Showing 1–50 of 60 results for author: Kubo, T

Three-Dimensional Niobium Coaxial Cavity with $\sim0.1\,$second Lifetime

Authors: Takaaki Takenaka, Takayuki Kubo, Imran Mahboob, Kosuke Mizuno, Hitoshi Inoue, Takayuki Saeki, Shiro Saito

Abstract: We report on the internal quality factor of a three-dimensional niobium quarter-wave coaxial cavity, with mid-temperature annealing, exhibiting $Q_{\rm int} \gtrsim 3\times10^9$ at the single-photon level below 20\,mK, which corresponds to an internal photon lifetime of $τ_{\rm int}\sim90\,\mathrm{ms}$. Moreover, $Q_{\rm int}$ of the mid-temperature annealed cavities remains almost unchanged even… ▽ More We report on the internal quality factor of a three-dimensional niobium quarter-wave coaxial cavity, with mid-temperature annealing, exhibiting $Q_{\rm int} \gtrsim 3\times10^9$ at the single-photon level below 20\,mK, which corresponds to an internal photon lifetime of $τ_{\rm int}\sim90\,\mathrm{ms}$. Moreover, $Q_{\rm int}$ of the mid-temperature annealed cavities remains almost unchanged even after several cooldown cycles and air exposure. These results suggest that stable low-loss niobium oxides might be formed by mid-temperature annealing on the surface of three-dimensional niobium cavity. This surface treatment could be applicable to the fabrication of 2D superconducting circuits and help improve the lifetime of Nb-based superconducting qubits. △ Less

Submitted 2 October, 2025; v1 submitted 2 October, 2025; originally announced October 2025.

Nonequilibrium nonlinear response theory of amplitude-dependent dissipative conductivity in disordered superconductors

Authors: Takayuki Kubo

Abstract: This work investigates amplitude-dependent nonlinear corrections to the dissipative conductivity in superconductors, using the Keldysh--Usadel theory of nonequilibrium superconductivity, which captures the nonequilibrium dynamics of both quasiparticles and the pair potential. Our rigorous formulation naturally incorporates both the direct nonlinear action of the photon field and indirect contribut… ▽ More This work investigates amplitude-dependent nonlinear corrections to the dissipative conductivity in superconductors, using the Keldysh--Usadel theory of nonequilibrium superconductivity, which captures the nonequilibrium dynamics of both quasiparticles and the pair potential. Our rigorous formulation naturally incorporates both the direct nonlinear action of the photon field and indirect contributions mediated by nonequilibrium variations in the pair potential, namely the Eliashberg effect and the Higgs mode. The third-harmonic current, often regarded as a hallmark of the Higgs mode in disordered superconductors, arises from both the direct photon action and the Higgs mode. Our numerical results are in excellent agreement with previous studies. In contrast, the first-harmonic current, and consequently the dissipative conductivity, receives contributions from all three mechanisms: the direct photon action, the Higgs mode, and the Eliashberg effect. It is shown that that the nonlinear correction to dissipative conductivity in dirty-limit superconductors can serve as a fingerprint of the Higgs mode, appearing as a resonance peak at a frequency near the superconducting gap \( Δ\). In addition, our results provide microscopic insight into amplitude-dependent dissipation at frequencies well below \( Δ\), which is particularly relevant for applied superconducting devices. In particular, the long-standing issue concerning the frequency dependence of the amplitude-dependent quality factor is explained as originating from the direct nonlinear action of the photon field, rather than from contributions by the Higgs mode and the Eliashberg effect. Our practical and explicit expression for the nonlinear conductivity formula makes our results accessible to a broad range of researchers. △ Less

Submitted 26 November, 2025; v1 submitted 11 September, 2025; originally announced September 2025.

