arXiv subjects

Toshihiro Kubo

Publications and source records attributed to Toshihiro Kubo.

15 recordsLinked to original sources

arXiv2021

Coexistence of parallel and series current paths in parallel-coupled double quantum dots in nonlinear transport regime

We investigated the electron transport properties of parallel-coupled double quantum dot (DQD) devices under magnetic fields. When a low magnetic field was applied, electron tunneling through parallel-coupled DQDs was observed. Under a high magnetic field, we observed both electron tunneling through parallel- and series-coupled DQDs under nonlinear transport conditions. In addition, the Pauli spin blockade was observed, indicating tunneling through the series-coupled DQDs. We attribute these behaviors to the magnetic-field-induced changes in the tunnel-couplings that allow the coexistence of the current paths of the parallel and series configurations.

cond-mat.mes-hall↗

arXiv2016

Quantum adiabatic pumping by modulating tunnel phase in quantum dots

In a mesoscopic system, under zero bias voltage, a finite charge is transferred by quantum adiabatic pumping by adiabatically and periodically changing two or more control parameters. We obtained expressions for the pumped charge for a ring of three quantum dots (QDs) by choosing the magnetic flux penetrating the ring as one of the control parameters. We found that the pumped charge shows a steplike behavior with respect to the variance of the flux. The value of the step heights is not universal but depends on the trajectory of the control parameters. We discuss the physical origin of this behavior on the basis of the Fano resonant condition of the ring.

cond-mat.mes-hall↗

arXiv2016

Interaction effect on adiabatic pump of charge and spin in quantum dot

We investigate the pumped charge and spin at zero-bias by adiabatic modulation of two control parameters using the full counting statistics with quantum master equation approach. First we study higher order effects of the pumping frequency in general Markov systems and show the equivalence between our approach and the real-time diagrammatic approach. An adiabatic modulation of the control parameters induces the Berry-Sinitsyn-Nemenman (BSN) phase. We show that the origin of the BSN phase is a non-adiabatic effect. The adiabatically pumped charge (spin) is given by a summation of (i) a time integral of the instantaneous steady charge (spin) current and (ii) a geometric surface integral of the BSN curvature, which results from the BSN phase. In quantum dots (QDs) weakly coupled to two leads, we show that (i) is usually dominant if the thermodynamic parameters are modulated although it is zero if the thermodynamic parameters are fixed to zero-bias. To observe the spin effects, we consider collinear magnetic fields, which relate to spins through the Zeeman effect, with different amplitudes applying to the QDs and the leads. For interacting one level QD, we calculate analytically the pumped charge and spin by modulating the magnetic fields and the coupling strengths to the leads in the weak and strong interacting limits. We show that the difference between these two limits appears through the averages of the numbers of the electron with up and down spin in the QD. For the quantum pump by the modulation of the magnetic fields of the QD and one lead, the energy-dependences of linewidth functions, which are usually neglected, are essential.

cond-mat.mes-hall↗

arXiv2016

Spin-1 Quantum Walks

We study the quantum walks of two interacting spin-1 bosons. We derive an exact solution for the time-dependent wave function that describes the two-particle dynamics governed by the one-dimensional spin-1 Bose-Hubbard model. We show that propagation dynamics in real space and mixing dynamics in spin space are correlated via the spin-dependent interaction in this system. The spin-mixing dynamics has two characteristic frequencies in the limit of large spin-dependent interactions. One of the characteristic frequencies is determined by the energy difference between two bound states, and the other frequency relates to the cotunneling process of a pair of spin-1 bosons. Furthermore, we numerically analyze the growth of the spin correlations in quantum walks. We find that long-range spin correlations emerge showing a clear dependence on the sign of the spin-dependent interaction and the initial state.

