Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

We study spin entanglement of the quasiparticles of the local Fermi liquid excited in nonlinear current through a quantum dot described by the Anderson impurity model with two degenerate orbitals coupled to each other via an exchange interaction. Applying the renormalized perturbation theory, we obtain the precise form of the cumulant generating function and cross-correlations for the currents with spin angled to arbitrary directions, up to third order in the applied bias voltage. It is found that the exchange interaction gives rise to spin-angle dependency in the cross-correlation between the currents through the two different orbitals and also brings an intrinsic cross-correlation of currents with three different angular momenta.

DOI： 10.1103/PhysRevB.97.045127

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

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 L. Tosi, P. Roura-Bas, and A. A. Aligia, J. Phys.: Condens. Matter 27, 335601 (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 interdot 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 interdot interaction U12　is greater than or of the order of the bandwidth of the coupled conduction bath
D, which might be difficult to achieve experimentally. By comparing the SU(4) Kondo results for a total dot occupation ntot=1 and 2, we conclude that the temperature dependence of the conductance is largely determined by the constraints of the Friedel sum rule rather than the SU(4) symmetry and suggest that an initial increase of the conductance with temperature is a distinguishing characteristic feature of an
ntot=1 universal SU(4) fixed point.

DOI： 10.1103/PhysRevB.93.235115

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

We examine the low-energy behavior of a double quantum dot in a regime where spin and pseudospin excitations are degenerate. The individual quantum dots are described by Anderson impurity models with an on-site interaction U which are capacitively coupled by an interdot interaction U12<U. The low-energy response functions are expressed in terms of renormalized parameters, which can be deduced from an analysis of the fixed point in a numerical renormalization group calculation. At the point where the spin and pseudospin degrees of freedom become degenerate, the free quasiparticle excitations have a phase shift of π/4 and a 4-fold degeneracy. We find, however, when the quasiparticle interactions are included, that the low-energy effective model has SU(4) symmetry only in the special case U12=U unless both U and U12 are greater than D, the half bandwidth of the conduction electron bath. We show that the gate voltage dependence of the temperature-dependent differential conductance observed in recent experiments can be described by a quasiparticle density of states with temperature-dependent renormalized parameters.

DOI： 10.1103/PhysRevB.88.245130

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

Studies of the effects of the Hund's rule coupling JH　in multiple orbit impurities or quantum dots using different models have led to quite different predictions for the Kondo temperature TK as a function of JH. We show that the differences depend on whether or not the models conserve orbital angular momentum about the impurity site. Using numerical renormalization-group calculations, we deduce the renormalized parameters for the Fermi liquid regime and show that, despite the differences between the models, the low-energy fixed point in the strong-correlation regime is universal, with a single energy scale
TK and just two renormalized interaction parameters, a renormalized single orbital term, ˜U=4TK, and a renormalized Hund's rule term, ˜JH=8TK/3.

DOI： 10.1103/PhysRevB.86.245131

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

We apply a combination of numerical renormalization group (NRG) and renormalized perturbation theory (RPT) to a model of two quantum dots (impurities) described by two Anderson impurity models hybridized to their respective baths. The dots are coupled via a direct Coulomb interaction U12 and a spin exchange interaction
J. The model has two types of quantum critical points, one at J=Jc to a local singlet state and one at U12=Uc12 to a locally charge ordered state. The renormalized parameters which determine the low energy behavior are calculated from the NRG. The results confirm the values predicted from the RPT on the approach to the critical points, which can be expressed in terms of a single energy scale T∗ in all cases. This includes cases without particle-hole symmetry, and cases with asymmetry between the dots, where there is also a transition at J=Jc. The results give a comprehensive quantitative picture of the behavior of the model in the low energy Fermi liquid regimes, and some of the conclusions regarding the emergence of a single energy scale may apply to a more general class of quantum critical points, such as those observed in some heavy fermion systems.

DOI： 10.1103/PhysRevB.86.125134

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

We study nonequilibrium current fluctuations through a quantum dot, which includes a ferromagnetic Hund’s 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 Hund’s rule coupling, and these pairs enhance the current fluctuations. In the fully screened higher-spin Kondo limit, the Fano factor takes a value Fb=(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.

