arXiv subjects

Nobuhiko Taniguchi

Publications and source records attributed to Nobuhiko Taniguchi.

13 recordsLinked to original sources

arXiv2024

Quantum coherent control of linear and nonlinear thermoelectricity on graphene nanostructure heat engines

We theoretically show how structural modifications and controlling quantum coherency can enhance linear and nonlinear thermoelectric performance in graphene nanostructure heat engines. Although graphene has emerged as a promising material for a nanoscale heat engine due to its high coherency and tunable electronic properties, its large lattice thermal transport often limits its thermal efficiency. Using the density-functional tight-binding method, we demonstrate that one can suppress lattice thermal transport degrading the thermal efficiency by deliberately manipulating the junction's bending angle at low temperatures. We further argue that applying an optimal local gate voltage unleashes its great potential in achieving excellent efficiency and reasonably high output power that persist in the fully nonlinear regime.

cond-mat.mes-hall↗

arXiv2020

Quantum control of nonlinear thermoelectricity at the nanoscale

We theoretically study how one can control and enhance nonlinear thermoelectricity by regulating quantum coherence in nanostructures such as a quantum dot system or a single-molecule junction. In nanostructures, the typical temperature scale is much smaller than the resonance width, which largely suppresses thermoelectric effects. Yet we demonstrate one can achieve a reasonably good thermoelectric performance by regulating quantum coherence. Engaging a quantum-dot interferometer (a quantum dot embedded in the ring geometry) as a heat engine, we explore the idea of thermoelectric enhancement induced by the Fano resonance. We develop an analytical treatment of fully nonlinear responses for a dot with or without strong interaction. Based on the microscopic model with the nonequilibrium Green function technique, we show how to enhance efficiency and/or output power as well as where to locate an optimal gate voltage. We also argue how to assess nonlinear thermoelectricity by linear-response quantities.

cond-mat.mes-hall↗

arXiv2018

Quantum thermodynamics of nanoscale steady states far from equilibrium

We develop an exact quantum thermodynamic description for a noninteracting nanoscale steady state that couples strongly with multiple reservoirs. It is demonstrated that there exists a steady-state extension of the thermodynamic function that correctly accounts for the multiterminal Landauer-Büttiker formula of quantum transport of charge, energy or heat, via the nonequilibrium thermodynamic relations. Its explicit form is obtained for a single bosonic or fermionic level in the wide-band limit, and corresponding thermodynamic forces (affinities) are identified. Nonlinear generalization of the Onsager reciprocity relations are derived. We suggest that the steady-state thermodynamic function is also capable of characterizing the heat current fluctuations of the critical transport where the thermal fluctuations dominate. It is also pointed out that the suggested nonequilibrium steady-state thermodynamic relations seemingly persist for a spin-degenerate single level with local interaction.

cond-mat.mes-hall↗

arXiv2017

Exact path-integral evaluation of locally interacting systems: The subtlety of operator ordering

We discuss how one calculates the coherent path integrals for locally interacting systems, where some inconsistencies with exact results have been reported previously. It is shown that the operator ordering subtlety that is hidden in the local interaction term modifies the Hubbard-Stratonovich transformation in the continuous time formulation, and it helps reproduce known results by the operator method. We also demonstrate that many-body effects in the strong interaction limit can be well characterized by the free-particle theory that is subject to annealed random potentials and dynamical gauge (or phase) fields. The present treatment expands the conventional paradigm of the one-particle description, and it provides a simple, viable picture for strongly correlated materials of either bosonic or fermionic systems.

quant-ph↗

arXiv2014

Multi-terminal Anderson impurity model in nonequilibrium: Analytical perturbative treatment

We study the nonequilibrium spectral function of the single-impurity Anderson model connecting with multi-terminal leads. The full dependence on frequency and bias voltage of the nonequilibrium self-energy and spectral function is obtained analytically up to the second-order perturbation regarding the interaction strength $U$. High and low bias voltage properties are analyzed for a generic multi-terminal dot, showing a crossover from the Kondo resonance to the Coulomb peaks with increasing bias voltage. For a dot where the particle-hole symmetry is not present, we construct a current-preserving evaluation of the nonequilibrium spectral function for arbitrary bias voltage. It is shown that finite bias voltage does not split the Kondo resonance in this order, and no specific structure due to multiple leads emerges. Overall bias dependence is quite similar to finite temperature effect for a dot with or without the particle-hole symmetry.

cond-mat.mes-hall↗

arXiv2012

Spin Current Generation as a Nonequilibrium Kondo Effect in a Spin-orbit Mesoscopic Interferometer

