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Takeo Kato

Publications and source records attributed to Takeo Kato.

At least 73 records · Page 4Linked to original sources

Geometrical optimization of pumping power under adiabatic parameter driving

Adiabatic pumping is a fundamental concept in the time-dependent transport of mesoscopic devices. To maximize pumping performance, i.e., the amount of pumping per unit time, it is necessary to carefully manage the driving speed, which should be sufficiently less than the {\it limited speed}, an upper bound of the driving speed below which non-adiabatic effects are negligible. In general, the amount of pumping increases as the contour of the driving parameter lengthens, however a long contour diminishes the pumping power because it requires more time per cycle under the limited speed constraint. We consider this trade-off carefully and show that there should exist an optimized period and contour to maximize the power of adiabatic pumping. We confirm this conclusion based on the results of charge pumping using a single-level quantum dot.

cond-mat.mes-hall↗

Sub-band picture of high-harmonic generation in solids

We propose a novel picture of high-harmonic generation (HHG) in solids based on the concept of temporally changing band structures. To demonstrate the utility of this picture, we focus on the high-order sideband generation (HSG) caused by strong terahertz (THz) and weak near-infrared (NIR) light in the context of pump-probe spectroscopy. We find that the NIR frequency dependence of the HSG indicates the existence of new energy levels (sub-bands) around the band-gap energy, which have multiple frequencies of THz light. This sub-band picture explains why the HSG intensity becomes a non-monotonic function of the THz light amplitude. The present analysis not only reveals the origin of the plateau structure in HHG spectra, but also provides a connection to other high-field phenomena.

cond-mat.mtrl-sci↗

Surface plasmon polaritons in thin-film Weyl semimetals

We theoretically investigate surface plasmon polaritons propagating in the thin-film Weyl semimetals. We show how the properties of surface plasmon polaritons are affected by hybridization between plasmons localized at the two metal-dielectric interfaces. Generally, this hybridization results in new mixed plasmon modes, which are called short-range surface plasmons and long-range surface plasmons, respectively. We calculate dispersion curves of these mixed modes for three principle configurations of the axion vector describing axial anomaly in Weyl semimetals. We show that the partial lack of the dispersion and the non-reciprocity can be controlled by fine-tuning of the thickness of the Weyl semimetals, the dielectric constants of the outer insulators, and the direction of the axion vector.

cond-mat.mes-hall↗

Quantum critical phenomena in heat transport via a two-state system

We present a theoretical study of the quantum critical behavior in heat transport via a two-state system with sub-ohmic reservoirs. We calculate the temperature dependence of the thermal conductance near the quantum phase transition via the continuous-time quantum Monte Carlo method and discuss its critical exponents. We also propose a superconducting circuit to realize the sub-ohmic spin-boson model that can be used to observe quantum critical phenomena.

cond-mat.mes-hall↗

Heat transport via a local two-state system near thermal equilibrium

Heat transport in spin-boson systems near the thermal equilibrium is systematically investigated. An asymptotically exact expression for the thermal conductance in a low-temperature regime wherein transport is described via a co-tunneling mechanism is derived. This formula predicts the power-law temperature dependence of thermal conductance $\propto T^{2s+1}$ for a thermal environment of spectral density with the exponent $s$. An accurate numerical simulation is performed using the quantum Monte Carlo method, and these predictions are confirmed for arbitrary thermal baths. Our numerical calculation classifies the transport mechanism, and shows that the noninteracting-blip approximation quantitatively describes thermal conductance in the incoherent transport regime.

cond-mat.mes-hall↗

mVMC - Open-source software for many-variable variational Monte Carlo method

mVMC (many-variable Variational Monte Carlo) is an open-source software based on the variational Monte Carlo method applicable for a wide range of Hamiltonians for interacting fermion systems. In mVMC, we introduce more than ten thousands variational parameters and simultaneously optimize them by using the stochastic reconfiguration (SR) method. In this paper, we explain basics and user interfaces of mVMC. By using mVMC, users can perform the calculation by preparing only one input file of about ten lines for widely studied quantum lattice models, and can also perform it for general Hamiltonians by preparing several additional input files. We show the benchmark results of mVMC for the Hubbard model, the Heisenberg model, and the Kondo-lattice model. These benchmark results demonstrate that mVMC provides ground-state and low-energy-excited-state wave functions for interacting fermion systems with high accuracy.

