arXiv subjects

Mikio Eto

Publications and source records attributed to Mikio Eto.

At least 19 recordsLinked to original sources

arXiv2026

Photon-Assisted Tunneling in Double Quantum Dot: Application of Scattering Theory

We theoretically examine the photon-assisted tunneling (PAT) in a double quantum dot (DQD) in parallel when one of the quantum dots (QDs) is irradiated by an AC field. First, we formulate the PAT in a single QD by solving the time-dependent Schr\"odinger equation using the scattering theory. The QD has an oscillating energy level, $\varepsilon(t)=\varepsilon_0+eV_{\mathrm{AC}}\cos\omega t$, and is connected to two leads by the tunnel coupling $\Gamma$. We show that the resonant tunneling takes place through energy levels of the polariton, $\varepsilon_0+N\hbar\omega$ ($N=0,\pm 1, \pm 2, \cdots$), when $\Gamma \ll \hbar\omega$ (PAT) and through the energy level $\varepsilon(t)$ when $\Gamma \gg \hbar\omega$ (adiabatic transport). Then, the scattering theory is applied to the PAT in the DQD in the presence of magnetic flux penetrating between the QDs. We observe the Aharonov--Bohm effect not only in the main peak ($N=0$) but also in subpeaks ($N \ne 0$), indicating coherent transport through the polariton states. Our theory is also applicable to the DQD in the three-terminal geometry. We demonstrate the phase measurement through the irradiated QD and show that the measured phase shift changes continuously from 0 to $\pi$ around both the main peak and subpeaks.

cond-mat.mes-hall↗

arXiv2025

Tuning of Weyl point emergence in multi-terminal Josephson junctions using quantum point contacts

Multi-terminal Josephson junction with three or more superconductors is an attractive quantum system to emerge and tune exotic electronic states. In four terminal Josephson junctions, the Weyl physics, namely topologically protected zero energy state, emerges without assuming any exotic materials. In this study, we consider the four-terminal Josephson junction with the quantum point contact structures between the mesoscopic normal region and four superconducting terminals. The quantum point contacts can tune electrically the number of conduction channels. We theoretically investigate an effect of the increase of channels on the emergence of Weyl points. The increase of channels causes the increase of Andreev bound states in the system, which increase the emergence probability of Weyl points. When all terminals have two channels, the emergence probability is up to 17\%, which is about four times larger than that for all single channel junctions. We consider the balance of the number of conduction channels in the four terminals. When the number of channels is unbalanced, the increase of emergence probability is suppressed.

cond-mat.mes-hall↗

arXiv2024

Superconducting Diode Effect in Double Quantum Dot Device

Superconducting diode effect (SDE) is theoretically examined in double quantum dot coupled to three superconducting leads, $L$, $R1$ and $R2$. Lead $L$ is commonly connected to two quantum dots (QD1, QD2) while lead $R1$ ($R2$) is connected to QD1 (QD2) only. The phase differences $\varphi_{1}$ between leads $L$ and $R1$ and $\varphi_{2}$ between leads $L$ and $R2$ are tuned independently. The critical current into lead $R1$ depends on its direction unless $\varphi_{2} = 0$, $\pi$, which is ascribable to the formation of Andreev molecule between the QDs. In the absence of electron-electron interaction $U$ in the QDs, the spectrum of the Andreev bound states forms Dirac cones in the $\varphi_{1}-\varphi_{2}$ plane if the energy levels in the QDs are tuned to the Fermi level in the leads. The SDE is enhanced to almost 30\% when $\varphi_{2}$ is set to the value at the Dirac points. In the presence of $U$, the SDE is still observed when $U$ is smaller than the superconducting energy gap in the leads. Our device should be one of the minimal models for the SDE since a similar device with a single QD does not show the SDE.

cond-mat.mes-hall↗

arXiv2023

Kondo Temperature Evaluated from Linear Conductance in Magnetic Fields

We theoretically and experimentally study the universal scaling property of the spin-1/2 Kondo state in the magnetic field dependence of bias-voltage linear conductance through a quantum dot at low temperatures. We discuss an efficient and reliable procedure to evaluate the Kondo temperature defined at the ground state from experimental or numerical data sets of the magnetic field dependence of the linear conductance or the magnetization of the quantum dot. This procedure is helpful for quantitative comparison of the theory and the experiment, and useful in Kondo-correlated systems where temperature control over a wide range is difficult, such as for cold atoms. We demonstrate its application to experimentally measured electric current through a carbon nanotube quantum dot.