Comments: 20 pages, 9 figures. The journal accepted version

Journal ref: Physical Review Applied 24, 064061 (2025)

Higgs-mode-induced instability and kinetic inductance in strongly dc-biased dirty-limit superconductors

Authors: Takayuki Kubo

Abstract: A perturbative ac field superposed on a dc bias ($J_b$) is known to excite the Higgs mode in superconductors. The dirty limit, where disorder enhances the Higgs resonance, provides an ideal setting for this study and is also relevant to many superconducting devices operating under strong dc biases. In this paper, we derive a general formula for the complex conductivity of disordered superconductor… ▽ More A perturbative ac field superposed on a dc bias ($J_b$) is known to excite the Higgs mode in superconductors. The dirty limit, where disorder enhances the Higgs resonance, provides an ideal setting for this study and is also relevant to many superconducting devices operating under strong dc biases. In this paper, we derive a general formula for the complex conductivity of disordered superconductors under an arbitrary dc bias using the Keldysh-Usadel theory of nonequilibrium superconductivity. This formula is relatively simple, making it more accessible to a broader research community. Our analysis reveals that in a strongly dc-biased dirty-limit superconductor, the Higgs mode induces an instability in the homogeneous superflow within a specific frequency window, making the high-current-carrying state vulnerable to ac perturbations. This instability, which occurs exclusively in the ${\rm ac} \parallel {\rm dc}$ configuration, leads to a non-monotonic dependence of kinetic inductance on frequency and bias strength. By carefully tuning the dc bias and the frequency of the ac perturbation, the kinetic inductance can be enhanced by nearly two orders of magnitude. In the weak dc bias regime, our formula recovers the well-known quadratic dependence, $L_k \propto 1+ C(J_b/J_{\rm dp})^2$, with coefficients $C=0.409$ for ${\rm ac} \parallel {\rm dc}$ and $C=0.136$ for ${\rm ac} \perp {\rm dc}$, where $J_{\rm dp}$ is the equilibrium depairing current density. These findings establish a robust theoretical framework for dc-biased superconducting systems and suggest that Higgs mode physics could be exploited in the design and optimization of superconducting detectors. Moreover, they may lead to a yet-to-be-explored detector concept based on Higgs mode physics. △ Less

Submitted 9 February, 2025; originally announced February 2025.

Comments: 15 pages, 8 figures

Journal ref: Physical Review Applied 23, 054091 (2025)

How High a Field Can Be and Has Been Achieved in Superconducting Bulk Niobium Cavities Across Different RRR Values?

Authors: Takayuki Kubo

Abstract: This Brief Note explores the relationship between residual resistivity ratio (RRR) and the maximum surface magnetic field in superconducting bulk niobium (Nb) cavities. Data from the 1980s to 2020s, covering RRR values from 30 to 500, are compared with theoretical performance limits, including the lower critical field (Bc1), superheating field (Bsh), and thermal runaway field (Brun). The results s… ▽ More This Brief Note explores the relationship between residual resistivity ratio (RRR) and the maximum surface magnetic field in superconducting bulk niobium (Nb) cavities. Data from the 1980s to 2020s, covering RRR values from 30 to 500, are compared with theoretical performance limits, including the lower critical field (Bc1), superheating field (Bsh), and thermal runaway field (Brun). The results show that modern Nb cavities are approaching Brun and the metastability region above Bc1 across the entire RRR range but remain below the fundamental limit at Bsh. Achieving Bsh requires not only advanced high-gradient surface processing but also improved thermal stability with low surface resistance, ultra-pure Nb, and optimized Kapitza conductance to ensure Brun > Bsh. △ Less

Submitted 23 October, 2024; originally announced October 2024.

Comments: 5 pages, 2 figures. This is the initial version submitted to a journal

Journal ref: Jpn. J. Appl. Phys. 64, 018002 (2025)

On the Applicability Ranges of Tc Formulas for Proximity-Coupled Thin SN and SS Bilayers

Authors: Takayuki Kubo

Abstract: This brief note revisits the well-established $T_c$ formulas for proximity-coupled thin superconductor-normal conductor (SN) and superconductor-superconductor (SS) bilayers, highlighting their relationships and clarifying their ranges of applicability. Since these formulas are often misapplied in practical contexts, this note provides guidance for their correct use in experimental and applied sett… ▽ More This brief note revisits the well-established $T_c$ formulas for proximity-coupled thin superconductor-normal conductor (SN) and superconductor-superconductor (SS) bilayers, highlighting their relationships and clarifying their ranges of applicability. Since these formulas are often misapplied in practical contexts, this note provides guidance for their correct use in experimental and applied settings. For SN bilayers, McMillan's formula is recommended for its broad applicability, with its SS counterpart offering similar reliability. △ Less

Submitted 10 October, 2024; originally announced October 2024.

