Yunori Nishikawa

Kondo effects in a triangular triple quantum dot with lower symmetries

The triangular triple quantum dot is an interesting system which can demonstrate various types of the Kondo effects, such as the one due to the local spin S=1 moment caused by the Nagaoka ferromagnetic mechanism and the SU(4) Kondo effect. We theoretically study the low-temperature properties and the Kondo energy scale of the triangular triple quantum dot, using the Wilson numerical renormalization group. We have explored a wide parameter region of the electron-filling and distortions which break the symmetry of an equilateral structure. Our results give a comprehensive overview of how the Kondo behavior varies in the different the regions in the wide parameter space of the triangular triple quantum dot.

Full Counting Statistics for Orbital-Degenerate Impurity Anderson Model with Hund's Rule Exchange Coupling

We study nonequilibrium current fluctuations through a quantum dot, which includes a ferromagnetic Hunds rule coupling J, in the low-energy Fermi liquid regime using the renormalized perturbation theory. The resulting cumulant for the current distribution in the particle-hole symmetric case shows that spin-triplet and spin-singlet pairs of quasiparticles are formed in the current due to the Hunds rule coupling, and these pairs enhance the current fluctuations. In the fully screened higher-spin Kondo limit, the Fano factor takes a value F(b)=(9M+6)/(5M+4) determined by the orbital degeneracy M. We also investigate the crossover between the small and large J limits in the two-orbital case M=2, using the numerical renormalization group approach.

Conditions for observing emergent SU(4) symmetry in a double quantum dot

We analyze conditions for the observation of a low energy SU(4) fixed point in capacitively coupled quantum dots. One problem, due to dots with different couplings to their baths, has been considered by Tosi, Roura-Bas and Aligia (2015). They showed how symmetry can be effectively restored via the adjustment of individual gates voltages, but they make the assumption of infinite on-dot and inter-dot interaction strengths. A related problem is the difference in the magnitudes between the on-dot and interdot strengths for capacitively coupled quantum dots. Here we examine both factors, based on a two site Anderson model, using the numerical renormalization group to calculate the local spectral densities on the dots and the renormalized parameters that specify the low energy fixed point. Our results support the conclusions of Tosi et al. that low energy SU(4) symmetry can be restored, but asymptotically achieved only if the inter-dot interaction

Analysis of low energy response and possible emergent SU(4) Kondo state in a double quantum dot

U_{12}

Convergence of energy scales on the approach to a local quantum critical point.

is greater than or of the order of the band width of the coupled conduction bath

Phase diagram and critical points of a double quantum dot

D

Hund's rule coupling in models of magnetic impurities and quantum dots

, which might be difficult to achieve experimentally. By comparing the SU(4) Kondo results for a total dot occupation

Renormalized parameters and perturbation theory for an n-channel Anderson model with Hund's rule coupling: Asymmetric case

n_{\rm tot}=1

Fermi Liquids and the Luttinger Integral

and

Current cross-correlation in the Anderson impurity model with exchange interaction

n_{\rm tot}=2