cond-mat.mes-hall↗

arXiv2020

Fano-Kondo resonance versus Kondo plateau in an Aharonov-Bohm ring with an embedded quantum dot

We theoretically examine the transport through an Aharonov-Bohm ring with an embedded quantum dot (QD), the so-called QD interferometer, to address two controversial issues regarding the shape of the Coulomb peaks and measurement of the transmission phase shift through a QD. We extend a previous model [B. R. Bulka and P. Stefanski, Phys. Rev. Lett. 86, 5128 (2001); W. Hofstetter, J. Konig, and H. Schoeller, ibid. 87, 156803 (2001)] to consider multiple conduction channels in two external leads, L and R. We introduce a parameter p_α (|p_α| \le 1) to characterize a connection between the two arms of the ring through lead α(=L, R), which is the overlap integral between the conduction modes coupled to the two arms. First, we study the shape of a conductance peak as a function of energy level in the QD, in the absence of electron-electron interaction U. We show an asymmetric Fano resonance for |p_{L,R}| = 1 in the case of single conduction channel in the leads and an almost symmetric Breit-Wigner resonance for |p_{L,R}| < 0.5 in the case of multiple channels. Second, the Kondo effect is taken into account by the Bethe ansatz exact solution in the presence of U. We precisely evaluate the conductance at temperature T=0 and show a crossover from an asymmetric Fano-Kondo resonance to the Kondo plateau with changing p_{L,R}. Our model is also applicable to the multi-terminal geometry of the QD interferometer. We discuss the measurement of the transmission phase shift through the QD in a three-terminal geometry by a "double-slit experiment." We derive an analytical expression for the relation between the measured value and the intrinsic value of the phase shift.

cond-mat.mes-hall↗

arXiv2019

Topological classification of the single-wall carbon nanotube

The single-wall carbon nanotube (SWNT) can be a one-dimensional topological insulator, which is characterized by a $\mathbb{Z}$-topological invariant, winding number. Using the analytical expression for the winding number, we classify the topology for all possible chiralities of SWNTs in the absence and presence of a magnetic field, which belongs to the topological categories of BDI and AIII, respectively. We find that the majority of SWNTs are nontrivial topological insulators in the absence of a magnetic field. In addition, the topological phase transition takes place when the band gap is closed by applying a magnetic field along the tube axis, in all the SWNTs except armchair nanotubes. The winding number determines the number of edge states localized at the tube ends by the bulk-edge correspondence, the proof of which is given for SWNTs in general. This enables the identification of the topology in experiments.

cond-mat.mes-hall↗

arXiv2016

Topological Phase Transition in Metallic Single-Wall Carbon Nanotube

The topological phase transition is theoretically studied in a metallic single-wall carbon nanotube (SWNT) by applying a magnetic field $B$ parallel to the tube. The $\mathbb{Z}$ topological invariant, winding number, is changed discontinuously when a small band gap is closed at a critical value of $B$, which can be observed as a change in the number of edge states owing to the bulk-edge correspondence. This is confirmed by numerical calculations for finite SWNTs of $\sim$ 1 $μ$m length, using a one-dimensional lattice model to effectively describe the mixing between $σ$ and $π$ orbitals and spin-orbit interaction, which are relevant to the formation of the band gap in metallic SWNTs.

cond-mat.mes-hall↗

arXiv2014

Critical current oscillation by magnetic field in semiconductor nanowire Josephson junction

We study theoretically the critical current in semiconductor nanowire Josephson junction with strong spin-orbit interaction. The critical current oscillates by an external magnetic field. We reveal that the oscillation of critical current depends on the orientation of magnetic field in the presence of spin-orbit interaction. We perform a numerical simulation for the nanowire by using a tight-binding model. The Andreev levels are calculated as a function of phase difference $φ$ between two superconductors. The DC Josephson current is evaluated from the Andreev levels in the case of short junctions. The spin-orbit interaction induces the effective magnetic field. When the external field is parallel with the effective one, the critical current oscillates accompanying the $0$-$π$ like transition. The period of oscillation is longer as the angle between the external and effective fields is larger.