Comments: 4 pages, 3 figures; the journal version includes supplementary materials with formula derivations and Mathematica code

Journal ref: Jpn. J. Appl. Phys. 64, 018001 (2025)

Static and Dynamic Electronic Properties of Weyl Semimetal NbP -- A Single Crystal $^{93}$Nb-NMR Study

Authors: Tetsuro Kubo, Hiroshi Yasuoka, Deepa Kasinathan, K. M. Ranjith, Marcus Schmidt, Michael Baenitz

Abstract: Nuclear magnetic resonance (NMR) techniques have been used to study the static and dynamic microscopic properties of the Weyl semimetal NbP. From a complete analysis of the angular dependence of the $^{93}$Nb-NMR spectra in a single crystal, the parameters for the electric quadrupole interactions and the magnetic hyperfine interactions were determined to be $ν_{\rm Q} = 0.61$\,MHz, $η= 0.20$,… ▽ More Nuclear magnetic resonance (NMR) techniques have been used to study the static and dynamic microscopic properties of the Weyl semimetal NbP. From a complete analysis of the angular dependence of the $^{93}$Nb-NMR spectra in a single crystal, the parameters for the electric quadrupole interactions and the magnetic hyperfine interactions were determined to be $ν_{\rm Q} = 0.61$\,MHz, $η= 0.20$, $(K_{XX}, K_{YY}, K_{ZZ}) = (- 0.06, 0.11, - 0.11)$\% at 4.5\,K. The temperature and field dependence of the $^{93}$Nb Knight shift revealed a characteristic feature of the shape of the density of states with nearly massless fermions. We clearly observed a quantum oscillation of the Knight shift associated with the band structure, whose frequency was in good agreement with the previous bulk measurements. The temperature dependence of the spin-lattice relaxation rate, $1 / T_{1} T$, showed an almost constant behavior for $30 < T < 180$\,K, while a weak temperature dependence was observed below $\sim 30$\,K. This contrasts with the behavior observed in TaP and TaAs, where the $1 / T_{1} T$ measured by the $^{181}$Ta nuclear quadrupole resonance (NQR) shows $1 / T_{1} T \propto T^{2}$ and $T^{4}$ above approximately 30\,K. In TaP, the temperature dependent orbital hyperfine interaction plays a signficant role in nuclear relaxation, whereas this contribution is not observed in TaAs. Two-component spin echo oscillations were observed. The shorter-period oscillation is attributed to the origin of quadrupole coupling, while the longer-period oscillation indicates the presence of indirect nuclear spin-spin coupling, as discussed in other Weyl semimetal like TaP. △ Less

Submitted 17 October, 2024; originally announced October 2024.

Significant Contributions of the Higgs Mode and Self-Energy Corrections to Low-Frequency Complex Conductivity in DC-Biased Superconducting Devices

Authors: Takayuki Kubo

Abstract: We investigate the complex conductivity of superconductors under a DC bias based on the Keldysh-Eilenberger formalism of nonequilibrium superconductivity. This framework allows us to account for the Higgs mode and impurity scattering self-energy corrections, which are known to significantly impact the complex conductivity under a bias DC, especially near the resonance frequency of the Higgs mode.… ▽ More We investigate the complex conductivity of superconductors under a DC bias based on the Keldysh-Eilenberger formalism of nonequilibrium superconductivity. This framework allows us to account for the Higgs mode and impurity scattering self-energy corrections, which are known to significantly impact the complex conductivity under a bias DC, especially near the resonance frequency of the Higgs mode. The purpose of this paper is to explore the effects of these contributions on the low-frequency complex conductivity relevant to superconducting device technologies. Our approach enables us to derive the complex conductivity formula for superconductors ranging from clean to dirty limits, applicable to any bias DC strength. Our calculations reveal that the Higgs mode and impurity scattering self-energy corrections significantly affect the complex conductivity even at low frequencies, relevant to superconducting device technologies. Specifically, we find that the real part of the low-frequency complex conductivity exhibits a bias-dependent reduction up to \(\hbar ω\sim 0.1\), a much higher frequency than previously considered. This finding allows for the suppression of dissipation in devices by tuning the bias DC. Additionally, through the calculation of the imaginary part of the complex conductivity, we evaluate the bias-dependent kinetic inductance for superconductors ranging from clean to dirty limits. The bias dependence becomes stronger as the mean free path decreases. Our dirty-limit results coincide with previous studies based on the so-called slow experiment scenario. This widely used scenario can be understood as a phenomenological implementation of the Higgs mode into the kinetic inductance calculation, now justified by our calculation based on the robust theory of nonequilibrium superconductivity, which microscopically treats the Higgs mode contribution. △ Less

Submitted 1 August, 2024; originally announced August 2024.