We study nonequilibrium generation of spin-dependent transport through a single-level quantum dot embedded in a ring with the Rashba spin-orbit coupling. We consider nonmagnetic systems, involving no magnetic field nor ferromagnetic leads. It is theoretically predicted that large spin-dependent current occurs as a combined effect of the Rashba spin-orbit interaction, the Kondo effect, and nonequilibrium effect, without using magnetic field or material. The phenomenon is viewed as a new nonequilibrium correlation effect that disappears when either interaction or finite bias is absent. We show how the Kondo physics is connected with such emergent spin phenomenon by employing the finite interaction slave-boson approach.

cond-mat.mes-hall↗

arXiv2010

Universal conductance enhancement and reduction of the two-orbital Kondo effect

We investigate theoretically the linear and nonlinear conductance through a nanostructure with two-fold degenerate single levels, corresponding to the transport through nanostructures such as a carbon nanotube, or double dot systems with capacitive interaction. It is shown that the presence of the interaction asymmetry between orbits/dots affects significantly the profile of the linear conductance at finite temperature, and, of the nonlinear conductance, particularly around half-filling, where the two-particle Kondo effect occurs. Within the range of experimentally feasible parameters, the SU(4) universal behavior is suggested, and comparison with relevant experiments is made.

cond-mat.mes-hall↗

arXiv2009

Thermal Symmetry Crossover and Universal Behaviors in Carbon Nanotube Dots

Motivated by recent experiments on electronic transport through a carbon nanotube, we investigate the role of the intra- and inter-orbital Coulomb interactions on the temperature evolution of the conductance. It is shown that small amount (~10%) of asymmetry between these Coulomb repulsions substantially deforms the conductance profile at finite temperature, particularly around half-filling. The nature of such thermal symmetry crossover is elucidated.

cond-mat.mes-hall↗

arXiv2004

Quantum Anomaly and Effective Field Description of a Quantum Chaotic Billiard

We investigate the effective field theory of a quantum chaotic billiard from a new perspective of quantum anomalies, which result from the absence of continuous spectral symmetry in quantized systems. It is shown that commutators of composite operators on the energy shell acquire anomalous part. The presence of the anomaly allows one to introduce effective dual fields as phase variables without any additional coarse-graining nor ensemble averaging in a ballistic system. The spectral Husimi function plays a role as the corresponding amplitude.

cond-mat.mes-hall↗

arXiv2001

Two-dimensional Non-Hermitian Delocalization Transition as a Probe for the Localization Length

When one applies a type of non-Hermitian effect, constant imaginary vector potential, to disordered systems, delocalization is induced even in two or lower dimension. By using the non-Hermitian induced transition as a probe, We propose a new procedure of estimating localization in arbitrary-dimensional systems. By examining numerically the two-dimensional non-Hermitian tight-biding model with onsite disorder, it is shown that the failure of absorbing the non-Hermitian effect, namely the breakdown of the imaginary gauge transformation, characterize the inverse localization length near the band center.

cond-mat.dis-nn↗

arXiv1998

Nonperturbative Renormalization Group Function for Quantum Hall Plateau Transitions Imposed by Global Symmetries

As a unified theory of integer and fractional quantum Hall plateau transitions, a nonperturbative theory of the two-parameter scaling renormalization group function is presented. By imposing global symmetries known as ``the law of corresponding states'', we seek a possible form of renormalization group flows. Asking for consistency with the result from weak-localization perturbation theory, such restriction is so intense that we can analytically determine its concrete form. Accordingly, the critical exponent $ν$ and the irrelevant scaling index $y$ are obtained analytically and turn out to be irrational. Their values ($ν\approx 2.1$, $y\approx 0.3$) agree favorably well with experiments and numerics.

cond-mat.mes-hall↗

arXiv1996

Distribution of the Absorption by Chaotic States in Quantum Dots

The mesoscopic fluctuations of the absorption at optical transitions from a low energy regular state to high energy chaotic states in an aggregate of semiconductor quantum dots is studied. We provide a universal dependence of the distribution of the absorption coefficient on the total number of dots and the ratio of the level broadening to the level spacing. The distribution remain broad even at large broadening, and the absorption spectrum should demonstrate a strong sensitivity to weak magnetic field in the region of large and weak absorption. The results can also apply to the absorption of Rydberg atoms in strong magnetic field at the pre-threshold ionization.

cond-mat↗

arXiv1995

Statistics of Oscillator Strengths in Chaotic Systems

The statistical description of oscillator strengths for systems like hydrogen in a magnetic field is developed by using the supermatrix nonlinear $σ$-model. The correlator of oscillator strengths is found to have a universal parametric and frequency dependence, and its analytical expression is given. This universal expression applies to quantum chaotic systems with the same generality as Wigner-Dyson statistics.

cond-mat↗