cond-mat.mes-hall↗

arXiv2014

Anomalous Josephson effect induced by spin-orbit interaction and Zeeman effect in semiconductor nanowires

We investigate theoretically the Josephson junction of semiconductor nanowire with strong spin-orbit (SO) interaction in the presence of magnetic field. By using a tight-binding model, the energy levels $E_n$ of Andreev bound states are numerically calculated as a function of phase difference $φ$ between two superconductors in the case of short junctions. The DC Josephson current is evaluated from the Andreev levels. In the absence of SO interaction, a $0$-$π$ transition due to the magnetic field is clearly observed. In the presence of SO interaction, the coexistence of SO interaction and Zeeman effect results in $E_n (-φ) \ne E_n (φ)$, where the anomalous Josephson current flows even at $φ=0$. In addition, the direction-dependence of critical current is observed, in accordance with experimental results.

cond-mat.mes-hall↗

arXiv2013

Lasing and antibunching of optical phonons in semiconductor double quantum dots

We theoretically propose optical phonon lasing in a double quantum dot (DQD) fabricated on a semiconductor substrate. No additional cavity or resonator is required. An electron in the DQD is found to be coupled to only two longitudinal optical phonon modes that act as a natural cavity. When the energy level spacing in the DQD is tuned to the phonon energy, the electron transfer is accompanied by the emission of the phonon modes. The resulting non-equilibrium motion of electrons and phonons is analyzed by the rate equation approach based on the Born-Markov-Secular approximation. We show that the lasing occurs for pumping the DQD via electron tunneling at rate much larger than the phonon decay rate, whereas a phonon antibunching is observed in the opposite regime of slow tunneling. Both effects disappear by an effective thermalization induced by the Franck-Condon effect in a DQD fabricated in a suspended carbon nanotube with strong electron-phonon coupling.

cond-mat.mes-hall↗

arXiv2013

Josephson Current through Semiconductor Nanowire with Spin-Orbit Interaction in Magnetic Field

We theoretically study the DC Josephson effect of a semiconductor nanowire (NW) with strong spin-orbit interaction when a magnetic field is applied parallel to the NW. We adopt a model of single scatterer in a quasi-one-dimensional system for the case of short junctions where the size of normal region is much smaller than the coherent length. In the case of single conduction channel in the model, we obtain analytical expressions for the energy levels of Andreev bound states, $E_n$, and supercurrent $I$, as a function of phase difference $φ$ between two superconductors. We show the 0-$π$ transition by tuning the magnetic field. In the case of more than one conduction channel, we find that $E_n (-φ) \ne E_n (φ)$ by the interplay between the spin-orbit interaction and Zeeman effect, which results in finite supercurrent at $φ=0$ (anomalous Josephson current) and direction-dependent critical current.

cond-mat.mes-hall↗

arXiv2012

Optical Phonon Lasing in Semiconductor Double Quantum Dots

We propose optical phonon lasing for a double quantum dot (DQD) fabricated in a semiconductor substrate. We show that the DQD is weakly coupled to only two LO phonon modes that act as a natural cavity. The lasing occurs for pumping the DQD via electronic tunneling at rates much higher than the phonon decay rate, whereas an antibunching of phonon emission is observed in the opposite regime of slow tunneling. Both effects disappear with an effective thermalization induced by the Franck-Condon effect in a DQD fabricated in a carbon nanotube with a strong electron-phonon coupling.

cond-mat.mes-hall↗

arXiv2012

Generation of spin-polarized current using multi-terminated quantum dot with spin-orbit interaction

We theoretically examine generation of spin-polarized current using multi-terminated quantum dot with spin-orbit interaction. First, a two-level quantum dot is analyzed as a minimal model, which is connected to $N$ ($\ge 2$) external leads via tunnel barriers. When an unpolarized current is injected to the quantum dot from a lead, a polarized current is ejected to others, similarly to the spin Hall effect. In the absence of magnetic field, the generation of spin-polarized current requires $N \ge 3$. The polarization is markedly enhanced by resonant tunneling when the level spacing in the quantum dot is smaller than the level broadening due to the tunnel coupling to the leads. In a weak magnetic field, the orbital magnetization creates a spin-polarized current even in the two-terminal geometry (N=2). The numerical study for generalized situations confirms our analytical result using the two-level model.

