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

非技术摘要：现代凝聚态物理学的核心问题之一是具有强电子相互作用的材料中相的演化。量子相变(QPT)通过掺杂电荷或加压在接近零的温度下实现，引起了人们的特别兴趣，因为它们涉及许多尚未深入探索的研究主题，如无序的作用，以及可被视为磁秩序的前驱现象的短时短程动态磁涨落。利用粒子加速器产生的一束放射性介子粒子并将介子注入固体中，介子自旋弛豫(MuSR)提供了一种测量固体磁性的新方法。在磁性相变的研究中，MuSR测量可以独立于有序区域内单个有序磁矩的大小来确定磁性有序区域的体积分数。由于这一特点，与测量体积积分响应的传统磁化或弹性中子散射研究相比，MuSR具有独特的优势。在这个项目中，Pi Uemura对呈现Mott金属-绝缘体转变的选定系统中的磁序进行了MuSR研究。莫特跃迁最初是由英国的尼维尔·莫特爵士提出的，他因其理论预测而获得诺贝尔奖。然而，金属-绝缘体转变、磁序和结构相变之间的相互作用还没有完全弄清楚。此外，本项目还研究了磁序接近消失的特定弱磁性金属的磁相变。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研究集中在(A)RENiO_3[RE=稀土]、V_2O_3和其他几个Mott转变系统，其中强关联驱动其他金属电荷的局域化和磁序；以及(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波动导致各种新的自旋现象。在Mott系统(A)中，PI旨在解开自旋、电荷和晶格的作用，并全面了解受带宽、电荷填充和/或电流控制的Mott相变的一般和系统特有的特征。离子交换膜研究(B)的目的是阐明磁体体积演化，阐明无序在典型的离子交换膜系统中的作用，并研究由于磁单极子和三维Skyrmion晶格引起的MnGe中新的拓扑相变。为了寻找即将发生的Mott相变的动态自旋电荷软模，我们还将对V_2O_3和(La，Sr)VO_3进行非弹性中子散射研究。由于缺乏一阶QPT/相分离的实验结果，目前对QPT的理论研究大多局限于二阶QPT。PI的最新结果表明，在MnSi、RENiO_3和V_2O_3中存在一阶QPT，而二阶QPT的恢复可能是由于(Mn，Fe)Si中的无序。目前的MSR研究结合其他方法将有助于对一阶QPT提出新的看法和猜想，这可能对其他相关系统的研究具有深远的意义，如高温超导和重费米子超导体。

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.