arXiv subjects

Kazuo Ueda

Publications and source records attributed to Kazuo Ueda.

At least 19 recordsLinked to original sources

arXiv2014

Continuous Mott Transition in a Two-Dimensional Hubbard Model

We investigate nonmagnetic metal-insulator transition in the 1/5-depleted square lattice Hubbard model at half-filling within the 8-site cellular dynamical mean field theory. We find that a metal-insulator transition without any signatures of the first order transition, a continuous Mott transition, takes place in a certain range of parameters. The nature of the continuous Mott transition is nothing but a Lifshitz transition driven by the on-site Coulomb interaction. The renormalized matrix elements of hoppings and the spin-spin correlation functions reveal that physics of this transition is strong enhancement of the dimerization due to the non-local effects of electron-electron interaction.

cond-mat.str-el↗

arXiv2013

SU(3) Dirac electrons in the 1/5-depleted square-lattice Hubbard model at 1/4 filling

We investigate the magnetic and metal-insulator (M-I) phase diagram of the 1/5-depleted square-lattice Hubbard model at 1/4 filling by the mean-field approximation. There exist three magnetic phases of nonmagnetic (N), antiferromagnetic (AF), and ferromagnetic (F) types, each realized for the large intrasquare hopping t1, intersquare hopping t2, and Coulomb interaction U, respectively. Within each magnetic phase, the M-I transition of Lifshitz type emerges and, finally, six kind of phases are identified in the U-t1/t2 plane. When t1=t2, we find that the Dirac cone and nearly flat band around the Gamma point form the SU(3) multiplet. The SU(3) effective theory well describes the phase transitions between NI, paramagnetic-metal (PM), and AF phases. The NI and AFI phases are characterized by different Berry phases as in polyacetylene or graphene.

cond-mat.str-el↗

arXiv2012

Dimensional Reduction and Odd-Frequency Pairing of the Checkerboard-Lattice Hubbard Model at 1/4-Filling

The ferromagnetism of the checkerboard lattice Hubbard model at quarter filling is one of the few exact ferromagnetic ground states known in the family of Hubbard models. When the nearest neighbor hopping, t1, is negligible compared with the second neighbor one, t2, the system reduces to a collection of Hubbard chains. We find that the 1D character is surprisingly robust as long as t1 < t2. This phenomenon of dimensional reduction due to the geometrical frustration leads to peculiar magnetic orders with 1D character for intermediate U and is responsible for odd-frequency superconducting states close to the magnetic boundary.

cond-mat.supr-con↗

arXiv2012

Electric Dipolar Kondo Effect Emerging from Vibrating Magnetic Ion

When a magnetic ion vibrates in a metal, it inevitably introduces a new channel of hybridization with conduction electrons and in general, the vibrating ion induces electric dipole moment. In such a situation, we find that magnetic and non-magnetic Kondo effects alternatively occur due to the screening of spin moment and electric dipole moment of vibrating ion. In particular, electric dipolar two-channel Kondo effect is found to occur for weak Coulomb interaction. We also show that magnetically robust heavy-electron state appears near the fixed point of electric dipolar two-channel Kondo effect. We believe that the {\it vibrating} magnetic ion opens a new door in the Kondo physics.

cond-mat.str-el↗

arXiv2011

Effect of Anharmonicity on the Kondo Phenomena of a Magnetic Ion Vibrating in a Confinement Potential

Effect of anharmonicity of a cage potential for a magnetic ion vibrating in a metal is investigated by the numerical renormalization group method. The cage potential is assumed to be one-dimensional and of the double-well type. In the absence of the Coulomb interaction, we find continuous crossover among the three limiting cases: Yu-Anderson-type Kondo regime, the double-well-type Kondo one, and the renormalized Fermi chain one. In the entire parameter space of the double-well potential, the ground state is described by a local Fermi liquid. In the Yu-Anderson-type Kondo regime, a quantum phase transition to the ground state with odd parity takes place passing through the two-channel Kondo fixed point when the Coulomb interaction increases. Therefore, the vibration of a magnetic ion in an oversized cage structure is a promising route to the two-channel Kondo effect.

cond-mat.str-el↗

arXiv2011

Nonlinear Transport through Quantum Dots Studied by the Time-Dependent DMRG

Recent developments on studies of transport through quantum dots obtained by applying the time-dependent density matrix renormalization group method are summarized. Some new aspects of Kondo physics which appear in nonequilibrium steady states are discussed both for the single dot case and for the serially coupled double-quantum-dot case.

