Strongly correlated electron systems, particularly compounds including transition-metal, rare-earth, or actinide elements, are the main subjects of our research. In these systems where electron-electron interactions are very strong, there appear a variety of interesting phenomena at low temperatures, and various magnetic orders, unconventional superconductivity and density waves are typical cases. The present targets of our study include novel types of quantum order and quantum fluctuations in frustrated spin and strongly correlated electronic systems. In these systems, many soft modes of fluctuations are coupled, which is characteristic to frustrated systems, and this affects the nature of quantum phase transitions, as well as electronic states and transport properties. We have recently studied electronic structure and electric transport properties of strongly correlated electrons on a triangular lattice. We have also studied the Kondo effect of a frustrated cluster of magnetic impurities and discovered novel quantum states.
Kondo effect of a tetrahedron cluster of magnetic impurities and its schematic ground state phase diagram. With increasing exchange interaction J between impurity spins, the ground state changes from the conventional local Fermi liquid state to a new state where an emergent scalar chirality of impurity spins is decoupled from conduction electrons. These two states are separated by a new non-Fermi liquid (NFL) quantum critical point where J is the order of Kondo temperature TK of individual spin.
Change in the self energy of impurity electron with the Kondo temperature. Larger hybridization V results in a higher Kondo temperature and corresponds to the case of weaker interaction between impurities. With decreasing V, the dependence on imaginary frequency ωn changes from Fermi liquid like to insulator like. Units of energy are the band width D of conduction electron, and Coulomb repulsion U and hopping t between impurity sites are fixed.