Yasutomo Uemura - US grants

This grant will support research in experimental condensed matter physics to measure some of the properties of high temperature superconductors. This grant was selected from more than 260 proposals submitted in response to a solicitation for proposals in High Temperature Superconductivity in April, 1989. This research may provide valuable insight into the physical processes responsible for the unusual superconducting behavior. This research is supported through the Condensed Matter Physics programs with a significant contribution from the university.

9510454 Uemura Muon spin relaxation (muSR) measurements are proposed to study ground state properties and spin fluctuations in geometrically frustrated spin systems, such as the Kagome lattice and triangular lattice antiferromagnets strontium chromium (x) gallium (12-x) oxygen (19) and lithium nickel oxygen (2), as well as in low-dimensional spin systems, including the doped Haldane spin chain system (yttrium, calcium) (2) barium nickel oxygen (5), the spin-Peierls system (copper, zinc) germanium oxygen (3), the one-dimensional organic conductor aluminum carbon (60), and the spin-ladder cuprates, strontium (n-1) copper (n+1) oxygen (2n). This proposal aims to reveal details of suggested spin-liquid ground states as well as to elucidate the process of destruction of the many-body singlet ground state due to the introduction of imperfections or doped charge carriers. The muSR experiments will be performed at the TRIUMF facility in Vancouver, Canada. %%% Certain configurations of atomic arrangements in solids, such as the triangular or Kagome lattice, tend to prevent the alignment of the magnetic moments of the atoms placed on it, promoting dynamic fluctuations of the moments (spins). Similarly, magnetic ordering is suppressed in low-dimensional arrangements of spins, such as a chain of magnetic moments. In these systems, novel magnetic behavior is expected at low temperatures, including the possibility for the occurrence of a "quantum spin liquid", which is a disordered liquid- like dynamic state of the moments due to quantum-mechanical effects. This proposal aims to reveal the nature of such exotic magnetic behavior by utilizing a novel technique called muon spin relaxation (muSR), which probes the magnetism within solids with radioactive elementary particles, muons, produced at a high-intensity proton accelerator (the TRIUMF facility in Vancouver , Canada).

9510453 Luke Muon spin relaxation will be used to study the interplay between various interactions and phenomena in rare earth and actinide compounds. Many of these systems exhibit interesting magnetic behavior due to the combination of the Kondo effect, Rudman-Kittel-Kasuya Yosida interaction and the effect of crystalline fields. In materials where these interactions are present, such phenomena as heavy fermion superconductivity and quadrupole Kondo effect are observed. Superconductivity and antiferromagnatic ordering coexist in uraniumplatinum-three compound. This antiferromagnetism contributes to multiple superconducting phases. This research will help elucidate the microscopic nature of the different superconducting states. In another compound, cerium-copper2-silicon2, superconductivity and magnetism do not coexist but compete for the ground state. This work will investigate the competition between the two states. The competition between Kondo screening and exchange interactions will be studied in cerium-copper-two- tin-two. The relative importance of the crystalline electric fields, Kondo screening and exchange interactions will be determined in uranium doped yttrium-palladium- three. Theoretical models based on the anomalous low temperature linear specific heat in neodymium-cerium-copper- oxygen compound will be tested by way of the measurement of spin fluctuations in this compound. %%% Muon Spin Relaxation experiments will be conducted on a number of rare earth and actinide compounds which exhibit a rich interplay between various interactions and phenomena such as superconductivity and magnetic ordering. Five different studies will be conducted: (1) the coexistence between antiferromagnetic ordering and the microscopic nature of the resulting multiple superconducting phases, (2) the competition between magnetic ordering and superconductivity in a compound where they do not coexist, (3) the competition between Kondo screening and exchange interaction, (4) the relative importance between crystalline electric fields, Kondo screening and exchange interactions, and (5) fluctuation effects in materials which exhibit anomalous thermodynamic behavior. Each of the above study will be conducted on the appropriate compound. ***

