Papers - Eto, Mikio

Superconducting Diode Effect in Double Quantum Dot Device

Takeuchi G., Eto M.

Journal of the Physical Society of Japan 94 ( 5 )  2025.05

ISSN  00319015

　View Summary

The superconducting diode effect (SDE) is theoretically examined in a double quantum dot coupled to three superconducting leads, L, R1, and R2. Lead L is commonly connected to two quantum dots (QD1 and QD2), whereas lead R1 (R2) is connected to QD1 (QD2) only. The phase differences φ1 between leads L and R1 and φ2 between leads L and R2 are tuned independently. The critical current into lead R1 depends on its direction unless’2 ¼ 0; π, which is ascribable to the formation of the Andreev molecule between the QDs. In the absence of the electron–electron interaction U in the QDs, the spectrum of the Andreev bound states forms Dirac cones in the φ1–φ2 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 φ2 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.

• Access to Document (DOI)

Numerical Simulation of Quantum Dot Interferometer in Kondo Regime

Zhang Y., Kato M., Sakano R., Eto M.

Journal of the Physical Society of Japan (Journal of the Physical Society of Japan)  93 ( 2 )  2024.02

ISSN  00319015

　View Summary

We propose a numerical method to simulate a transport experiment using a quantum dot interferometer made of two quantum wires in parallel [S. Takada et al., Phys. Rev. Lett. 113, 126601 (2014)]. The wires are partly tunnel-coupled to each other to form a mesoscopic ring with an embedded quantum dot. Our method consists of two stages. In the first stage, we represent the experimental system with a tight-binding model by discretizing the space. The conductance around a Coulomb peak is evaluated as a function of magnetic field in four-terminal geometry, where the Coulomb interaction is irrelevant. We show clear Aharonov–Bohm (AB) oscillations despite the multiple conduction channels and magnetic field inside the wires. In the second stage, we adopt a model of double quantum dot (DQD) in parallel. The model parameters are chosen to reproduce the Coulomb peak and AB oscillations obtained in the first stage in the absence of the Coulomb interaction U. Finally, we calculate the conductance in the Kondo valley using the DQD model in the presence of U. We observe phase locking at π=2, which is consistent with experimental results.

• Access to Document (DOI)

Kondo temperature evaluated from linear conductance in magnetic fields

Sakano R., Hata T., Motoyama K., Teratani Y., Tsutsumi K., Oguri A., Arakawa T., Ferrier M., Deblock R., Eto M., Kobayashi K.

Physical Review B (Physical Review B)  108 ( 20 )  2023.11

ISSN  24699950

　View Summary

We theoretically and experimentally study the universal scaling property of the spin-12 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 measure electric current through a carbon nanotube quantum dot.

• Access to Document (DOI)

Kondo Effect and Phase Measurement in Double Quantum Dot in Parallel

Zhang Y., Sakano R., Eto M.

Journal of the Physical Society of Japan (Journal of the Physical Society of Japan)  91 ( 1 )  2022.01

ISSN  00319015

　View Summary

We propose a double quantum dot (DQD) in parallel as a tractable model for a mesoscopic ring with an embedded quantum dot (QD), which functions as a QD interferometer. One of the DQDs (QD 1) represents the embedded QD with energy level ϵ1 and Coulomb interaction U, whereas the other (QD 2) acts as a reference arm with an enlarged linewidth due to the tunnel coupling to external leads. The conductance at temperature T = 0 is formulated simply in terms of the retarded and advanced Green's functions of the DQD, which are exactly evaluated using the Bethe ansatz solution for the Kondo effect. Using this model, we address two controversial issues concerning the QD interferometer. The first issue concerns the shape of the conductance peak as a function of ϵ1. We show a crossover from an asymmetric Fano resonance (Fano-Kondo effect) to a symmetric Breit-Wigner resonance (Kondo plateau) in the absence (presence) of U, by decreasing the connection between the QDs through the leads. The second is on the measurement of the transmission phase shift through QD 1 by a "double-slit experiment", which is impossible in the two-terminal geometry because of the Onsager's reciprocity theorem. In a three-terminal geometry, we discuss a possible observation of phase locking at π=2 in the Kondo regime.

• Access to Document (DOI)

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

Eto M., Sakano R.

Physical Review B (Physical Review B)  102 ( 24 )  2020.12

Research paper (scientific journal), Joint Work, Lead author, Accepted,  ISSN  24699950

　View Summary

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. Stefański, Phys. Rev. Lett. 86, 5128 (2001)PRLTAO0031-900710.1103/PhysRevLett.86.5128; W. Hofstetter, J. König, and H. Schoeller, ibid. 87, 156803 (2001)PRLTAO0031-900710.1103/PhysRevLett.87.156803] to consider multiple conduction channels in two external leads, L and R. We introduce a parameter pα (|pα|≤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 |pL,R|=1 in the case of single conduction channel in the leads and an almost symmetric Breit-Wigner resonance for |pL,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 pL,R. Our model is also applicable to the multiterminal 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.