cond-mat.str-el↗

DC-current induced domain wall in a chiral $p$-wave superconductor

We study theoretically the impact of an applied DC-current on a mesoscopic chiral $p$-wave superconductor. Performing quasiclassical calculations on a two-dimensional system, with an external magnetic flux to generate a DC current, we show that the current can trigger a transition to a state with a domain wall between regions of different chiralities. The system shows an hysteretic behavior, as different domain wall configurations are possible for a given current. This domain wall creation mechanism can give new insights on recent experiments observing anomalous current variations in Sr${}_2$RuO${}_4$ junctions.

cond-mat.supr-con↗

Interaction effect on reservoir-parameter-driven adiabatic charge pumping via a single-level quantum dot system

We formulate adiabatic charge pumping via a single-level quantum dot (QD) induced by reser- voir parameter driving, i.e., temperature and electrochemical potential driving. Our formulation describes arbitrary strength of dot-reservoir coupling and Coulomb interaction in the QD, and is applicable to the low-temperature regime, where the Kondo effect becomes important. The adia- batic charge pumping is expressed by the Berry connection and is related to delayed response of the QD. We calculate the pumped charge by the renormalized perturbation theory, and discuss how the Coulomb interaction affects the charge pumping.

cond-mat.mes-hall↗

First-principles description of van der Waals-bonded spin-polarized systems using vdW-DF$+U$ method---application to solid oxygen at low pressure

The description of the molecular solid phase of O$_2$, especially its ground-state antiferromagnetic insulating phase, is known to be quite unsatisfactory within the conventional local and semilocal density functional approximations used in the Kohn-Sham formalism of density functional theory. The recently-developed van der Waals functionals that take into account nonlocal correlations have also shown subpar performance in this regard. The difficulty lies in the subtle balance between the van der Waals interactions and the exchange coupling between the antiferromagnetic and ferromagnetic molecule pairs in the molecular crystal. Here, we report that the DFT$+U$ approach used in combination with the vdW-DF functional performs surprisingly well in this regard, and discuss the reasoning behind this behavior. We also apply this approach to study the recently-reported magnetic field-induced $θ$ phase of solid O$_2$.

cond-mat.mtrl-sci↗

Effects of Coulomb interaction on photon-assisted current noises through a quantum dot

We study photon-assisted transport in a single-level quantum dot system under a periodically oscillating field. Photon-assisted current noises in the presence of the Coulomb interaction are calculated based on a gauge-invariant formulation of time-dependent transport. We derive the vertex corrections within the self-consistent Hartree-Fock approximation in terms the Floquet-Green's functions (Floquet-GFs), and examine the effects of the Coulomb interaction on the photon-assisted current noises. Moreover, we introduce a concept of an effective temperature to characterize nonequilibrium properties under the influence of the AC field. The vertex corrections are suppressed by the rise of the effective temperature, whereas characteristic resonant structures appear in the frequency spectra of the vertex corrections. The present result provides a useful viewpoint for understanding photon-assisted transport in interacting electron systems.

cond-mat.mes-hall↗

Spatial effect on stochastic dynamics of bistable evolutionary games

We consider the lifetimes of metastable states in bistable evolutionary games (coordination games), and examine how they are affected by spatial structure. A semiclassical approximation based on a path integral method is applied to stochastic evolutionary game dynamics with and without spatial structure, and the lifetimes of the metastable states are evaluated. It is shown that the population dependence of the lifetimes is qualitatively different in these two models. Our result indicates that spatial structure can accelerate the transitions between metastable states.

q-bio.PE↗

Dephasing in single-electron generation due to environmental noise probed by Hong Ou Mandel interferometry

We consider the effect of dephasing on a quantum dot which injects single electrons on a chiral edge channel of the quantum Hall effect. Dephasing is described by the coupling of the dot to a bosonic bath which represents the electromagnetic environment. Using the input-output formalism of quantum optics, we derive the density matrix of the edge degrees of freedom. Results are illustrated by computing the zero frequency current-current correlations when two such single electron emitters achieve a collision at the location of a quantum point contact, in the same spirit as the Hong Ou Mandel experiment of quantum optics. Such correlations are directly linked to the quantum mechanical purity. We show that as observed in a recent experiment, the effect of dephasing leads to a non-vanishing of the Hong Ou Mandel dip when the time delay between the two electron wave packets is zero. Generalizations to time filtered wave packets as well as to asymmetric, detuned injection between opposite edges are obtained.