w:\awards\awards96\*.doc 9802000 Uemura This experimental research project is devoted to unusual magnetic behavior in pure and doped spin-gap systems, and in spin-liquid systems, as studied by muon spin relaxation. Of principal interest are low-dimensional and/or geometrically frustrated spin systems. Of interest are conditions for the formation/absence of spin freezing, the role of frustration, and effects of charge doping and magnetic dilution. In more detail, four topics and systems of interest are: 1) questions of ordering temperature and/or ordered moment size in quasi 1-d zig-zag spin chain SrCuO2 and LixV2O5; 2) charge/vacancy doping effects in 2-leg ladders Sr2(Cu,Zn)4O6, LaCuO2.5, and (Ca,Mg)(V,Ti)2O5; 3) ground state of pure and perturbed geometrically frustrated systems LiV2O4, (Y,Sc)Mn2, CePt2Sn2; and 4) magnetic properties of linear chains and 2-leg ladders in the hybrid (La,Sr,Ca)14Cu24O41. The muon spin relaxation experiments are performed at TRIUMF, magnetic susceptibility measurements at Columbia and neutron scattering studies at JAERI. This research program is interdisciplinary in nature and involves one or more graduate or postgraduate students, who receive excellent training in preparation for careers in industry, government laboratories or academia. %%% This experimental basic research project is devoted to unusual magnetic behavior in crystalline specimens of a variety of inorganic chemical compounds. The questions of interest are the local behavior of the magnetic species in the samples, and an advanced experimental method called "muon spin relaxation" is used in this work. The measurement has to be made at a special accelerator facility where the muons (mu mesons), are produced and injected into the sample of interest. The measurement reveals the magnetic field strength inside the sample at those loca tions, often near internal magnetic moments, where the muons become trapped before they undergo decay. Of interest are conditions for the formation/absence of spin freezing, the role of frustration, and effects of charge doping and magnetic dilution. In more detail, four topics and systems of interest are: 1) questions of ordering temperature and/or ordered moment size in quasi 1-d zig- zag spin chain SrCuO2 and LixV2O5; 2) charge/vacancy doping effects in 2-leg ladders Sr2(Cu,Zn)4O6, LaCuO2.5, and (Ca,Mg)(V,Ti)2O5; 3) ground state of pure and perturbed geometrically frustrated systems LiV2O4, (Y,Sc)Mn2, CePt2Sn2; and 4) magnetic properties of linear chains and 2-leg ladders in the hybrid (La,Sr,Ca)14Cu24O41.The muon spin relaxation experiments are performed at TRIUMF, magnetic susceptibility measurements at Columbia and neutron scattering studies at JAERI. The results of this research will be of importance in magnetism, and in physical theory. They may lead to new methods that can be applied in chemistry and in nanoscale electronic devices. This research program is interdisciplinary in nature and involves one or more graduate or postgraduate students, who receive excellent training in preparation for careers in industry, government laboratories or academia. ***

This individual investigator award funds research that aims to apply the muon spin relaxation (MuSR) technique to the study of superconductivity and magnetism in a variety of systems including; high-Tc cuprate, intercalated HfNCl, other superconductors, and low-dimensional and/or geometrically frustrated spin systems. The MuSR method will allow accurate determination of the magnetic field penetration depth in type-II superconductors, and of spontaneous magnetic fields and spin fluctuations in magnetic materials. This project will also include attempts of charge doping via the formation of field effect transistors (FET). The MuSR and FET results will elucidate the evolution from insulators to superconductors to simple metals in superconductors, and from spin singlet states to frozen spin states in magnetic systems. Emerging pictures will be considered from a unified point of view regarding crossover from gapped to ungapped states near quantum phase transitions. The project will also support highly motivated graduate students at Columbia, give them the unique experience of international collaboration, and promote a greater role of female PhD students / researchers in experimental physics.
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During the recent 15 years, several novel superconductors with very high transition temperatures have been discovered. They include high-Tc copper oxides and intercalated HfNCl. Muon spin relaxation (MuSR) is a new and powerful experimental method for studying superconducting and magnetic properties of solids by using a beam of sub-atomic particles (Mu-meson) produced by high-intensity proton accelerators. This individual investigator award will support research in which the Columbia University group will apply the MuSR technique to studies of several high-Tc superconductors and relevant magnetic materials to elucidate how charges and spins are correlated to exhibit a variety of novel phenomena in solids. This project will provide information crucial for increasing transition temperatures, which would help in the application of high-Tc superconductors to commercial electronic devices and power cables. The project will also support highly motivated graduate students at Columbia, give them the unique experience of international collaboration, and promote a greater role of female PhD students / researchers in experimental physics.
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0314058
Uemura

