Norio Kawakami

Orbital-selective mott transitions in the degenerate Hubbard model.

We investigate the Mott transitions in two-band Hubbard models with different bandwidths. Applying dynamical mean field theory, we discuss the stability of itinerant quasiparticle states in each band. We demonstrate that separate Mott transitions occur at different Coulomb interaction strengths in general, which merge to a single transition only under special conditions. This kind of behavior may be relevant for the physics of the single-layer ruthenates, Ca2-xSrxRuO4.

Luttinger anomaly exponent of momentum distribution in the Hubbard chain

Abstract The exponent θ of the power-law anomaly for the momentum distribution in the one-dimensional repulsive Hubbard model is calculated by the Bethe ansatz method. We express the exponent of the 4kF oscillation part of the asymptotic density correlator in terms of the fractional charge of the massless holon excitation. The exponent θ is then obtained assuming the scaling law based on the Luttinger liquid nature of the system. For U→∞ and 0, the present formulation reproduces the correct values θ→ 1 8 and 0, respectively, for any filling. Furthermore our results agree nicely with the Monte Carlo calculation for finite Coulomb interaction. We explicitly show that near half-filling θ takes the value close to 1 8 as long as the Hubbard gap exists.

Exact expression of the ground-state energy for the symmetric anderson model

Abstract The exact expression of the ground-state energy for the symmetric Anderson model is obtained with the use of the Wiegmann approach. It is found that some of the quasi-momenta appearing in Wiegmanns paper are necessarily complex to obtain the expression of the ground-state energy.

Magnetoresistance of the Heavy-Electron Ce Compounds

The temperature-dependent magnetoresistance of the heavy-electron Ce compounds is calculated with the use of the periodic Anderson model. It is pointed out that the magnetoresistance has a minimum-structure and may change its sign from negative to positive as the temperature is decreased. The obtained results are qualitatively in good agreement with the experimental evidences of CeAl 3 and CeCu 6 .

Superconductivity Induced by Longitudinal Ferromagnetic Fluctuations in UCoGe

From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T(Curie)∼2.5  K and T(SC)∼0.6  K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H∥c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results, combined with model calculations, strongly suggest that the longitudinal FM spin fluctuations tuned by H∥c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.

Interference effects on Kondo-assisted transport through double quantum dots

We systematically investigate electron transport through double quantum dots with particular emphasis on interference induced via multiple paths of electron propagation. By means of the slave-boson mean-field approximation, we calculate the conductance, the local density of states, and the transmission probability in the Kondo regime at zero temperature. It is clarified how the Kondo-assisted transport changes its properties when the system is continuously changed among the serial, parallel and T-shaped double dots. The obtained results for the conductance are explained in terms of the Kondo resonances influenced by interference effects. We also discuss the impacts due to the spin-polarization of ferromagnetic leads.

Colossal enhancement of upper critical fields in noncentrosymmetric heavy fermion superconductors near quantum criticality: CeRhSi3 and CeIrSi3.

Strong-coupling effects on the upper critical fields Hc2 along the c axis in the noncentrosymmetric heavy fermion superconductors near quantum criticality CeRhSi3 and CeIrSi3 are examined. For sufficiently large spin-orbit interactions due to the lack of inversion symmetry, Hc2 is mainly determined by the orbital depairing effects. From microscopic calculations taking into account the strong spin fluctuations, we show that Hc2 is extremely enhanced as the system approaches the quantum critical point, resulting in Hc2 approximately 30 T even in the case of a low transition temperature Tc(H = 0) approximately 1 K, which explains well the huge Hc2 observed in the recent experiments.

Numerical Renormalization Group Approach to a Quantum Dot Coupled to Normal and Superconducting Leads

We study transport through a quantum dot coupled to normal and superconducting leads using the numerical renormalization group method. We show that the low-energy properties of the system are described by the local Fermi liquid theory despite of the superconducting correlations penetrated into the dot due to a proximity effect. We calculate the linear conductance due to the Andreev reflection in the presence of the Coulomb interaction. It is demonstrated that the maximum structure appearing in the conductance clearly characterizes a crossover between two distinct spin-singlet ground states, i.e. the superconducting singlet state and the Kondo singlet state. It is further elucidated that the gate-voltage dependence of the conductance shows different behavior in the superconducting singlet region from that in the Kondo singlet region.

Luttinger liquid properties of highly correlated electron systems in one dimension

An exact description is given of the long-distance behaviour of the one-dimensional t-J model at t=J. The authors employ the Bathe ansatz method and the finite-size scaling technique in conformal field theory. The charge and spin degrees of freedom are separated, and described by two independent c=1 conformal theories. The critical exponents for the charge, spin, electron and superconducting correlation functions are obtained for arbitrary band filling. The authors then make detailed comparison of the t-J model with the repulsive Hubbard model with emphasis on the Luttinger liquid properties. Analysing the electron filling dependence they observe the enhancement of the superconducting correlations compared with the highly correlated Hubbard model. The effect of the external magnetic field at and near half-filling is also discussed.

Thermodynamic Quantities of the One-Dimensional Hubbard Model at Finite Temperatures

The thermodynamic quantities of the 1D Hubbard model are investigated exactly by means of the Bethe Ansatz solution at finite temperatures. For an arbitrary electron concentration, the magnetic susceptibility, the charge susceptibility and the specific heat are calculated numerically as a function of temperature. On the basis of the obtained results, the characteristic behavior, which appears in the charge excitation near half-filling, is discussed, and is understood to be closely related to the gap formation of the Mott-type insulator.