cond-mat.mes-hall↗

Enhancement and termination of the superconducting proximity effect due to atomic-scale defects visualized by scanning tunneling microscopy

Using low-temperature scanning tunneling microscopy and spectroscopy, we have studied the proximity effect at the interfaces between superconducting Pb island structures and metallic Pb-induced striped-incommensurate phase formed on a Si(111) substrate. Our real-space observation revealed that the step structures on the two-dimensional metallic layer exhibit significant roles on the propagation of the superconducting pair correlation; the proximity effect is terminated by the steps, and in the confined area by the interface and the steps the effect is enhanced. The observed results are explained quantitatively with an elastic reflection of electrons at the step edges based on calculations with the quasi-classical Green's function formulation using Usadel equation.

cond-mat.mes-hall↗

Kondo signature in heat transfer via a local two-state system

We study the Kondo effect in heat transport via a local two-state system. This system is described by the spin-boson Hamiltonian with Ohmic dissipation, which can be mapped onto the Kondo model with anisotropic exchange coupling. We calculate thermal conductance by the Monte Carlo method based on the exact formula. Thermal conductance has a scaling form κ= (k_B^2 T_K/\hbar) f(α,T/T_K ), where T_K and αindicate the Kondo temperature and dimensionless coupling strength, respectively. Temperature dependence of conductance is classified by the Kondo temperature as κ\propto (T/T_K )^3 for T\ll T_K and κ\propto (k_B T / \hbarω_c)^{2α-1} for T\gg T_K. Similarities to the Kondo signature in electric transport are discussed.

cond-mat.stat-mech↗

Relaxor behavior and morphotropic phase boundary in a simple model

A simple model to reproduce strong enhancement of dielectric response near the morphotropic phase boundary (MPB) is proposed. This model consists of long-range dipole-dipole interaction and compositional chemical disorder incorporated by the variation in lengths of dipole moments. By applying Monte Carlo simulation, we show that there appears a ferroelectric boundary phase between two types of antiferroelectric phases at an optimal strength of randomness. In the boundary phase, ferroelectric domain becomes remarkably large and flexible to external electric fields, leading to huge dielectric response. This observation indicates that huge dielectric response near the MPB originates from {\it local} polarization rotation under suppressed anisotropy by phase competition.

cond-mat.mtrl-sci↗

Experimental Verification of Comparability between Spin-Orbit and Spin-Diffusion Lengths

We experimentally confirmed that the spin-orbit lengths of noble metals obtained from weak anti-localization measurements are comparable to the spin diffusion lengths determined from lateral spin valve ones. Even for metals with strong spin-orbit interactions such as Pt, we verified that the two methods gave comparable values which were much larger than those obtained from recent spin torque ferromagnetic resonance measurements. To give a further evidence for the comparability between the two length scales, we measured the disorder dependence of the spin-orbit length of copper by changing the thickness of the wire. The obtained spin-orbit length nicely follows a linear law as a function of the diffusion coefficient, clearly indicating that the Elliott-Yafet mechanism is dominant as in the case of the spin diffusion length.

cond-mat.mes-hall↗

Properties of a single photon generated by a solid-state emitter: effects of pure dephasing

We investigate the properties of a single photon generated by a solid-state emitter subject to strong pure dephasing. We employ a model in which all the elements of the system, including the propagating fields, are treated quantum mechanically. We analytically derive the density matrix of the emitted photon, which contains full information about the photon, such as its pulse profile, power spectrum, and purity. We visualize these analytical results using realistic parameters and reveal the conditions for maximizing the purity of generated photons.

quant-ph↗

Full counting statistics for SU(N) impurity Anderson model

We analyze the full counting statistics of a multiorbital Kondo effect in a quantum dotwith the SU(N) symmetry in the framework of the renormalized perturbation theory. The current probability distribution function is calculated for an arbitrary dot-site Coulomb repulsion $U$ in the particle-hole symmetric case. The resulting cumulant up to the leading nonlinear term of applied bias voltages indicates two types of electron transfer, respectively carrying charge $e$ and $2e$, with different $N$-dependences. The cross correlation between different orbital currents shows exponential enhancement with respect to $U$, which directly addresses formation of the orbital-singlet state.

cond-mat.mes-hall↗