This award supports a three-month collaborative research project between Professor Yasutomo Uemura at Columbia University in New York and Professor Shin-ichi Uchida at the University of Tokyo in Japan. They will be undertaking research on High-Tc Superconductors and other Spin-gap Magnetic Systems. They will be collaborating on the muon spin relaxation, transport and optical measurements of high-Tc cuprate superconductors (HTSC). Muen spin relaxation is a powerful probe of magnetism and superconductivity. It is sensitive to spin fluctuations in a wide range of frequencies, and spin freezing of periodic/random/dilute spin configurations with an average ordered moment. It also provides volume-differential information for systems with coexisting regions with/without spin freezing. Magnetic field penetration depth of type-11 Superconductors can be determined accurately by muon spin relaxation. The collaborators will work on analyses of existing data. The experimental results will be compared to detailed computer simulation to estimate the volume fraction of the specimen subject to static spin freezing in high magnetic field. In addition to the effect of applied magnetic field as a single perturbation, they will also study what would happen when a high magnetic field is applied to systems which are already subject to other perturbations, such as Zn-doping or magnetic nano-island formation.

The project brings together the efforts of two laboratories that have complementary expertise and research capabilities. The U.S. researcher provides expertise in the data analyses and interpretations and the Japanese collaborator is a world leader in optical and transport physics. Superconducting materials are fertile areas for research crucial to a number of applications areas such as the transmission of electricity, advanced electric motors, the storage of energy via flywheels, and advanced opto-electronic devices and sensors. Through the exchange of ideas and technology, this project will broaden our base of basic knowledge and promote international understanding and cooperation. Results of the research will be disseminated at scientific meetings and in scientific journals.

This Inter-American Materials Collaboration (CIAM) project aims to study quantum phase transitions and quantum critical behaviors of correlated electron systems by developing a new spectrometer for high-pressure measurements and performing collaborative muon spin relaxation (mSR) experiments at TRIUMF (Vancouver), together with complementary studies of transport (Toronto), magnetization (Kentucky), and Moessbauer and Perturbed Angular Correlation (PAC) (CBPF-Brazil). mSR has advantages over other microscopic methods (neutron scattering and NMR), in measurements of (1) volume fraction of magnetically ordered regions; (2) magnetic orderof very small / random / dilute moments; and (3) the absolute values of the magnetic field penetration depth in superconducting systems. With these unique capabilities, this project would elucidate: (a) the crossover from itinerant electron ferromagnetism to correlated paramagnetism in MnSi and ZrZn2; (b) interplay of magnetism and superconductivity in CeCu2Si2; (c) competition between the "hidden order" and antiferromagnetic states in URu2Si2; (d) magnetic-ordering systematics of intermetallic Ce compounds; and (e) pressure-induced Mott transition in Ca2RuO4. This project would also promote the participation of experienced as well as young scientists from the US, Canada, Brazil, and Argentina, including several scientists from under-represented groups, in extensive international collaboration. The proposed work will be performed at unique facilities in Vancouver, Kentucky, and Rio de Janeiro, while enriching graduate and undergraduate teaching resources of Columbia Univesity, University of Kentucky, and other participating institutions in Canada, Brazil, and Argentina.

Muon Spin Relaxation (mSR), Moessbauer effect, and Perturbed Angular Correlations (PAC) are powerful methods based on technologies and principles from particle/nuclear physics applied to the study of magnetism and superconductivity in solid state physics. The present project is an Inter-American Materials Collaboration (CIAM) in which scientists and students from the US, Canada, Brazil, and Argentina work together. A team of researchers from Columbia University and University of Kentucky will develop capabilities for measuring mSR signals at high applied pressure and perform mSR experiments using TRIUMF, a Canadian accelerator facility in Vancouver. The project also involves Moessbauer and PAC measurements at CBPF-Rio in Brazil, and other complementary studies in Argentina, Toronto, New York, and Kentucky. These techniques will be applied to topical subjects of modern solid-state physics. The goal is to understand novel phenomena exhibited by materials systems in which strong electron correlations play a major role in determining the superconducting and magnetic properties. This project will support senior, junior scientists and students from the US, Canada, Brazil, and Argentina, including several scientists from under-represented groups, while enriching graduate and undergraduate teaching resources of the involved institutions.