Comments: 12 pages, 8 figures. This is the initial version submitted to a journal and is currently under revision

Journal ref: Physical Review Applied 22, 044042 (2024)

Metastable ordered states induced by low temperature annealing of δ-Ag2/3V2O5

Authors: T. Kubo, K. Kojima, N. Katayama, T. Runčevski, R. E. Dinnebier, A. S. Gibbs, M. Isobe, H. Sawa

Abstract: In δ-Ag2/3V2O5 with charge degrees of freedom in V, it is known that the charge ordering state and physical properties of V that appear at low temperatures depend strongly on the ordering state of Ag. In this study, we focused on the Ag ions in the interlayer and studied the structure using synchrotron radiation powder diffraction in dependence on temperature. We found that when the sample is slow… ▽ More In δ-Ag2/3V2O5 with charge degrees of freedom in V, it is known that the charge ordering state and physical properties of V that appear at low temperatures depend strongly on the ordering state of Ag. In this study, we focused on the Ag ions in the interlayer and studied the structure using synchrotron radiation powder diffraction in dependence on temperature. We found that when the sample is slowly cooled from room temperature and ordering occurs at the Ag sites, V4+/V5+ charge ordering of V and subsequent V4+-V4+ structural dimers are produced. Although quenching the sample from room temperature suppresses the ordering of Ag, annealing at around 160 K promotes partial ordering of Ag and allows a metastable phase to be realized. This metastable phase is maintained even when the temperature is lowered again, producing a remarkable change in low-temperature properties. These results indicate that the ordered state of Ag, which is the key to control the charge-ordered state and physical properties, can be controlled by low-temperature annealing. The results of this study may provide a methodology for the realization of metastable states in a wide range of material groups of vanadium compounds, where competition among various charge ordered states underlies the physical properties. △ Less

Submitted 11 March, 2024; originally announced March 2024.

Comments: 10 pages, 6 figures

On-Surface Synthesis of Silole and Disilacyclooctaene Derivatives

Authors: Kewei Sun, Lauri Kurki, Orlando J. Silveira, Tomohiko Nishiuchi, Takashi Kubo, Ondřej Krejčí, Adam S. Foster, Shigeki Kawai

Abstract: Sila-cyclic rings are a class of organosilicon cyclic compounds and have abundant application in organic chemistry and materials science. However, it is still challenging to synthesize compounds with sila-cyclic rings in solution chemistry due to their low solubility and high reactivity. Recently, on-surface synthesis was introduced into organosilicon chemistry as 1,4- disilabenzene bridged nanost… ▽ More Sila-cyclic rings are a class of organosilicon cyclic compounds and have abundant application in organic chemistry and materials science. However, it is still challenging to synthesize compounds with sila-cyclic rings in solution chemistry due to their low solubility and high reactivity. Recently, on-surface synthesis was introduced into organosilicon chemistry as 1,4- disilabenzene bridged nanostructures were obtained via coupling between bromo-substituted molecules and silicon atoms on Au(111). Here, we extend this strategy for syntheses of silole derivatives and graphene nanoribbons with eight-membered sila-cyclic rings from 2,2',6,6'- tetrabromobiphenyl and 1,4,5,8-tetrabromonaphthalene on Au(111), respectively. Their structures and electronic properties were investigated by a combination of scanning tunneling microscopy/spectroscopy and density functional theory calculations. This work demonstrates a generality of this synthesis strategy to fabricate various silicon incorporated nanostructures. △ Less

Submitted 19 December, 2023; originally announced December 2023.

