Nonequilibrium transport through a quantum dot coupled to normal and superconducting leads

We study the interacting quantum dot coupled to the normal and superconducting leads by means of a continuous-time quantum Monte Carlo method in the Keldysh-Nambu formalism. Deducing the steady current through the quantum dot under a finite voltage, we examine how the gap magnitude in the superconducting lead and the interaction strength at the quantum dot affect transport properties. It is clarified that the Andreev reflection and Kondo effect lead to nonmonotonic behavior in the nonequilibrium transport at zero temperature.Comment: 6 pages, 3 figures, conference paper of SCES 201

Quantum Monte Carlo study of nonequilibrium transport through a quantum dot coupled to normal and superconducting leads

We investigate the nonequilibrium phenomena through the quantum dot coupled to the normal and superconducting leads using a weak-coupling continuous-time Monte Carlo method. Calculating the time evolution of particle number, double occupancy, and pairing correlation at the quantum dot, we discuss how the system approaches the steady state. We also deduce the steady current through the quantum dot beyond the linear response region. It is clarified that the interaction decreases the current in the Kondo-singlet dominant region. On the other hand, when the quantum dot is tightly coupled to the superconducting lead, the current is increased by the introduction of the Coulomb interaction, which originates from the competition between the Kondo and proximity effects. Transient currents induced by the interaction quench are also addressed.Comment: 8 pages, 7 figure

Cluster mean-field approach with density matrix renormalization group: Application to the hard-core bosonic Hubbard model on a triangular lattice

We introduce a new numerical method for the solution of self-consistent equations in the cluster mean-field theory. The method uses the density matrix renormalization group method to solve the associated cluster problem. We obtain an accurate critical value of the supersolid-superfluid transitions in the hard-core bosonic Hubbard model on a triangular lattice, which is comparable with the recent quantum Monte Carlo results. This algorithm is applicable to more general classes of models with a larger number of degrees of freedom.Comment: 6 pages, 4 figures, SCES 201

Stability of FFLO states in optical lattices with bilayer structure

We investigate the stability of the superfluid state in a bilayer fermionic optical lattice system with a confining potential, using the Bogoliubov de-Gennes equations. It is clarified that in the imbalanced case, the introduction of the interlayer hopping stabilizes the radial Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, while makes the angular FFLO state unstable. We also discuss the system size dependence of the superfluid ground state. It is clarified that in a certain ring region the A-FFLO state is indeed realized in a large system.Comment: 6 pages, 11 figure

Frustration-induced phase transitions in the spin-S orthogonal-dimer chain

We investigate quantum phase transitions in the frustrated orthogonal-dimer chain with an arbitrary spin $S \geq 1/2$. When the ratio of the competing exchange couplings is varied, first-order phase transitions occur 2S times among distinct spin-gap phases. The introduction of single-ion anisotropy further enriches the phase diagram. The phase transitions described by the present model possess most of the essential properties inherent in frustrated quantum spin systems.Comment: 4 pages, 9 figure