arXiv subjects

Yoshihide Tanaka

Publications and source records attributed to Yoshihide Tanaka.

6 recordsLinked to original sources

arXiv2007

Kondo effect in asymmetric Josephson couplings through a quantum dot

Asymmetry in the Josephson couplings between two superconductors through a quantum dot is studied based on a single impurity Anderson model using the numerical renormalization group (NRG). Specifically, we examine how the difference between the couplings Γ_L and Γ_R affects the ground state, which is known to show a quantum phase transition between a nonmagnetic singlet and a magnetic doublet depending on the various parameters; the Coulomb interaction U, onsite potential ε_d, level width Γ_L, Γ_R, caused by the hybridization, and superconducting gaps Δ_L and Δ_R for the leads on the left and right. Our results show that whether the local moment is fully screened or not depends substantially on the asymmetry in the couplings Γ_L \neq Γ_R. It tends to make the singlet ground state stable, while the size and phase difference of the two superconducting gaps tend to suppress the screening. We also discuss some general symmetry properties of the system and their relation to the current conservation.

cond-mat.mes-hall↗

arXiv2006

Kondo screening of a high-spin Nagaoka state in a triangular quantum dot

We study transport through a triangle triple quantum dot connected to two noninteracting leads using the numerical renormalization group (NRG). The triangle has a high-spin ground state of S=1 caused by a Nagaoka ferromagnetism, when it is isolated and has one extra electron introduced into a half-filling. The results show that the conduction electrons screen the local moment via two separate stages with different energy scales. The half of the S=1 is screened first by one of the channel degrees, and then at very low temperature the remaining half is fully screened to form a Kondo singlet. The transport is determined by two phase shifts for quasi-particles with even and odd parities, and then a two-terminal conductance in the series configuration is suppressed $g_{\rm series} \simeq 0$, while plateau of a four-terminal parallel conductance reaches a Unitary limit value $g_{\rm parallel} \simeq 4e^2/h$ of two conducting modes.

cond-mat.mes-hall↗

arXiv2004

Quantum phase transition in a minimal model for the Kondo effect in a Josephson junction

We propose a minimal model for the Josephson current through a quantum dot in a Kondo regime. We start with the model that consists of an Anderson impurity connected to two superconducting (SC) leads with the gaps $Δ_α=|Δ_α| e^{i θ_α}$, where $α= L, R$ for the lead at left and right. We show that, when one of the SC gaps is much larger than the others $|Δ_L| \gg |Δ_R|$, the starting model can be mapped exactly onto the single-channel model, which consists of the right lead of $Δ_R$ and the Anderson impurity with an extra onsite SC gap of $Δ_d \equiv Γ_L e^{i θ_L}$. Here $θ_L$ and $Γ_L$ are defined with respect to the starting model, and $Γ_L$ is the level width due to the coupling with the left lead. Based on this simplified model, we study the ground-state properties for the asymmetric gap, $|Δ_L| \gg |Δ_R|$, using the numerical renormalization group (NRG) method. The results show that the phase difference of the SC gaps $ϕ\equiv θ_R -θ_L$, which induces the Josephson current, disturbs the screening of the local moment to destabilize the singlet ground state typical of the Kondo system. It can also drive the quantum phase transition to a magnetic doublet ground state, and at the critical point the Josephson current shows a discontinuous change. The asymmetry of the two SC gaps causes a re-entrant magnetic phase, in which the in-gap bound state lies close to the Fermi level.

cond-mat.mes-hall↗

arXiv2003

Effects of Electron Correlation on the Transport through a Quantum Dot Superlattice

We study correlation effects on the transport through a quantum dot superlattice using a two-dimensional Hubbard model connected to two noninteracting leads. To calculate the zero-temperature conductance away from half-filling, we have used the non-magnetic solution of the Hartree-Fock approximation for the unperturbed Green's function, and then take into account the electron correlation beyond the mean field theory through the second-order self-energy corrections with respect to the residual interaction. When the value of the onsite potential $ε_d$ or the repulsion $U$ is changed, the conductance shows a number of peaks corresponding to the resonance states passing through the Fermi energy. The behavior of the resonance states for correlated electrons can be investigated using the eigenvalue of the effective Hamiltonian for a quasi-particle of a local Fermi liquid. We calculate the eigenvalue using the second-order self-energy, and show explicitly that it can be compared with the conductance peak.

cond-mat.mes-hall↗

arXiv2002

Effects of disorder on conductance through small interacting systems

We study the effects of disorders on the transport through small interacting systems based on a two-dimensional Hubbard cluster of finite size connected to two noninteracting leads. This system can be regarded as a model for the superlattice of quantum dots or atomic network of the nanometer size. We calculate the conductance at T=0 using the order $U^2$ self-energy in an electron-hole symmetric case. The results show that the conductance is ensitive to the randomness when the resonance states are situated near the Fermi energy.

cond-mat.mes-hall↗

arXiv2001

Perturbation Study of the Conductance through an Interacting Region Connected to Multi-Mode Leads

We study the effects of electron correlation on transport through an interacting region connected to multi-mode leads based on the perturbation expansion with respect to the inter-electron interaction. At zero temperature the conductance defined in the Kubo formalism can be written in terms of a single-particle Green's function at the Fermi energy, and it can be mapped onto a transmission coefficient of the free quasiparticles described by an effective Hamiltonian. We apply this formulation to a two-dimensional Hubbard model of finite size connected to two noninteracting leads. We calculate the conductance in the electron-hole symmetric case using the order $U^2$ self-energy. The conductance shows several maximums in the $U$ dependence in some parameter regions of $t_y/t_x$, where $t_x$ ($t_y$) is the hopping matrix element in the $x$- ($y$-) directions. This is caused by the resonance occurring in some of the subbands, and is related with the $U$ dependence of the eigenvalues of the effective Hamiltonian.

cond-mat.mes-hall↗