DOI： 10.1103/PhysRevLett.108.266401

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

We find the emergence of strong correlations and universality on the approach to the quantum critical points of a two-impurity Anderson model. The two impurities are coupled by an interimpurity exchange interaction J and direct interaction U12 and are hybridized with separate conduction channels. The low energy behavior is described in terms of renormalized parameters. We show that on the approach to the transitions to a local singlet and a local charged ordered state, the quasiparticle weight factor z→0
, and the renormalized parameters can be expressed in terms of a single energy scale T∗. The values of the renormalized interaction parameters in terms of T∗
can be predicted from the condition of continuity of the spin and charge susceptibilities, and correspond to strong correlation. These predictions are confirmed by the numerical renormalization group calculations, including the case when the on site interaction U=0.

DOI： 10.1103/PhysRevLett.108.056402

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

We study the low-energy properties and characteristic Kondo energy scale of a triangular triple quantum dot, connected to two non-interacting leads, in a wide parameter range of a gate voltage and distortions which lower the symmetry of an equilateral structure, using the numerical renormalization group approach. For large Coulomb interactions, the ground states with different characters can be classified according to the plateaus of Θ≡(δe−δo)(2/π), where δe and δo are the phase shifts for the even and odd partial waves. At these plateaus of Θ, both Θ and the occupation number Ntot≡(δe+δo)(2/π) take values close to integers, and thus the ground states can be characterized by these two integers. The Kondo effect with a local moment with total spin S=1 due to a Nagaoka mechanism appears on the plateau, which can be identified by Θ≃2.0 and Ntot≃4.0. For large distortions, however, the high-spin moment disappears through a singlet-triplet transition occurring within the four-electron region. It happens at a crossover to the adjacent plateaus for Θ≃0.0 and Θ≃4.0, and the two-terminal conductance has a peak in the transient regions. For weak distortions, the SU(4) Kondo effect also takes place for Ntot≃3.0. It appears as a sharp conductance valley between the S=1/2 Kondo ridges on both sides. We also find that the characteristic energy scale T∗　reflect these varieties of the Kondo effect. Particularly,　T ∗ is sensitive to the distribution of the charge and spin in the triangular triple dot.

DOI： 10.1103/PhysRevB.83.205304

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

We explore the predictions of the renormalized perturbation theory for the n-channel Anderson model, both with and without Hund’s rule coupling, in the regime away from particle-hole symmetry. For the model with n=2 we deduce the renormalized parameters from numerical renormalization-group calculations and plot them as a function of the local occupation of the impurity site nd. From the exact relations in terms of the renormalized parameters, we calculate the orbital, spin and charge susceptibilities, Wilson ratios, and quasiparticle density of states at T=0, in the different parameter regimes, which gives a comprehensive overview of the low-energy behavior of the model. We compare the difference in Kondo behaviors at the points, where nd=1 and nd=2. Some unexpected features of the results are that a strong on-site interaction gives significant renormalization and suppression charge susceptibility in the intermediate valence regimes, and the peaks in the spin susceptibility away from the particle-hole symmetric case do not occur at integer values nd=1,3

DOI： 10.1103/PhysRevB.82.245109

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

We extend the renormalized perturbation theory for the single-impurity Anderson model to the n-channel model with a Hund’s rule coupling, and show that the exact results for the spin, orbital, and chargesusceptibilities, as well as the leading low-temperature dependence for the resistivity, are obtained by working to second order in the renormalized couplings. A universal relation is obtained between the renormalized parameters, independent of n, in the Kondo regime. An expression for the dynamic spin susceptibility is also derived by taking into account repeated quasiparticle scattering, which is asymptotically exact in the low-frequency regime and satisfies the Korringa-Shiba relation. The renormalized parameters, including the renormalized Hund’s rule coupling, are deduced from numerical renormalization-group calculations for the model for the case n=2. The results confirm explicitly the universal relations between the parameters in the Kondo regime. Using these results, we evaluate the spin, orbital, and charge susceptibilities, temperature dependence of the low-temperature resistivity, and dynamic spin susceptibility for the particle-hole symmetric regime of the n=2 model.

DOI： 10.1103/PhysRevB.82.115123

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Single Work   International / domestic magazine：International journal  

We theoretically investigate a momentum-dependent resonant inelastic X-ray scattering (RIXS) in broad band materials. Taking 3C-SiC, Si and Ge as typical examples of broad band materials, we calculate BINS spectra by changing transferred momenta.. The RIXS spectra at the Si K edge of 3C-SiC have strong transferred momentum dependence. On the other hand, the RIXS spectra at the K edge of Ge have almost no transferred momentum dependence. In the case of RIXS at the K edge of Si, the spectra have weak transferred momentum dependence. Through an explanation of the transferred momentum dependence for 3C-SiC, Si and Ge, we point out that a RIXS can be utilized to study valence band dispersion for broad band materials. We illustrate how to extract the valence bands dispersion from the transferred momentum dependence of the spectra, taking the RIXS at the Si K edge of 3C-SiC as a typical example.