quant-ph↗

arXiv2015

Coulomb interaction-induced Aharonov-Bohm oscillations

We study the Coulomb interaction-induced Aharonov-Bohm (AB) oscillations in the linear response transport through a remote quantum dot which has no tunnel coupling but has Coulomb coupling with the quantum dot embedded in an AB interferometer. We show that the Coulomb interaction-induced AB effect is characterized by a charge susceptibility of a remote quantum dot in a weak interaction regime. In a strong but finite interaction regime, around the particle-hole symmetric point, there exists the region where the visibility of the induced AB oscillations becomes one although the visibility of the original AB oscillations in the interferometer is low.

cond-mat.mes-hall↗

arXiv2013

Power dependence of electric dipole spin resonance

We develop a formalism of electric dipole spin resonance (EDSR) based on slanting magnetic field, where we especially investigate the microwave amplitude dependence. With increasing microwave amplitude, the Rabi frequency increases linearly for a spin confined in a harmonic potential. How- ever, when the spin is confined in the double-well potential, the Rabi frequency shows sub-linear dependence with increasing the microwave amplitude.

cond-mat.mes-hall↗

arXiv2012

Backaction Dephasing by a Quantum Dot Detector

We derive an analytical expression for the backaction dephasing rate, which characterizes the disturbance induced by coupling with an environment containing a quantum dot detector (QDD). In this letter, we show that charge noise induces backaction dephasing in an explicit form. In the linear transport regime through a QDD, this backaction dephasing induced by charge noise can be explained as a relaxation by an inelastic electron-electron scattering in Fermi liquid theory. In the low bias voltage regime, the increase or decrease of dephasing rate depends on the QDD energy level, the linewidth functions, and how to apply the bias voltage. Unlike quantum point contact, the dephasing rate would be insensitive to the bias voltage in a high bias voltage regime because of the saturation of charge noise in a QDD.

cond-mat.mes-hall↗

arXiv2011

Phase and amplitude of Aharonov-Bohm oscillations in nonlinear three-terminal transport through a double quantum dot

We study three-terminal linear and nonlinear transport through an Aharonov-Bohm interferometer containing a double quantum dot using the nonequilibrium Green's function method. Under the condition that one of the three terminals is a voltage probe, we show that the linear conductance is symmetric with respect to the magnetic field (phase symmetry). However, in the nonlinear transport regime, the phase symmetry is broken. Unlike two-terminal transport, the phase symmetry is broken even in noninteracting electron systems. Based on the lowest-order nonlinear conductance coefficient with respect to the source-drain bias voltage, we discuss the direction in which the phase shifts with the magnetic field. When the higher harmonic components of the Aharonov-Bohm oscillations are negligible, the phaseshift is a monotonically increasing function with respect to the source-drain bias voltage. To observe the Aharonov-Bohm oscillations with higher visibility, we need strong coupling between the quantum dots and the voltage probe. However, this leads to dephasing since the voltage probe acts as a Büttiker dephasing probe. The interplay between such antithetic concepts provides a peak in the visibility of the Aharonov-Bohm oscillations when the coupling between the quantum dots and the voltage probe changes.

cond-mat.mes-hall↗

arXiv2010

Kondo effects and shot noise enhancement in a laterally coupled double quantum dot

The spin and orbital Kondo effects and the related shot noise for a laterally coupled double quantum dot are studied taking account of coherent indirect coupling via a reservoir. We calculate the linear conductance and shot noise for various charge states to distinguish between the spin and orbital Kondo effects. We find that a novel antiferromagnetic exchange coupling can be generated by the coherent indirect coupling, and it works to suppress the spin Kondo effect when each quantum dot holds just one electron. We also show that we can capture the feature of the pseudospin Kondo effect from the shot noise measurement.

cond-mat.mes-hall↗

arXiv2010

Coherent Manipulation of Individual Electron Spin in a Double Quantum Dot Integrated with a Micro-Magnet