cond-mat.str-el↗

arXiv2011

Kondo Effect of a Magnetic Ion Vibrating in a Harmonic Potential

To discuss Kondo effects of a magnetic ion vibrating in the sea of conduction electrons, a generalized Anderson model is derived. The model includes a new channel of hybridization associated with phonon emission or absorption. In the simplest case of the localized electron orbital with the s-wave symmetry, hybridization with p-waves becomes possible. Interesting interplay among the conventional s-wave Kondo effect and the p-wave one and the Yu-Anderson type Kondo effect is found and the ground state phase diagram is determined by using the numerical renormalization group method. Two different types of stable fixed points are identified and the two-channel Kondo fixed points are generically realized on the boundary.

cond-mat.str-el↗

arXiv2010

Kondo Effect of a Vibrating Magnetic Impurity

A generalized Anderson model for a magnetic ion in a harmonic potential is formulated. The model is investigated by the numerical renormalization group(NRG) method. In addition to the conventional s-wave screening, the model exhibits phonon assisted p-wave Kondo effect as well as Yu and Anderson type Kondo effect. It is shown that the s-wave Kondo and the Yu-Anderson Kondo belong to the same fixed point. At the boundary between the s-wave and p-wave Kondo regions line of fixed points of the two channel Kondo effect is identified.

cond-mat.str-el↗

arXiv2010

Nonequilibrium Current in the One Dimensional Hubbard Model at Half-Filling

Nonlinear transport in the one dimensional Hubbard model at half-filling under a finite bias voltage is investigated by the adaptive time-dependent density matrix renormalization group method. For repulsive on-site interaction, dielectric breakdown of the Mott insulating ground state to a current-carrying nonequilibrium steady state is clearly observed when the voltage exceeds the charge gap. It is found that by increasing the voltage further the current-voltage characteristics are scaled only by the charge gap and the scaling curve exhibits almost linear dependence on the voltage whose slope is suppressed by the electron correlation. In the case of attractive interaction the linear conductance is the perfect one $2e^2/h$ which agrees with the prediction by the Luttinger liquid theory.

cond-mat.str-el↗

arXiv2010

Anharmonicity in one-dimensional electron-phonon system

We investigate the effect of anharmonicity on the one-dimensional half-filled Holstein model by using the determinant quantum Monte Carlo method. By calculating the order parameters we find that with and without anharmonicity there is always an transition from a disorder phase to a dimerized phase. Moreover, in the dimerized phase a lattice dimerization and a charge density wave coexist. The anharmonicity represented by the quartic term suppresses the dimerization as well as the charge density wave, while a double-well potential favors the dimerization. In addition, by calculating the correlation exponents we show that the disorder phase is metallic with gapless charge excitations and gapful spin excitations while in the dimerized phase both excitations are gapful.

cond-mat.str-el↗

arXiv2009

Perturbative Approach to the Nonequilibrium Kondo Effect in a Quantum Dot

The theory of quantum transport through a dot under a finite bias voltage is developed using perturbation theory in the Keldysh formalism. It is found that the Kondo resonance splits into double peaks when the voltage exceeds the Kondo temperature, $eV>k_B T_K $, which leads to the appearance of a second peak in conductance, in addition to the zero-bias peak. The possible relevance of the new peak to the 0.7 conductance anomaly observed in quantum point contact is discussed.

cond-mat.mes-hall↗

arXiv2008

Green's function of fully anharmonic lattice vibration

Motivated by the discovery of superconductivity in beta-pyrochlore oxides, we study property of rattling motion coupled with conduction electrons. We derive the general expression of the Green's function of fully anharmonic lattice vibration within the accuracy of the second order perturbation of electron-ion interaction by introducing self-energy, vertex-correction, and normalization factor for each transition. Using the expression, we discuss the characteristic properties of the spectral function in the entire range from weakly anharmonic potential to double-well case, and calculate NMR relaxation rate due to the two phonon Raman process.

cond-mat.str-el↗

arXiv2008

Insulator to superfluid transition in coupled photonic cavities in two dimensions