Melting of excitonic insulator phase by an intense terahertz pulse in Ta$_2$NiSe$_5$

Authors: Naoki Takamura, Tatsuya Miyamoto, Ryohei Ikeda, Tetsushi Kubo, Masaki Yamamoto, Hiroki Sato, Yang Han, Takayuki Ito, Tetsu Sato, Akitoshi Nakano, Hiroshi Sawa, Hiroshi Okamoto

Abstract: In this study, the optical response to a terahertz pulse was investigated in the transition metal chalcogenide Ta$_2$NiSe$_5$, a candidate excitonic insulator. First, by irradiating a terahertz pulse with a relatively weak electric field (0.3 MV/cm), the spectral changes in reflectivity near the absorption edge due to third-order optical nonlinearity were measured and the absorption peak character… ▽ More In this study, the optical response to a terahertz pulse was investigated in the transition metal chalcogenide Ta$_2$NiSe$_5$, a candidate excitonic insulator. First, by irradiating a terahertz pulse with a relatively weak electric field (0.3 MV/cm), the spectral changes in reflectivity near the absorption edge due to third-order optical nonlinearity were measured and the absorption peak characteristic of the excitonic phase just below the interband transition was identified. Next, by irradiating a strong terahertz pulse with a strong electric field of 1.65 MV/cm, the absorption of the excitonic phase was found to be reduced, and a Drude-like response appeared in the mid-infrared region. These responses can be interpreted as carrier generation by exciton dissociation induced by the electric field, resulting in the partial melting of the excitonic phase and metallization. The presence of a distinct threshold electric field for carrier generation indicates exciton dissociation via quantum-tunnelling processes. The spectral change due to metallization by the electric field is significantly different from that due to the strong optical excitation across the gap, which can be explained by the different melting mechanisms of the excitonic phase in the two types of excitations. △ Less

Submitted 11 September, 2023; originally announced September 2023.

Comments: 66 pages, 11 figures, 2 tables

Comparative $^{181}$Ta-NQR Study of Weyl Monopnictides TaAs and TaP: Relevance of Weyl Fermion Excitations

Authors: Tetsuro Kubo, Hiroshi Yasuoka, Balázs Dóra, Deepa Kasinathan, Yurii Prots, Helge Rosner, Takuto Fujii, Marcus Schmidt, Michael Baenitz

Abstract: Based on our first detailed $^{181}$Ta nuclear quadrupole resonance (NQR) studies from 2017 on the Weyl semimetal TaP, we now extended our NQR studies to another Ta-based monopnictide TaAs. In the present work, we have determined the temperature-dependent $^{181}$Ta-NQR spectra, the spin-lattice relaxation time $T_{1}$, and the spin-spin relaxation time $T_{2}$. We found the following characterist… ▽ More Based on our first detailed $^{181}$Ta nuclear quadrupole resonance (NQR) studies from 2017 on the Weyl semimetal TaP, we now extended our NQR studies to another Ta-based monopnictide TaAs. In the present work, we have determined the temperature-dependent $^{181}$Ta-NQR spectra, the spin-lattice relaxation time $T_{1}$, and the spin-spin relaxation time $T_{2}$. We found the following characteristic features that showed great contrast to what was found in TaP: (1) The quadrupole coupling constant and asymmetry parameter of EFG, extracted from three NQR frequencies, have a strong temperature dependence above $\sim$80 K that cannot be explained by the density functional theory calculation incorporating the thermal expansion of the lattice. (2) The temperature dependence of the spin-lattice relaxation rate, $1/T_{1} T$, shows a $T^{4}$ power law behavior above $\sim$30 K. This is a great contrast with the $1/T_{1} T \propto T^{2}$ behavior found in TaP, which was ascribed to the magnetic excitations at the Weyl nodes with a temperature-dependent orbital hyperfine coupling. (3) Regarding the nuclear spin-spin interaction, we found the spin-echo signal decays with the pulse separation simply by a Lorentzian function in TaAs, but we have observed spin-echo modulations in TaP that is most likely due to the indirect nuclear spin-spin coupling via virtually excited Weyl fermions. From our experimental findings, we conclude that the present NQR results do not show dominant contributions from Weyl fermion excitations in TaAs. △ Less

Submitted 29 July, 2023; originally announced July 2023.