This Materials-World-Network project aims to support studies of quantum phase transitions (QPTs) by the muon spin relaxation (MuSR) measurements to be performed at high-intensity accelerator facilities, such as TRIUMF in Vancouver, Canada and Paul Scherrer Institut (PSI) in Villigen, Switzerland, by an international team of researchers. The team includes senior and junior scientists from Columbia University (USA), McMaster University and TRIUMF (Canada), PSI (Switzerland), Kyoto and Tohoku Universities (Japan), Brazilian Center for Research in Physics (CBPF, Brazil) and Centro Atomico Bariloche (CAB, Argentina). For many decades, studies of phase transitions have been performed exclusively by varying temperatures (thermal phase transitions). Recent developments of materials and technologies have opened a new window to investigate quantum evolutions of phases at low temperatures by varying pressure or composition as a tuning parameter. Many novel concepts and unexplored features of QPTs are being revealed in studies of heavy-fermion systems, frustrated / low-dimensional magnets and exotic superconductors. Taking advantage of unique capability of MuSR to detect magnetic order of even very small / random magnetic moments, to estimate volume fraction of magnetically ordered regions in a situation involving phase separation, and to characterize dynamic spin fluctuations in a very wide time window complementary to other techniques, the present project will elucidate magnetic QPT's in frustrated square-lattice J1-J2 spin systems, doped magnetic semiconductors (Ga,Mn)As and (Fe,Co)Si, novel ruthenium oxide compounds, quasi one-dimensional spin system which exhibits Bose Condensation of magnons, as well as UGe2 and other heavy-fermion systems. Extensive collaboration of leading researchers in materials development (Kyoto, Tohoku), MuSR measurements (Columbia, McMaster), MuSR instrumentation (TRIUMF, PSI), Moessbauer effect (CBPF), and heavy-fermion physics (CAB), at the world's strongest meson facilities (TRIUMF and PSI) with close involvement of leading theorist team members (Affleck, Maekawa, Millis), will advance basic understandings of the role of competing states and soft dynamic modes near quantum phase boundaries. Such progress could contribute to long-awaited determination of mechanisms for novel superconductors as well as to development of ideal magnetic semiconductors suitable for application to spin-sensitive transistors. Involvement of students and postdoctoral researchers from all participating institutions provides valuable training and international research experience to young scientists in a challenging area of cutting-edge research in condensed matter physics.

This project uses large accelerator facilities and principles from particle/nuclear physics to study a variety of materials including novel superconductors to understand properties related to magnetism and quantum physics not accessible to other techniques in this research area. An international team of researchers will conduct the main part of their measurements at TRIUMF, a Canadian accelerator facility in Vancouver, British Columbia. The research is focused on understanding novel condensed matter phenomena in a variety of materials systems including semiconductors and oxide compounds that exhibit interesting magnetic behavior. Students and junior scientists from the US, Canada, Brazil, Argentina, Switzerland and Japan work together in this project and gain valuable international research experience. In addition, web-based courses and seminars for graduate, undergraduate students are planned to be developed and shared among all participating institutions using the technology already available at Columbia University.

This award is jointly funded by the Division of Materials Research and the Office of Multidisciplinary Activities in the Mathematical and Physical Sciences Directorate and by the Office of International Science and Engineering ?East Asia and pacific Program.

This PIRE project forms an international consortium of leading superconductivity researchers from the U.S., Japan, Canada, UK, and China to investigate novel superconductors to clarify superconducting mechanisms and properties and develop novel superconducting materials. In conventional electrical systems heat is generated by friction as electrons collide with atoms and impurities in the wire, a property that is ideal for appliances such as toasters or irons but not for most other electrical applications. Superconductivity can be thought of as "frictionless" electricity whereby electrons glide unimpeded between atoms, thus vastly improving the conductor's energy efficiency. To date this has only been achieved at extremely low temperatures; the challenge is to harness this phenomenon at or near room temperature and at high electrical currents. This project will fill gaps in our current understanding of superconductivity, reconcile current theories, and advance the development of better materials for fast-performing devices and cost-saving electric motors, generators, and power transmission lines.