DOI： 10.4028/www.scientific.net/MSF.645-648.235

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

We theoretically demonstrate that a resonant inelastic X-ray scattering (RIXS) with a sizable momentum transfer can be utilized to study valence band dispersion for broad band materials. We take RIXS at the Si K edge of 3C-SiC as a typical example. The RIXS spectra are calculated by systematically changing the transferred momentum, an incident photon polarization and an incident photonenergy, on the basis of an ab initio calculation. We find that the spectra depend heavily on both the transferred momentum and the incident photon polarization, and the peaks in the spectra correspond to the energies of the valence bands. We conclude that the information on the energy dispersion of valence bands can be extracted from the transferred momentum dependence of the RIXS spectra. These findings lead to further application for RIXS when investigating the band structure of broad band materials.

DOI： 10.1016/j.physb.2009.12.011

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

We study the low-energy properties of a triangular triple quantum dot connected to two noninteracting leads in a wide parameter range, using the numerical renormalization group (NRG). Various kinds of Kondo effects take place in this system depending on the electron filling Ntot, or the level position ϵd of the triple dot. The SU(4) Kondo behavior is seen in the half-filled case Ntot≃3.0 at the dip of the series conductance, and it causes a charge redistribution between the even and odd orbitals in the triangle. We show generally that the quasiparticle excitations from a local Fermi-liquid ground state acquire a channel symmetry at zero points of the two-terminal conductance, in the case the system has time-reversal and inversion symmetries. It causes the SU(4) behavior at low energies, while the orbital degeneracy in the triangle determines the high-energy behavior. At four-electron filling Ntot≃4.0, a local S=1 moment emerges at high temperatures due to a Nagaoka ferromagnetic mechanism. It is fully screened by the electrons from the two conducting channels via a two-stage Kondo effect, which is caused by a difference in the charge distribution in the even and odd orbitals.

DOI： 10.1103/PhysRevB.80.155330

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

DOI： 10.1088/1742-6596/150/2/022066

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

We study theoretically heavy fermion superconductors CeMIn5 (M = Co, Rh, Ir). CeCoIn5 and CeIrIn5 that become superconducting at ambient pressure with Tc = 2.3 K and 0.4 K, respectively. On the other hand, CeRhIn5 is an antiferromagnet at ambient pressure and becomes superconducting under pressures greater than 1.6 GPa. With regards to the superconductivity, the existence of line nodes is indicated by nuclear-quadrupole-resonance (NQR), thermal conductivity, specific heat and electrical resistivity measurements. However, the pairing symmetry between dx2−y2 and dxy is controversial. Therefore, we investigate the gap structure of CeMIn5 by a detailed calculation. We introduce a three-dimensional periodic Anderson model (3D-PAM) in order to reproduce the band characteristics of CeMIn5. Thus, we identify the gap structure of CeMIn5 as the dx2−y2 symmetry by solving the Èliashberg equations. In addition, we discuss the pressure dependence of Tc and show that two factors determine Tc. One factor is the momentum dependence of quasi-particle interaction and the other factor is the wavefunction renormalization factor. Thus, we have explained the superconductivity in CeMIn5 using the Fermi liquid theory.

DOI： 10.1088/0953-8984/19/40/406219

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

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 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 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 , while plateau of a four-terminal parallel conductance reaches a unitary limit value of two conducting modes.

DOI： 10.1016/j.jmmm.2006.10.890

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

The electronic structure of in the spin-Peierls (SP) phase with the space group of Bbcm is calculated for the first time within the local-density approximation (LDA) and the scheme where the local 3d–3d Coulomb interaction U is introduced on the Cu sites. Although the insulating antiferromagnetic solution is obtained in the LDA, the obtained bandgap of and a magnetic moment of are much smaller than the experimental bandgap of and the magnetic moment of , respectively. With the value of determined using an electron spectroscopy, the calculation yields a charge-transfer bandgap of and gives a magnetic moment of . The essential electronic and magnetic properties of in the SP phase can be described by means of the scheme.

