Akihisa Koga

Superconductivity without inversion symmetry: MnSi versus CePt3Si.

Superconductivity in materials without spatial inversion symmetry is studied. We show that in contrast to common belief, spin-triplet pairing is not entirely excluded in such systems. Moreover, paramagnetic limiting is analyzed for both spin-singlet and -triplet pairing. The lack of inversion symmetry reduces the effect of the paramagnetic limiting for spin-singlet pairing. These results are applied to MnSi and CePt3Si.

Spin, charge, and orbital fluctuations in a multiorbital Mott insulator

Theoretische Physik, ETH-H¨onggerberg, 8093 Zu¨rich, Switzerland(Dated: February 2, 2008)The two-orbital degenerate Hubbardmodel with distinct hopping integrals is studied by combiningdynamical mean-field theory with quantum Monte Carlo simulations. The role of orbital fluctuationsfor the nature of the Mott transition is elucidated by examining the temperature dependence of spin,charge and orbital susceptibilities as well as the one-particle spectral function. We also consider theeffect of the hybridization between the two orbitals, which is important particularly close to the Motttransition points. The introduction of the hybridization induces orbital fluctuations, resulting in theformation of a Kondo-like heavy-fermion behavior, similarly to f electron systems, but involvingelectrons in bands of comparable width.

Stability of a metallic state in the two-orbital Hubbard model

Electron correlations in the two-orbital Hubbard model at half-filling are investigated by combining dynamical mean field theory with the exact diagonalization method. We systematically study how the interplay of the intra- and interband Coulomb interactions, together with the Hund coupling, effects the metal-insulator transition. It is found that if the intra- and interband Coulomb interactions are nearly equal, the Fermi-liquid state is stabilized due to orbital fluctuations up to fairly large interactions, while the system is immediately driven to the Mott insulating phase away from this condition. The effects of the isotropic and anisotropic Hund coupling are also addressed.

Frustrated Heisenberg antiferromagnet on the pyrochlore lattice

Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan(Dated: February 1, 2008)We investigate quantum phase transitions for the s = 1/2 antiferromagnetic Heisenberg model ona pyrochlore lattice. By means of a series expansion starting from isolated tetrahedra, the ground-state phase diagram is determined. When the ratio of the two competing exchange couplings isvaried, the first-order (second-order) quantum phase transition occurs between the two spin gapphases (the spin-gap and the antiferromagnetic phases). We also discuss some properties expectedfor the s = 1 pyrochlore spin system.

First-order quantum phase transition in the orthogonal-dimer spin chain

We investigate the low-energy properties of the orthogonal-dimer spin chain characterized by a frustrated dimer-plaquette structure. When the competing antiferromagnetic couplings are varied, the first-order quantum phase transition occurs between the dimer and the plaquette phases, which is accompanied by nontrivial features due to frustration: besides the discontinuity in the lowest excitation gap at the transition point, a sharp level-crossing occurs for the spectrum in the plaquette phase. We further reveal that the plateau in the magnetization curve at 1/4 of the full moment dramatically changes its character in the vicinity of the critical point. It is argued that the first-order phase transition in this system captures some essential properties found in the two-dimensional orthogonal-dimer model proposed for

Low temperature properties of the infinite-dimensional attractive Hubbard model

\rm SrCu_2(BO_3)_2

Field-induced phase transitions in a Kondo insulator

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Ground-State Phase Diagram for the Three-Dimensional Orthogonal-Dimer System

Theoretische Physik, ETH Zurich, 8093 Zu¨rich, Switzerland(Dated: June 24, 2011)We investigate the attractive Hubbard model in infinite spatial dimensions by combining dynam-ical mean-field theory with a strong-coupling continuous-time quantum Monte Carlo method. Bycalculating the superfluid order parameter and the density of states, we discuss the stability of thesuperfluid state. In the intermediate coupling region above the critical temperature, the density ofstates exhibits a heavy fermion behavior with a quasi-particle peak in the dense system, while a dipstructure appears in the dilute system. The formation of the superfluid gap is also addressed.I. INTRODUCTION

Finite-temperature Mott transitions in the multiorbital Hubbard model

We study the magnetic-field effect on a Kondo insulator by exploiting the periodic Anderson model with the Zeeman term. The analysis using dynamical mean field theory combined with quantum Monte Carlo simulations determines the detailed phase diagram at finite temperatures. At low temperatures, the magnetic field drives the Kondo insulator to a transverse antiferromagnetic phase, which further enters a polarized metallic phase at higher fields. The antiferromagnetic transition temperature

Supersolid State of Ultracold Fermions in Optical Lattice

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