A system of coupled photonic cavities on a two-dimensional square lattice is systematically investigated using the stochastic series expansion quantum Monte Carlo method. The ground state phase diagram contains insulating phases with integer polariton densities surrounded by a superfluid phase. The finite-size scaling of the superfluid density is used to determine the phase boundaries accurately. We find that the critical behavior is that of the generic, density-driven Mott-superfluid transition with dynamic exponent $z=2$, with no special multicritical points with $z=1$ at the tips of the insulating-phase lobes (as exist in the case of the Bose-Hubbard model). This demonstrates a limitation of the description of polaritons as structureless bosons.

cond-mat.other↗

arXiv2008

Time-dependent DMRG Study on Quantum Dot under a Finite Bias Voltage

Resonant tunneling through quantum dot under a finite bias voltage at zero temperature is investigated by using the adaptive time-dependent density matrix renormalization group(TdDMRG) method. Quantum dot is modeled by the Anderson Hamiltonian with the 1-D nearest-neighbor tight-binding leads. Initially the ground state wave function is calculated with the usual DMRG method. Then the time evolution of the wave function due to the slowly changing bias voltage between the two leads is calculated by using the TdDMRG technique. Even though the system size is finite, the expectation values of current operator show steady-like behavior for a finite time interval, in which the system is expected to resemble the real nonequilibrium steady state of the infinitely long system. We show that from the time intervals one can obtain quantitatively correct results for differential conductance in a wide range of bias voltage. Finally we observe an anomalous behavior in the expectation value of the double occupation operator at the dot $ $ as a function of bias voltage.

cond-mat.mes-hall↗

arXiv2007

Theoretical study of NMR relaxation due to rattling phonons

We calculate the NMR relaxation rate due to quadrupolar coupling of the nucleus to a local, strongly anharmonic phonon mode. As a model potential for a rattling motion we consider a square-well potential. We calculate the free phonon Green's function analytically and derive the low and high temperature limits of the NMR relaxation rate. It is shown that the temperature dependence of the NMR relaxation rate possesses a peak in contrast to harmonic phonons but in qualitative agreement with a recent NMR study on KOs2O6. We discuss the influence of phonon renormalization due to electron-phonon interaction.

cond-mat.supr-con↗

arXiv2007

NMR relaxation and resistivity from rattling phonons in pyrochlore superconductors

We calculate the temperature dependence of NMR relaxation rate and electrical resistivity for coupling to a local, strongly anharmonic phonon mode. We argue that the two-phonon Raman process is dominating NMR relaxation. Due to the strong anharmonicity of the phonon an unusual temperature dependence is found having a low temperature peak and becoming constant towards higher temperatures. The electrical resistivity is found to vary like T^2 at low temperatures and following a sqrt{T} behavior at high temperatures. Both results are in qualitative agreement with recent observations on beta-pyrochlore oxide superconductors.

cond-mat.supr-con↗

arXiv2007

Insulating charge density wave for a half-filled SU(N) Hubbard model with an attractive on-site interaction in one dimension

We study a one-dimensional SU(N) Hubbard model with an attractive on-site interaction and $N>2$ at half-filling on the bipartite lattice using density-matrix renormalization-group method and a perturbation theory. We find that the ground state of the SU(N) Hubbard model is a charge density wave state with two-fold degeneracy. All the excitations are found to be gapful, resulting in an insulating ground state, on contrary to that in the SU(2) case. Moreover, the charge gap is equal to the Cooperon gap, which behaves as $-2Nt^2/(N-1)U$ in the strong coupling regime. However, the spin gap $Δ_{s}$ and the quasiparticle gap $Δ_{1}$ as well open exponentially in the weak coupling region, while in the strong coupling region, they linearly depend on $U$ such that $Δ_{s}\sim -U(N-1)$ and $Δ_{1}\sim -U(N-1)/2$.

cond-mat.str-el↗

arXiv2007

Low-energy excitations of the one-dimensional half-filled SU(4) Hubbard model with an attractive on-site interaction: Density-matrix renormalization-group calculations and perturbation theory

We investigate low-energy excitations of the one-dimensional half-filled SU(4) Hubbard model with an attractive on-site interaction U < 0 using the density matrix renormalization group method as well as a perturbation theory. We find that the ground state is a charge density wave state with a long range order. The ground state is completely incompressible since all the excitations are gapful. The charge gap which is the same as the four-particle excitation gap is a non-monotonic function of U, while the spin gap and others increase with increasing |U| and have linear asymptotic behaviors.

cond-mat.str-el↗