DOI： 10.1016/j.jmmm.2006.10.489

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

The ground-state properties of quartet quantum-dot arrays are studied using the numerical renormalization group (NRG) method with a four-site Hubbard model connected to two noninteracting leads. Specifically, we calculate the conductance and local charge in the dots from the many-body phase shifts, which can be deduced from the fixed-point eigenvalues of NRG. As a function of the on-site energy ϵd which corresponds to the gate voltage, the conductance shows alternatively wide peaks and valleys. Simultaneously, the total number of electrons Nel in the four dots shows a quantized staircase behavior due to a large Coulomb interaction
U. The conductance plateaus of the unitary limit emerging for odd Nel are caused by the Kondo effect. The valleys of the conductance emerge for even Nel
, and their width becomes substantially large at half-filling. It can be regarded as a kind of the Mott-Hubbard insulating behavior manifesting in a small system. These structures of the plateaus and valleys become weak for large values of the hybridization strength Γ between the chain and leads. We also discuss the parallel conductance for the array connected to four leads.

DOI： 10.1103/PhysRevB.73.125108

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

CeMIn 5 (M=Co, Rh, Ir) are heavy fermion compounds with a HoCoGa 5 -type structure. CeCoIn 5 and CeIrIn 5 become superconducting at ambient pressure with T c =2.3 and 0.4 K, respectively. On the other hand, CeRhIn 5 is an antiferromagnet at ambient pressure, and becomes superconducting under pressures greater than 1.6 GPa. Nuclear-quadrupole-resonance (NQR), thermal conductivity, specific heat, and electrical resistivity measurements indicate that the superconductivity in CeMIn 5 has line nodes. However, the pairing symmetry between d x 2 - y 2 and d x y is controversial based on the angle-dependent measurements of thermal conductivity and specific heat under the magnetic field, respectively. Therefore, we investigate the gap structure of CeMIn 5 by a detailed calculation. In order to reproduce the band structure characteristics of CeMIn 5 , we introduce a three-dimensional (3D) periodic Anderson model (PAM) and solve the Eliashberg equations. Thus, we identify the gap structure of CeMIn 5 as the d x 2 - y 2 symmetry. In addition, we discuss the pressure dependence of T c and show that two opposing factors determine T c . One factor is the momentum dependence of quasi-particle interaction and the other factor is the wave-function renormalization factor.

DOI： 10.1143/JPSJ.75.024713

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

We phenomenologically investigate a possible mechanism of the banding of angular velocity in a current-driven vortex lattice in type-II superconductors. It is shown that the plastic motion occurs above some critical value of the applied current, resulting in a splitting in the vortex lattice. The present model can qualitatively reproduce major features of the experimental results reported by Lopez et al. [Phys. Rev. Lett. 82, 1277 (1999)].

DOI： 10.1103/PhysRevB.73.064511

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

The pairing symmetry for CeCoIn 5 is controversial between d x 2 - y 2 and d x y from the angle-dependent measurements of thermal conductivity and specific heat under the magnetic field, respectively. Therefore we investigate the gap structure of CeCoIn 5 , and identify it as d x 2 - y 2 symmetry. In addition, we show that two opponent factors determine the pressure dependence of T c : the momentum dependence of quasi-particle interaction, and the wave-function renormalization factor.

DOI： 10.1143/JPSJS.75S.250

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

We describe a formulation to deduce the phase shifts, which determine the ground-state properties of interacting quantum-dot systems with the inversion symmetry, from the fixed-point eigenvalues of the numerical renormalization group (NRG). Our approach does not assume the specific form of the Hamiltonian nor the electron–hole symmetry, and it is applicable to a wide class of quantum impurities connected to noninteracting leads. We apply the method to a triple dot which is described by a three-site Hubbard chain connected to two noninteracting leads, and calculate the dc conductance away from half-filling. The conductance shows the typical Kondo plateaus of the Unitary limit g ≃2 e 2 / h in some regions of the onsite energy ε d , at which the total number of electrons N el in the three dots is odd, i.e., N el ≃1, 3 and 5. In contrast, the conductance shows a wide minimum in the regions of ε d corresponding to even number of electrons N el ≃2 and 4. We also discuss the parallel conductance of the triple dot connected transversely to four leads, and show that it can be deduced from the two phase shifts defined in the two-lead case.

DOI： 10.1143/JPSJ.74.2554

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

We investigate theoretically the resonant inelastic X-ray scattering (RIXS) at the K edge of Si on the basis of an ab initio calculation. We calculate the RIXS spectra with systematically varying transfered-momenta, incident-photon energy and incident-photon polarization. We confirm the anisotropy of the experimental spectra by Y. Ma et al. [Phys. Rev. Lett. 74 (1995) 478], providing a quantitative explanation of the spectra.