The project links leading materials experimentalists and eminent theorists in a study of FeAs, CuO, CeCoIn5, and URu2Si2 superconductors using powerful experimental probing techniques including neutron scattering, muon spin relaxation, X-ray scattering, Raman spectroscopy, and scanning tunneling microscopy. These advanced methods allow elucidation of the phase diagrams of these important new materials of which some significant aspects are currently unknown. The PIRE team will explore the parameters affecting the highest temperature at which a certain material is superconducting and ways of increasing that temperature so that superconductivity will not require such expensive refrigeration. Some anomalies in the superfluid density and specific heat discontinuities, inconsistent with the standard theory of superconductivity, will also be investigated both experimentally and theoretically.

International collaboration is essential for this work because it will provide U.S. scientists and students with access to critical world-class accelerator-based facilities available in the UK and Canada but not in the U.S., to high quality specimens fabricated in China and Japan, and to first-rate scientific expertise from all countries. Combining and comparing the results of multiple probes on the same high-quality specimens will significantly improve the accuracy of data. Face to face collaboration of theorists and experimentalists focused on key concepts will facilitate the translation of mathematical theory into realistic and effective models and materials. The project places great emphasis on training students and early career scientists. Students and postdoctoral researchers will undertake 3-6 month research visits to work on superconducting mechanisms at foreign sites, where they will also receive language and cultural training. The project will actively recruit minority students into the sciences via workshops for high-school students and teachers from disadvantaged schools in New York and via an outreach program on superconductivity and scanning tunneling microscopy. High school and undergraduate students will gain valuable beam-time experience through the project, and female students, who are as a group underrepresented in the physical sciences, will be provided valuable mentoring from four female leading scientists on the team. The PIRE team will also develop a contemporary, internet-based set of solid state physics lectures and a text book on introductory solid state physics that reflect current knowledge in condensed matter physics and related experimental techniques.

The project will strengthen and internationalize materials research programs at the U.S. institutions and engage more U.S. students in international research collaborations. It will place Columbia University and its students and faculty at the core of a research and education partnership with extensive research collaborations, teaching cooperation, and frequent reciprocal research visits for participating faculty and students. Impacts extend beyond the PI and his institution, including providing U.S. students with research opportunities at two Department of Energy U.S. National Laboratories (Oak Ridge and Los Alamos) and training of early career scientists at the UK's ISIS and Canada's TRIUMF facilities, both of which will build the core workforce for new probing facilities currently under construction in the U.S. and Japan. This PIRE project will build upon an existing Inter American materials science network (CIAM) and forge a foundation for long-term research and educational collaborations among scientists and institutions in the five participating nations, all advancing the state-of-the-art in superconductivity and its applications.

Participating U.S. institutions include Columbia University (NY), University of Tennessee at Knoxville, and the Department of Energy's Oak Ridge (TN) and Los Alamos (NM) National Laboratories. Foreign institutions include Institute of Physics - Chinese Academy of Sciences, University of Bristol (UK), the UK Science and Technology Facilities Council's ISIS facility, McMaster University (Canada), TRIUMF Canada's National Laboratory for Particle and Nuclear Physics, Tokyo University (Japan), Osaka University (Japan), Tohoku University (Japan), and the National Institute of Advanced Industrial Science and Technology (AIST) (Japan).

This award is co-funded by the Office of International Science and Engineering and the Division of Materials Research.

****Technical Abstract****
Muon spin relaxation (MuSR) measurements provide unique information on the ordered volume fraction, moment size, slow spin fluctuations, and critical behavior for studies of magnetic ordering, and allow reliable determination of the superfluid density in type-II superconductors. The present project aims to study exotic spin and charge correlations in novel superconducting and magnetic systems by extensive MuSR measurements at TRIUMF (Canada) and PSI (Switzerland). The present research covers: (1) quantum critical behaviors and Skyrmion spin correlations in weak itinerant ferro/helical magnets MnSi and FeGe, (2) novel ferromagnetic system Li(Zn,Mn)As generated by doping charge (with excess Li) and spin (with Mn) into the I-II-V semiconductor LiZnAs, and (3) various FeAs, CuO and heavy fermion superconductors, with a special focus on strange/unexplained behaviors in the overdoped regions. Success of the present project will advance understanding of very strong and unconventional spin-charge couplings found in many of these systems, and possibly contribute to development of new superconducting and spin-tronics systems and devices. The MuSR measurements will be performed in extensive international collaborations, providing opportunities for development of graduate, undergraduate and post-doctoral students and researchers in a unique multi-national environment and facilities. Outreach plans include development of on-line lecture courses, and involvement of high school students.