cond-mat.mes-hall↗

arXiv2011

Optical Aharonov-Bohm Effect on Wigner Molecules in Type-II Semiconductor Quantum Dots

We theoretically examine the magnetoluminescence from a trion and a biexciton in a type-II semiconductor quantum dot, in which holes are confined inside the quantum dot and electrons are in a ring-shaped region surrounding the quantum dot. First, we show that two electrons in the trion and biexciton are strongly correlated to each other, forming a Wigner molecule: Since the relative motion of electrons is frozen, they behave as a composite particle whose mass and charge are twice those of a single electron. As a result, the energy of the trion and biexciton oscillates as a function of magnetic field with half the period of the single-electron Aharonov-Bohm oscillation. Next, we evaluate the photoluminescence. Both the peak position and peak height change discontinuously at the transition of the many-body ground state, implying a possible observation of the Wigner molecule by the optical experiment.

cond-mat.mes-hall↗

arXiv2010

Scaling analysis of Kondo screening cloud in a mesoscopic ring with an embedded quantum dot

The Kondo effect is theoretically studied in a quantum dot embedded in a mesoscopic ring. The ring is connected to two external leads, which enables the transport measurement. Using the "poor man's" scaling method, we obtain analytical expressions of the Kondo temperature T_K as a function of the Aharonov-Bohm phase ϕby the magnetic flux penetrating the ring. In this Kondo problem, there are two characteristic lengths. One is the screening length of the charge fluctuation, L_c=\hbar v_F/ |ε_0|, where v_F is the Fermi velocity and ε_0 is the energy level in the quantum dot. The other is the screening length of spin fluctuation, i.e., size of Kondo screening cloud, L_K=\hbar v_F/ T_K. We obtain different expressions of T_K(ϕ) for (i) L_c \ll L_K \ll L, (ii) L_c \ll L \ll L_K, and (iii) L \ll L_c \ll L_K, where L is the size of the ring. T_K is markedly modulated by ϕin cases (ii) and (iii), whereas it hardly depends on ϕin case (i). We also derive logarithmic corrections to the conductance at temperature T\gg T_K and an analytical expression of the conductance at T\ll T_K, on the basis of the scaling analysis.

cond-mat.mes-hall↗

arXiv2010

Quantum Dot Spin Filter in Resonant Tunneling and Kondo Regimes

A quantum dot with spin-orbit interaction can work as an efficient spin filter if it is connected to N (> 2) external leads via tunnel barriers. When an unpolarized current is injected to a quantum dot from a lead, polarized currents are ejected to other leads. A two-level quantum dot is examined as a minimal model. First, we show that the spin polarization is markedly enhanced by resonant tunneling when the level spacing in the dot is smaller than the level broadening. Next, we examine the many-body resonance induced by the Kondo effect in the Coulomb blockade regime. A large spin current is generated in the presence of the SU(4) Kondo effect when the level spacing is less than the Kondo temperature.

cond-mat.mes-hall↗

arXiv2010

Excitonic Aharonov-Bohm Effect in Isotopically Pure 70Ge/Si Type-II Quantum Dots

We report on a magneto-photoluminescence study of isotopically pure 70Ge/Si self-assembled type-II quantum dots. Oscillatory behaviors attributed to the Aharonov-Bohm effect are simultaneously observed for the emission energy and intensity of excitons subject to an increasing magnetic field. When the magnetic flux penetrates through the ring-like trajectory of an electron moving around each quantum dot, the ground state of an exciton experiences a change in its angular momentum. Our results provide the experimental evidence for the phase coherence of a localized electron wave function in group-IV Ge/Si self-assembled quantum structures.

cond-mat.mes-hall↗

arXiv2009

Enhanced spin Hall effect by tuning antidot potential: Proposal for a spin filter

We propose an efficient spin filter including an antidot fabricated on semiconductor heterostructures with strong spin-orbit interaction. The antidot creates a tunable potential on two-dimensional electron gas in the heterostructures, which may be attractive as well as repulsive. Our idea is based on the enhancement of extrinsic spin Hall effect by resonant scattering when the attractive potential is properly tuned. Numerical studies for three- and four-terminal devices indicate that the efficiency of the spin filter can be more than 50% by tuning the potential to the resonant condition.

cond-mat.mes-hall↗