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：现代凝聚态物理学中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的演化。量子相变（QPT），通过掺杂电荷或施加压力在接近零的温度下实现，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，以及可以被视为磁序的前兆现象的短时间和短范围动态磁波动。 利用粒子加速器产生的放射性μ子粒子束，将μ子注入固体，μ子自旋弛豫（Muon Spin Relaxation，MuSR）提供了一种测量固体磁性的新方法。 在磁相变的研究中，MuSR测量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。由于这一特点，MuSR具有独特的优势，超过传统的磁化或弹性中子散射研究，测量体积积分响应。在这个项目中，PI Uemura在选定的系统中对磁序进行了MuSR研究，这些系统表现出Mott金属-绝缘体转变。 莫特转变最初是由英国的内维尔·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、磁有序和结构相变之间的相互作用尚未完全阐明。 此外，本计画也探讨特定弱磁性金属在磁序消失时的磁性相变。这些研究可能会导致更好地理解微妙的相变现象所造成的组成电子之间的强相互作用。 技术摘要：计划中的MuSR研究重点是（A）RENiO 3 [RE=稀土]、V2 O3和其他几种Mott转变系统，其中强相关性驱动其他金属电荷的局部化和磁序;以及（B）（Mn，Fe）Si和其他一些流动电子磁体（IEM），其中竞争相互作用和自旋波动导致各种新颖的自旋现象。 在Mott系统（A）中，PI的目标是解开自旋、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跃迁的通用和系统特定特征。IEM研究（B）的目标是阐明磁体积的演变，澄清在原型IEM系统中的无序的作用，并研究由于磁单极子和三维Skyrmion晶格在MnGe中的新的拓扑相变。V_2O_3和（La，Sr）VO_3的非弹性中子散射研究也将被用来寻找即将发生的Mott跃迁的动态自旋-电荷软模。 迄今为止，由于缺乏一级QPT /相分离的实验结果，大多数QPT的理论研究仅限于二级QPT。PI的最新结果显示了MnSi、RENiO 3和V2 O3中的一阶QPT的明确证据，以及可能由于（Mn，Fe）Si中的无序而导致的二阶QPT的恢复。目前的MuSR研究与其他方法相结合，将有助于一阶QPT的新观点和新方法，这可能对其他相关系统，如高Tc和重费米子超导体的研究产生深远的影响。

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.