****Non Technical Abstract****
Over the last 30 years, solid state physics has developed with discoveries of many novel and unconventional materials and phenomena. They include high-Tc CuO superconductors, doped ferromagnetic semiconductors, and systems existing in the boundary of magnetically ordered and disordered (or suprconducting) states. The key element of these systems is strong interaction between spins (origin of magnetism) and charges (required for metallic, semiconducting, or superconducting behaviors). Another notable development of modern solid state physics is the use of large accelerator facilities which produce high-intensity beams of X-rays, neutrons and muons. The present project will use positive muons obtained at dedicated high-intensity accelerator facilities in Canada (TRIUMF) and Switzerland (PSI) to elucidate detailed relationship between magnetic (spin) and conductive (charge) phenomena. Rsearch advances may contribute to development of novel materials for superconducting and / or spin-sensitive electronics (spintronics) devices. The muon measurements will be performed in international collaboration including gropus from the US, Canada, Japan, China, Brazil, UK and Switzerland, which will help development of students and researchers in a unique multi-national environment. Educational/outreach activities include the development of on-line lecture courses for solid state physics, associated with text books and lab courses. In cooperation with TRIUMF's outreach activities, the project supports involvements of high school students and physics teachers in forefront experimental research using very large accelerator facilities.

NON-TECHNICAL SUMMARY
This DMREF project aims to make breakthroughs in understanding and designing novel superconductors, magnetic semiconductors, and other magnetic materials. The research can lead to development of materials with higher transition temperatures suitable for applications to electronic devices with novel functionalities. To achieve this goal, collaborative research will be performed by five Principal Investigators (PIs) specializing in a variety of techniques and methods, including neutron scattering (Dai) and muon spin relaxation (Uemura) as advanced magnetic probes, synthesis and charge transport of nano-scale systems (Ni and Kim), and theory and computational material design (Kotliar). The team members will unite their forces and expertise to characterize high-quality specimens with multiple experimental probes, to explore electric field-effect doping of charge carriers using nano-scale devices, to interpret the results using advanced computational models, and to design and synthesize new materials. As demonstrated in recent discoveries of ferromagnetic semiconductors that have crystal structures identical to those of Fe-based high-Tc superconductors, encounters and coherent collaboration between experts from different research communities will lead to unanticipated breakthroughs. Since 2011, the PIs from Columbia and Rice have organized live/video lecture courses for entry-level graduate students "Frontiers of Condensed Matter Physics" seeking broader impact, and have accumulated about 100 video lectures of leading scientists describing modern studies of solid state physics. The present project will allow adding a new series to this course involving faculty members from Columbia, Rice, Harvard, Rutgers, and UCLA and connecting their classrooms with a web-based technology for simultaneous broadcast.

TECHNICAL SUMMARY
Condensation and pairing mechanisms of high-Tc cuprate and iron-based superconductors have not yet been established. However, there are growing signatures pointing toward the important role played by magnetic interactions. With multi-probe experimental researchers using neutrons, muons, transport, and scanning tunneling microscopy (STM), supplemented by quantitative comparison to advanced computation, the present DMREF project will shed new light on the quest for understanding unconventional superconductors. In conjunction with the predictive powers of advanced computational methods, a better understanding of the physical mechanisms at work will contribute to the ability to design materials with higher transition temperatures. Carrier doping using electric field effects will provide a new route to search for novel superconductors, less sensitive to disorder effects associated with conventional doping with chemical substitutions. Transport results on Fermi-level tuning via electrolyte gate voltage will be directly compared to advanced theoretical computations on electronic structures. Additionally, the formation of interfaces of unconventional superconductors and their magnetic derivatives, and engineering of phase changes via charge-doping with field-effect gating, will result in devices with novel functionality, leading to an as yet unexplored interdisciplinary research front. This project will provide a unique collaborative experience involving leading researchers, graduate students, and postdocs with multiple research fields and techniques that will make important contributions to the development of the future leaders of modern physics research.

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.