DOI： 10.1143/JPSJ.73.3171

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

To discuss the possibility that the superconductivities in Na x CoO 2 · y H 2 O are induced by electron correlation, we investigate the possible pairing symmetry based on the single-band Hubbard model whose dispersion of the bare energy band is obtained using the band structure calculation of Na x CoO 2 · y H 2 O. The superconducting transition temperature is estimated by solving the Éliashberg equation. In this equation, both normal and anomalous self-energies are calculated up to the third-order terms with respect to the Coulomb repulsion. In the case of spin-singlet pairing, the candidate of pairing symmetry (the maximum eigen value λ max SS of Éliashberg's equation) belongs to the d -wave (E 2 representation of D 6 group). In the case of spin-triplet pairing, the candidate of pairing symmetry (the maximum eigen value λ max ST of Éliashberg's equation) belongs to the f y ( y 2 -3 x 2 ) -wave (B 1 representation of D 6 group). It is found that λ max SS ≃λ max ST and the transition temperatures of unconventional pairing state are estimated to be low compared with the observed temperature within our simple model.

DOI： 10.1143/JPSJ.73.1127

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

Stimulated by the recent finding of the superconductivity in Na 0.35 CoO 2 .1.3H 2 O, we investigate superconducting instabilities on a 2D triangular lattice in the repulsive Hubbard model. Using the third-order perturbation expansion with respect to the on-site repulsion U , we evaluate the linearized Dyson–Gor'kov equation. We find that f -wave spin-triplet pairing is the most stable in a wide range of the next-nearest-neighbor hopping integral t ' and electron number density n . The introduction of t ' is crucial for adjusting the van Hove singularities to the neighborhood of the Fermi surface crossing around the K point. In this case, the bare spin susceptibility shows a broad peak around the Γ point. These conditions stabilize f -wave pairing. Although f -wave pairing is also given by the fluctuation-exchange approximation, the transition temperature is too low to be observed. This is because the depairing effect by the spin fluctuation is overestimated. Thus, the third-order vertex corrections are important for the spin-triplet superconductivity, similar to the case of Sr 2 RuO 4 .

DOI： 10.1143/JPSJ.73.17

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

We study the resonant inelastic x-ray scattering (RIXS) at the K edge of Ge. We measure RIXS spectra with systematically varying momenta in the final state. The spectra are a measure of exciting an electron–hole pair. We find a single-peak structure (except the elastic peak) as a function of photon energy, which is nearly independent of final-state momenta. We analyze the experimental data by means of the band structure calculation. The calculation reproduces well the experimental shape, clarifying the implication of the spectral shape.

DOI： 10.1143/JPSJ.73.970

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：Domestic journal  

Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

We investigate an anomalously enhanced jump of the electronic specific heat at the superconducting transition point observed in CeCoIn5 within the fluctuation-exchange approximation. The strong antiferromagnetic spin fluctuation leads to strong-coupling superconductivity with a relatively high transition temperature. Simultaneously, the jump of the specific heat becomes large. This tendency also becomes stronger as the antiferromagnetic spin fluctuation becomes stronger.

DOI： 10.1088/0953-8984/15/28/360

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

Within third-order perturbation theory, we discuss the possibility of spin-triplet and spin-singlet superconductivities in a two-dimensional Hubbard model on a triangular lattice for some model parameters corresponding to the heavy-fermion superconductors UM2Al3 (M = Pd,Ni) and organic superconductors under uniaxial strain. In the model of the heavy-fermion superconductors UM2Al3 (M = Pd,Ni), when we vary the symmetry in the dispersion of the bare energy band from D2 to D6, spin-singlet superconductivity in the D2-symmetric system is suppressed and we obtain spin-triplet superconductivity in the nearly D6-symmetric system. We point out the possibility that the results obtained correspond to the difference between the superconductivity of UPd2Al3 and that of UNi2Al3. In the model of organic superconductors under uniaxial strain, when we increase the diagonal hopping integral to above 1.2, we obtain the tendency for spin-triplet superconductivity to emerge. In all cases mentioned above, it is found that the vertex correction terms, which are not included in the interaction mediated by the spin fluctuation, are essential for realizing the spin-triplet pairing.

DOI： 10.1088/0953-8984/15/28/357

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

We discuss the possibility of spin-triplet (ST) superconductivity in two-dimensional Hubbard model on triangular lattice within the third order perturbation theory. We obtain ST superconducting state. It is found that the vertex correction terms which are not included in the interaction mediated by the spin-fluctuation is essential for realizing the ST pairing for our searched values of parameters. We argue that the present mechanism of ST superconductivity can be applied to heavy-fermion superconductors UNi2Al3, UPt3.