Comments: 8 pages, 10 figures

Journal ref: J. Phys. Soc. Jpn. 92, 094706 (2023)

Local Probe Structure Isomerization in a One-Dimensional Molecular Array

Authors: Shigeki Kawai, Orlando J. Silveira, Lauri Kurki, Zhangyu Yuan, Tomohiko Nishiuchi, Takuya Kodama, Kewei Sun, Oscar Custance, Jose L. Lado, Takashi Kubo, Adam S. Foster

Abstract: Synthesis of one-dimensional molecular arrays with tailored stereoisomers is challenging yet has a great potential for application in molecular opto-, electronic- and magnetic-devices, where the local array structure plays a decisive role in the functional properties. Here, we demonstrate construction and characterization of dehydroazulene isomer and diradical units in three-dimensional organometa… ▽ More Synthesis of one-dimensional molecular arrays with tailored stereoisomers is challenging yet has a great potential for application in molecular opto-, electronic- and magnetic-devices, where the local array structure plays a decisive role in the functional properties. Here, we demonstrate construction and characterization of dehydroazulene isomer and diradical units in three-dimensional organometallic compounds on Ag(111) with a combination of low-temperature scanning tunneling microscopy and density functional theory calculations. Tip-induced voltage pulses firstly result in the formation of a diradical species via successive homolytic fission of two C-Br bonds in the naphthyl groups, which are subsequently transformed into chiral dehydroazulene moieties. The delicate balance of the reaction rates among the diradical and two stereoisomers, arising from an in-line configuration of tip and molecular unit, allows directional azulene-to-azulene and azulene-to-diradical local probe structural isomerization in a controlled manner. Furthermore, our theoretical calculations suggest that the diradical moiety hosts an open-shell singlet with antiferromagnetic coupling between the unpaired electrons, which can undergo an inelastic spin transition of 91 meV to the ferromagnetically coupled triplet state. △ Less

Submitted 4 October, 2023; v1 submitted 28 May, 2023; originally announced May 2023.

Tuning critical field, critical current, and diode effect of narrow thin-film superconductors through engineering inhomogeneous Pearl length

Authors: Takayuki Kubo

Abstract: We explore critical field and critical current behavior in inhomogeneous narrow thin-film superconducting strips. Formulations are developed to calculate free energy, critical field, and critical current for strips with inhomogeneous Pearl length distributions. Our findings show that inhomogeneities, specifically a shorter Pearl length in the middle of the strip, significantly enhance the critical… ▽ More We explore critical field and critical current behavior in inhomogeneous narrow thin-film superconducting strips. Formulations are developed to calculate free energy, critical field, and critical current for strips with inhomogeneous Pearl length distributions. Our findings show that inhomogeneities, specifically a shorter Pearl length in the middle of the strip, significantly enhance the critical field $B_{c1}$. This has practical implications for achieving complete flux expulsion. While narrow strips have traditionally been considered the most effective approach to improve $B_{c1}$ and eliminate trapped vortices, our results suggest that engineered inhomogeneities offer an alternative method to enhance $B_{c1}$ and improve flux expulsion without reducing strip width, providing greater design flexibility for superconducting devices. Additionally, we find that for the purpose of increasing the critical current, utilizing an inhomogeneous film with a reduced Pearl length in the middle of the strip is more advantageous. The enhancement in critical current arises from the current suppression effect at the edges induced by the inhomogeneous distribution of superfluid density. Furthermore, we demonstrate that an inhomogeneous film with a left-right asymmetric Pearl length distribution enables control over the nonreciprocity of the critical current, highlighting the potential of engineering inhomogeneous Pearl length distributions to implement devices exhibiting the superconducting diode effect. Our results provide concrete examples of how manipulating the inhomogeneity of Pearl length can enhance the performance of superconducting devices. Various methods such as doping nonuniform impurities or creating a temperature gradient can be employed to implement an inhomogeneous Pearl length distribution. △ Less

Submitted 17 May, 2023; originally announced May 2023.

Comments: 17 pages, 10 figures

Journal ref: Physical Review Applied 20, 034033 (2023)

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

Authors: Tsuyoshi Hatano, Toshihiro Kubo, Shinichi Amaha, Yasuhiro Tokura, Seigo Tarucha

Abstract: 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… ▽ More 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. △ Less

Submitted 12 October, 2021; originally announced October 2021.

