn,Fe)Si 中的无序而导致二级 QPT 的恢复。目前的MuSR研究与其他方法相结合将有助于对一阶QPT提出新的观点和猜想，这可能对高温和重费米子超导体等其他相关系统的研究产生深远的影响。