• Access to Document (DOI)

Topological classification of the single-wall carbon nanotube

Okuyama R., Izumida W., Eto M.

Physical Review B (Physical Review B)  99 ( 11 )  2019.03

Research paper (scientific journal), Last author, Accepted,  ISSN  24699950

　View Summary

© 2019 American Physical Society. The single-wall carbon nanotube (SWNT) can be a one-dimensional topological insulator, which is characterized by a 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.

• Access to Document (DOI)

Fano Resonance in Transport Through Double Quantum Dot in Parallel

Zhang Y., Eto M.

Physica Status Solidi (B) Basic Research (Physica Status Solidi (B) Basic Research)  256 ( 6 )  2019

ISSN  03701972

　View Summary

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim The transport properties of double quantum dot (DQD) in parallel are theoretically examined. This system simulates a mesoscopic ring with an embedded quantum dot, so-called Aharonov–Bohm (AB) interferometer, by enlarging the line width in one of the quantum dots. We find that: 1) the Coulomb peak shows an asymmetric shape of Fano resonance when the DQD is coupled to external leads with single conduction channel; 2) the peak becomes more symmetric for the leads with multiple channels; and 3) the resonant shape is characterized by a single parameter which is determined by the structure of the leads. Our theory may explain the experimental results on the transport through the AB interferometer; an asymmetric Fano resonance was observed by one group while a symmetric Breit–Wigner resonance was observed by the other groups.

• Access to Document (DOI)

Light Controlled Optical Aharonov-Bohm Oscillations in a Single Quantum Ring

Kim H., Park S., Okuyama R., Kyhm K., Eto M., Taylor R., Nogues G., Dang L., Potemski M., Je K., Kim J., Kyhm J., Song J.

Nano Letters (Nano Letters)  18 ( 10 ) 6188 - 6194 2018.10

ISSN  15306984

　View Summary

© 2018 American Chemical Society. We found that optical Aharonov-Bohm oscillations in a single GaAs/GaAlAs quantum ring can be controlled by excitation intensity. With a weak excitation intensity of 1.2 kW cm -2 , the optical Aharonov-Bohm oscillation period of biexcitons was observed to be half that of excitons in accordance with the period expected for a two-exciton Wigner molecule. When the excitation intensity is increased by an order of magnitude (12 kW cm -2 ), a gradual deviation of the Wigner molecule condition occurs with decreased oscillation periods and diamagnetic coefficients for both excitons and biexcitons along with a spectral shift. These results suggest that the effective orbit radii and rim widths of electrons and holes in a single quantum ring can be modified by light intensity via photoexcited carriers, which are possibly trapped at interface defects resulting in a local electric field.

• Access to Document (DOI)

Topology in single-wall carbon nanotube of zigzag and armchair type

R. Okuyama, W. Izumida, and M. Eto

Journal of Physics: Conference Series (Journal of Physics: Conference Series)  969 ( 1 ) 012137/1-6 2018.04

Research paper (international conference proceedings), Joint Work, Accepted,  ISSN  17426588

　View Summary

単層カーボンナノチューブのトポロジーを、ジグザク型およびアームチェア型に対して調べた。

• Access to Document (DOI)

Enhanced current fluctuation in Coulomb blockade regime of multilevel quantum dot

Atsushi Iwasaki and M. Eto

Journal of Physics: Conference Series 864   012034/1-4 2017.04

Research paper (international conference proceedings), Joint Work, Accepted

　View Summary

クーロンブロッケード領域にある多準位量子ドットに対して電流揺らぎを定式化し、その増大機構を明らかにした。

• Access to Document (DOI)

Spin-dependent transport in multi-terminal Aharonov-Casher ring with quantum dot

Taku Matsunaga, Rui Sakano, and M. Eto

Journal of Physics: Conference Series 864   012059/1-4 2017.04

Research paper (international conference proceedings), Joint Work, Accepted

　View Summary

量子ドットを埋め込んだ微小リングにおけるアハラノフ・キャッシャー効果を定式化し、多端子測定によるスピン分極電流の生成を明らかにした。

• Access to Document (DOI)

Topological Phase Transition in Metallic Single-Wall Carbon Nanotube

Rin Okuyama, Wataru Izumida, and Mikio Eto

Journal of the Physical Society of Japan 86 ( 1 ) 013702/1-4 2017.01

Research paper (scientific journal), Joint Work, Accepted

　View Summary

金属型の単層カーボンナノチューブのトポロジカル絶縁体としての性質を理論的に調べた。そのトポロジーは「巻き付き数」と呼ばれる整数で特徴付けられること、多くの場合に巻き付き数がゼロでない値を取ること、を有効１次元モデルを構築することで数値的に示した。さらに、チューブに平行な磁場を引火することで巻き付き数が変化する「トポロジカル相転移」が生じることを予言した。その相転移は、チューブの端状態の数の変化として実験で観測可能である。

