Yasuyuki Kato

We study the behavior of the quarter-filled Kondo-lattice model on a triangular lattice by combining a zero-temperature variational approach and finite-temperature Monte Carlo simulations. For intermediate coupling between itinerant electrons and classical moments S(j), we find a thermodynamic phase transition into an exotic spin ordering with uniform scalar spin chirality and (S(j))=0. The state exhibits a spontaneous quantum Hall effect. We also study how its properties are affected by the app

Unbiased quantum Monte Carlo simulations are performed on the nearest-neighbor spin-1/2 pyrochlore XXZ model with an antiferromagnetic longitudinal and the weak ferromagnetic transverse exchange couplings, J and J_{⊥}. The specific heat exhibits a broad peak at T_{CSI}~0.2J associated with a crossover to a classical Coulomb liquid regime showing a suppressed spin-ice monopole density, a broadened pinch-point singularity, and the Pauling entropy for |J_{⊥}|≪J, as in classical spin ice. On further

We study the Bose-Hubbard model with an external harmonic field, which is effective for modeling a cold atomic Bose gas trapped in an optical lattice. We modify the directed-loop algorithm to simulate large systems efficiently. As a demonstration we carry out the simulation of a system consisting of 1. 8 x 10{5} particles on a 64{3} lattice. These numbers are comparable to those in the pioneering experimental work by Greiner [Nature (London) 415, 39 (2002)]. Furthermore, we observe coherence bet

We investigate the spin excitation spectra in chiral and polar magnets by the linear spin-wave theory for an effective spin model with symmetric and antisymmetric long-range interactions. In one dimension, we obtain the analytic form of the dynamical spin structure factor for proper-screw and cycloidal helical spin states with uniform twists, which shows a gapless mode with strong intensity at the helical wave number. When introducing spin anisotropy in the symmetric interactions, we numerically

We develop a theory of the coexistence of superconductivity (SC) and antiferromagnetism (AFM) in CeCoIn(5). We show that in Pauli-limited nodal superconductors the nesting of the quasiparticle pockets induced by Zeeman pair breaking leads to incommensurate AFM with the magnetic moment normal to the field. We compute the phase diagram and find a first order transition to the normal state at low temperatures, the absence of normal state AFM, and the coexistence of SC and AFM at high fields, in agr

Quantum criticality near a tricritical point is studied in the two-component Bose-Hubbard model on square lattices. The existence of a quantum tricritical point on a boundary of a superfluid-insulator transition is confirmed by quantum Monte Carlo simulations. Moreover, we analytically derive the quantum tricritical behaviors on the basis of an effective field theory. We find two significant features of the quantum tricriticality that are its characteristic chemical potential dependence of the s