Comments: 18 pages, 7 figures

Journal ref: Applied Physics Express 14, 105001 (2021)

Effects of nonmagnetic impurities and subgap states on the kinetic inductance, complex conductivity, quality factor and depairing current density

Authors: Takayuki Kubo

Abstract: We investigate how a combination of a nonmagnetic-impurity scattering rate $γ$ and finite subgap states parametrized by Dynes $Γ$ affects various physical quantities relevant to to superconducting devices: kinetic inductance $L_k$, complex conductivity $σ$, surface resistance $R_s$, quality factor $Q$, and depairing current density $J_d$. All the calculations are based on the Eilenberger formalism… ▽ More We investigate how a combination of a nonmagnetic-impurity scattering rate $γ$ and finite subgap states parametrized by Dynes $Γ$ affects various physical quantities relevant to to superconducting devices: kinetic inductance $L_k$, complex conductivity $σ$, surface resistance $R_s$, quality factor $Q$, and depairing current density $J_d$. All the calculations are based on the Eilenberger formalism of the BCS theory. We assume the device materials are extreme type-II $s$-wave superconductors. It is well known that the optimum impurity concentration ($γ/Δ_0 \sim 1$) minimizes $R_s$. Here, $Δ_0$ is the pair potential for the idealized ($Γ\to 0$) superconductor for the temperature $T\to 0$. We find the optimum $Γ$ can also reduce $R_s$ by one order of magnitude for a clean superconductor ($γ/Δ_0 < 1$) and a few tens $\%$ for a dirty superconductor ($γ/Δ_0 > 1$). Also, we find a nearly-ideal ($Γ/Δ_0 \ll 1$) clean-limit superconductor exhibits a frequency-independent $R_s$ for a broad range of frequency $ω$, which can significantly improve $Q$ of a very compact cavity with a few tens of GHz frequency. As $Γ$ or $γ$ increases, the plateau disappears, and $R_s$ obeys the $ω^2$ dependence. The subgap-state-induced residual surface resistance $R_{\rm res}$ is also studied, which can be detected by an SRF-grade high-$Q$ 3D resonator. We calculate $L_k(γ, Γ,T)$ and $J_d(γ, Γ,T)$, which are monotonic increasing and decreasing functions of $(γ, Γ,T)$, respectively. Measurements of $(γ, Γ)$ of device materials can give helpful information on engineering $(γ, Γ)$ via materials processing, by which it would be possible to improve $Q$, engineer $L_k$, and ameliorate $J_d$. △ Less

Submitted 1 October, 2021; originally announced October 2021.

Comments: 15 pages, 15 figures

Journal ref: Physical Review Applied 17, 014018 (2022)

Superheating fields of semi-infinite superconductors and layered superconductors in the diffusive limit: structural optimization based on the microscopic theory

Authors: Takayuki Kubo

Abstract: We investigate the superheating fields $H_{sh}$ of semi-infinite superconductors and layered superconductors in the diffusive limit by using the well-established quasiclassical Green's function formalism of the BCS theory. The coupled Maxwell-Usadel equations are self-consistently solved to obtain the spatial distributions of the magnetic field, screening current density, penetration depth, and pa… ▽ More We investigate the superheating fields $H_{sh}$ of semi-infinite superconductors and layered superconductors in the diffusive limit by using the well-established quasiclassical Green's function formalism of the BCS theory. The coupled Maxwell-Usadel equations are self-consistently solved to obtain the spatial distributions of the magnetic field, screening current density, penetration depth, and pair potential. We find the superheating field of a semi-infinite superconductor in the diffusive limit is given by $H_{sh} = 0.795 H_{c0}$ at the temperature $T \to 0$. Here $H_{c0}$ is the thermodynamic critical-field at the zero temperature. Also, we evaluate $H_{sh}$ of layered superconductors in the diffusive limit as functions of the layer thicknesses ($d$) and identify the optimum thickness that maximizes $H_{sh}$ for various materials combinations. Qualitative interpretation of $H_{sh}(d)$ based on the London approximation is also discussed. The results of this work can be used to improve the performance of superconducting rf resonant cavities for particle accelerators. △ Less

Submitted 22 June, 2020; originally announced June 2020.