• Access to Document (DOI)

Observation of a Biexciton Wigner Molecule by Fractional Optical Aharonov-Bohm Oscillations in a Single Quantum Ring

Hee Dae Kim, Rin Okuyama, Kwangseuk Kyhm, Mikio Eto, Robert A. Taylor, Aurelien L. Nicolet, Marek Potemski, Gilles Nogues, Le Si Dang, Ku-Chul Je, Jongsu Kim, Ji-Hoon Kyhm, Kyu Hyoek Yoen, Eun Hye Lee, Jun Young Kim, Il Ki Han, Wonjun Choi, and Jindong Song

NANO Letters (American Chemical Society)  16   27 - 33 2015.12

Research paper (scientific journal), Joint Work, Accepted

　View Summary

リング形をした半導体量子ドットにおける光学実験によって、バイエキシトンのウィグナー分子形成の観測に成功した。磁場をかけたときの光学アハラノフ・ボーム効果を測定し、ウィグナー分子に対する理論計算の結果と定量的な一致を見た。

• Access to Document (DOI)

会議だより：第21回２次元電子系国際会議（EP2DS-21）

江藤 幹雄

固体物理 (アグネ技術出版)  50 ( 12 ) 833 - 838 2015.12

Research paper (scientific journal), Single Work, Accepted

　View Summary

2015年8月に仙台で開かれた第２１回２次元電子系国際会議の報告。低次元物理学の最近の研究についてレヴューした。

Spin Blocker Using the Interband Rashba Effect in Symmetric Double Quantum Wells

S. Souma, A. Sawada, H. Chen, Y. Sekine, M. Eto, and T. Koga

Physical Review Applied (American Physical Society)  4 ( 3 ) 034010/1-9 2015.09

Research paper (scientific journal), Joint Work, Accepted

　View Summary

２重量子井戸を利用した、スピン分極電流を生成する新しいデバイスを提案した。２つの量子井戸に反対称に電場を与え、両者に符号が互いに異なるスピン軌道相互作用を発生させる。これに外部磁場をかけることで、一方のスピンのみが共鳴的に反射されるアイディアを提案し、その効率を数値計算によって確かめた。

• Access to Document (DOI)

量子力学と方程式

江藤 幹雄

数理科学（特集「物理と方程式: 現象はいかに記述されるか」） (サイエンス社)  623   26 - 31 2015.05

Research paper (scientific journal), Single Work

　View Summary

量子力学において、物理現象が方程式によってどのように記述されるか、について解説を行った。

Study of coupled QED-cavities using the self-consistent Mori projector method

S. Okubo, M. Eto, P. Degenfeld-Schonburg, and M. J. Hartmann

Journal of Physics: Conference Series 568   012014/1-5 2014.12

Research paper (international conference proceedings), Joint Work, Accepted

　View Summary

QED-cavity結合系の非平衡特性を理論的に調べた。少数系での時間発展を密度行列を用いて定式化した。無限周期系に対して自己無撞着Mori射影法を適用し、定常状態を導出した。

• Access to Document (DOI)

Critical current in semiconductor nanowire Josephson junctions in the presence of magnetic field

T. Yokoyama, M. Eto, and Yu. V. Nazarov

Journal of Physics: Conference Series 568   052035/1-5 2014.12

Research paper (international conference proceedings), Joint Work, Accepted

　View Summary

半導体ナノワイヤのジョセフソン接合を理論的に考察し、超伝導臨界電流の磁場依存性を明らかにした。

• Access to Document (DOI)

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

T. Yokoyama, M. Eto, and Yu. V. Nazarov

Physical Review B 89 ( 19 ) 195407/1-14 2014.05

Research paper (scientific journal), Joint Work, Accepted

　View Summary

半導体ナノワイヤに超伝導電極を結合した系において、スピン軌道相互作用のジョセフソン効果への影響を数値計算によって詳細に調べた。現実のナノワイヤを離散化し、擬1次元タイト・バインディング・モデルによって不純物散乱とスピン起動相互作用を表した。磁場中のゼーマン効果がある場合、超伝導体間の位相差がない場合に超伝導電流の流れる異常ジョセフソン効果、および臨界電流の方向依存性を明らかにした。

• Access to Document (DOI)

Effect of spin-orbit interaction on supercurrent in semiconductor nanowire

T. Yokoyama, M. Eto, and Yu. V. Nazarov

JPS Conference Proceedings 1   012023/1-4 2014.03

Research paper (international conference proceedings), Joint Work, Accepted

　View Summary

半導体ナノワイヤを流れる超伝導電流におけるスピン軌道相互作用の効果を明らかにした。

• Access to Document (DOI)