We report the coherent manipulation of electron spins in a double quantum dot integrated with a micro-magnet. We performed electric dipole spin resonance experiments in the continuous wave (CW) and pump-and-probe modes. We observed two resonant CW peaks and two Rabi oscillations of the quantum dot current by sweeping an external magnetic field at a fixed frequency. Two peaks and oscillations are measured at different resonant magnetic field, which reflects the fact that the local magnetic fields at each quantum dot are modulated by the stray field of a micro-magnet. As predicted with a density matrix approach, the CW current is quadratic with respect to microwave (MW) voltage while the Rabi frequency (ν_Rabi) is linear. The difference between the ν_Rabi values of two Rabi oscillations directly reflects the MW electric field across the two dots. These results show that the spins on each dot can be manipulated coherently at will by tuning the micro-magnet alignment and MW electric field.

cond-mat.mes-hall↗

arXiv2007

Microwave band on-chip coil technique for single electron spin resonance in a quantum dot

Microwave band on-chip microcoils are developed for the application to single electron spin resonance measurement with a single quantum dot. Basic properties such as characteristic impedance and electromagnetic field distribution are examined for various coil designs by means of experiment and simulation. The combined setup operates relevantly in the experiment at dilution temperature. The frequency responses of the return loss and Coulomb blockade current are examined. Capacitive coupling between a coil and a quantum dot causes photon assisted tunneling, whose signal can greatly overlap the electron spin resonance signal. To suppress the photon assisted tunneling effect, a technique for compensating for the microwave electric field is developed. Good performance of this technique is confirmed from measurement of Coulomb blockade oscillations.

cond-mat.mes-hall↗

arXiv2006

Electron transport through Aharonov-Bohm interferometer with laterally coupled double quantum dots

We theoretically investigate electron transport through an Aharonov-Bohm interferometer containing laterally coupled double quantum dots. We introduce the indirect coupling parameter $α$, which characterizes the strength of the coupling via the reservoirs between two quantum dots. $|α|=1$ indicates the strongest coupling, where only a single mode contributes to the transport in the system. Two conduction modes exist in a system where $|α|\neq 1$. The interference effects such as the Fano resonance and the Aharonov-Bohm oscillation are suppressed as the absolute value of the parameter $α$ decreases from 1. The linear conductance does not depend on the flux when $α=0$ since it corresponds to independent coupling of the dots to the reservoir modes.

cond-mat.mes-hall↗

arXiv2005

Electron Transport in Magnetic-Field-Induced Quasi-One-Dimensional Electron Systems in Semiconductor Nanowhiskers

Many-body effects on tunneling of electrons in semiconductor nanowhiskers are investigated in a magnetic quantum limit. We consider the system with which bulk and edge states coexist. We show that interaction parameters of edge states are much smaller than those of bulk states and the tunneling conductance of edge states hardly depends on temperature and the singular behavior of tunneling conductance of bulk states can be observed.

cond-mat.mes-hall↗

arXiv2004

Many-Body Effects on Tunneling of Electrons in Magnetic-Field-Induced Quasi One-Dimensional Electron Systems in Semiconductor Nanowhiskers

Effects of the electron-electron interaction on tunneling in a semiconductor nanowhisker are studied in a magnetic quantum limit. We consider the system with which bulk and edge states coexist. In bulk states, the temperature dependence of the transmission probability is qualitatively similar to that of a one-dimensional electron system. We investigate contributions of edge states on transmission probability in bulk states. Those contributions can be neglected within our approximation which takes into account only most divergent terms at low temperatures.

cond-mat.mes-hall↗

arXiv2003

Many-Body Effects on the Transmission Probability through a Tunnel Junction in a Strong Magnetic Field

We investigate effects of electron-electron interaction on the transmission probability of electrons through a tunnel junction in a strong magnetic field. We start with the Hartree-Fock approximation, and we show that the coulomb interaction, which gives rise to the divergence of Fock correction, should be replaced by the dynamically screened interaction. We also show that we should use the bare coulomb interaction for Hartree term. We take into account higher order contributions using a simple renormalization group approach. The temperature dependence of the transmission probability is qualitatively similar to that of a one-dimensional system.

cond-mat.mes-hall↗