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

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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.