DOI： 10.1016/S0921-4534(02)02623-0

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

We discuss the possibility of spin-triplet superconductivity in a two-dimensional Hubbard model on a triangular lattice within the third-order perturbation theory. When we vary the symmetry in the dispersion of the bare energy band from D 2 to D 6 , spin-singlet superconductivity in the D 2 -symmetric system is suppressed and we obtain spin-triplet superconductivity in near the D 6 -symmetric system. In this case, it is found that the vertex terms, which are not included in the interaction mediated by the spin fluctuation, are essential for realizing the spin-triplet pairing. We point out the possibility that obtained results correspond to the difference between the superconductivity of UNi 2 Al 3 and that of UPd 2 Al 3 .

DOI： 10.1143/JPSJ.71.2629

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

We reformulate the Éliashberg's equation for the superconducting transition within the quasi-particle description, which takes account of the heavy electron mass. We discuss the superconductivity of CeIr x Co 1- x In 5 by using such a renormalized formula. On the basis of an effective two-dimensional Hubbard model which represents the most heavy quasi-two dimensional Fermi surface with f -character of CeIr x Co 1- x In 5 , both normal and anomalous self-energies are calculated up to third order with respect to the renormalized on-site repulsion U between quasi-particles. The superconducting transition temperature is obtained by solving the Éliashberg's equation. Reasonable transition temperatures are obtained for moderately large U . It is found that the momentum and frequency dependence of spin fluctuations given by RPA-like terms gives rise to the d-wave pairing state, while the vertex correction terms are important for obtaining reasonable transition temperatures and for differentiating n -dependence of T c . These obtained results seem to explain the x -dependence of T c in CeIr x Co 1- x In 5 and confirm the usefulness of the perturbation theory with respect to U .

DOI： 10.1143/JPSJ.71.1140

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work  

We investigate the superconducting mechanism and the transition temperature of heavy fermion superconductor UPd 2 Al 3 on the basis of a single band two-dimensional Hubbard model on triangular lattice, which represents the most heavy band of UPd 2 Al 3 . Both normal and anomalous self-energies are calculated up to third order with respect to the Coulomb repulsion U between itinerant electrons. The superconducting transition temperature is obtained by solving the Éliashberg's equation. Reasonable transition temperatures are obtained for moderately large U . It is found that the momentum and frequency dependence of spin fluctuations given by RPA-like terms gives rise to the d-wave pairing state, while the vertex correction terms are important for obtaining reasonable transition temperatures. These results seem to show that the superconductivity in UPd 2 Al 3 can be explained by the perturbation theory with respect to U .

DOI： 10.1143/JPSJ.71.237

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Kind of work：Joint Work   International / domestic magazine：International journal  

DOI： 10.1016/S0022-3697(02)00064-1

Publishing type：Research paper, summary (national, other academic conference)  

DOI： 10.11316/jpsgaiyo.68.2.4.0_634_2

CiNii Article

Publishing type：Research paper, summary (national, other academic conference)  

DOI： 10.11316/jpsgaiyo.68.2.4.0_634_1

CiNii Article

Publishing type：Research paper, summary (national, other academic conference)  

DOI： 10.11316/jpsgaiyo.67.1.4.0_723_4

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： 10.11316/jpsgaiyo.65.1.4.0_706_4

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： 10.11316/jpsgaiyo.64.1.4.0_700_4

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： 10.11316/jpsgaiyo.64.2.4.0_577_4

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： 10.11316/jpsgaiyo.64.1.4.0_693_1

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： 10.11316/jpsgaiyo.63.1.4.0_678_4

CiNii Article

Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： 10.11316/jpsgaiyo.63.2.4.0_584_1

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DOI： 10.11316/jpsgaiyo.60.2.4.0_528_1

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DOI： 10.11316/jpsgaiyo.58.2.3.0_509_1

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DOI： 10.11316/jpsgaiyo.58.1.3.0_535_1

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DOI： 10.11316/jpsgaiyo.57.1.3.0_537_1

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Publishing type：Research paper, summary (national, other academic conference)  

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Presentation type：Oral presentation (invited, special)  

Venue：Osaka City University  

Presentation type：Oral presentation (general)  

Venue： 岡山大学  

Audience： Teachers

教員免許状更新講習にて「量子論への招待」という題目で講義を行った。またそのためのテキストを執筆した。