Comments: 9 pages, 5 figures

Journal ref: Supercond. Sci. Technol. 34, 045006 (2021)

Enhanced exciton-exciton collisions in an ultra-flat monolayer MoSe2 prepared through deterministic flattening

Authors: T. Hotta, A. Ueda, S. Higuchi, M. Okada, T. Shimizu, T. Kubo, K. Ueno, T. Taniguchi, K. Watanabe, R. Kitaura

Abstract: Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with ultra-flat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-force-dependent charges of n ~ 2 x 10^12 cm^-2 uN^-1,… ▽ More Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with ultra-flat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-force-dependent charges of n ~ 2 x 10^12 cm^-2 uN^-1, resulting in strong intensity of trions in photoluminescence spectra of MoSe2 at low temperature. We found that a hBN/MoSe2/hBN prepared with the "modified nano-squeezing method" shows a strong excitonic emission with negligible trion peak, and the residual linewidth of the exciton peak is only 2.2 meV, which is comparable to the homogeneous limit. Furthermore, in this high-quality sample, we found that formation of biexciton occurs even at extremely low excitation power (Phi ~ 2.3 x 10^19 cm^-2 s^-1) due to the enhanced collisions between excitons. △ Less

Submitted 4 June, 2020; originally announced June 2020.

Journal ref: ACS Nano 2020

Microscopic mechanism of van der Waals heteroepitaxy in the formation of MoS2/hBN vertical heterostructures

Authors: Mitsuhiro Okada, Mina Maruyama, Susumu Okada, Jamie H. Warner, Yusuke Kureishi, Yosuke Uchiyama, Takashi Taniguchi, Kenji Watanabe, Tetsuo Shimizu, Toshitaka Kubo, Hisanori Shinohara, Ryo Kitaura

Abstract: Recent works have revealed that van der Waals (vdW) epitaxial growth of 2D materials on crystalline substrates, such as hexagonal boron nitride (hBN), leads to formation of self-aligned grains, which results in defect-free stitching between the grains. However, how the weak vdW interaction causes strong limitation on orientation of grains is still not understood yet. In this work, we have focused… ▽ More Recent works have revealed that van der Waals (vdW) epitaxial growth of 2D materials on crystalline substrates, such as hexagonal boron nitride (hBN), leads to formation of self-aligned grains, which results in defect-free stitching between the grains. However, how the weak vdW interaction causes strong limitation on orientation of grains is still not understood yet. In this work, we have focused on investigation of mechanism of self-alignment of MoS2 grains in vdW epitaxial growth on hBN. Through calculation based on density functional theory and the Lennard-Jones potential, we found that interaction energy between MoS2 and hBN strongly depends both on size and orientation of MoS2. We also found that, when size of MoS2 is ca. 40 nm, rotational energy barrier can exceed ~ 1 eV, which should suppress rotation to limit orientation of MoS2 even at growth temperature. △ Less

Submitted 5 May, 2020; v1 submitted 27 April, 2020; originally announced April 2020.

Journal ref: ACS Omega 5, 49, 31692, 2020

Superfluid flow in disordered superconductors with Dynes pair-breaking scattering: depairing current, kinetic inductance, and superheating field

Authors: Takayuki Kubo

Abstract: We investigate the effects of Dynes pair-breaking scattering rate $Γ$ on the superfluid flow in a narrow thin-film superconductor and a semi-infinite superconductor by self-consistently solving the coupled Maxwell and Usadel equations for the BCS theory in the diffusive limit for all temperature $T$, all $Γ$, and all superfluid momentum. We obtain the depairing current density $j_d(Γ, T)$ and the… ▽ More We investigate the effects of Dynes pair-breaking scattering rate $Γ$ on the superfluid flow in a narrow thin-film superconductor and a semi-infinite superconductor by self-consistently solving the coupled Maxwell and Usadel equations for the BCS theory in the diffusive limit for all temperature $T$, all $Γ$, and all superfluid momentum. We obtain the depairing current density $j_d(Γ, T)$ and the current-dependent nonlinear kinetic inductance $L_k(j_s, Γ, T)$ in a narrow thin-film and the superheating field $H_{sh}(Γ, T)$ and the current distribution in a semi-infinite superconductor, taking the nonlinear Meissner effect into account. The analytical expressions for $j_d(Γ,T)|_{T=0}$, $L_k(j_s, Γ, T)|_{T=0}$, and $H_{sh}(Γ, T)|_{T=0}$ are also derived. The theory suggests $j_d$ and $H_{sh}$ can be ameliorated by reducing $Γ$, and $L_k$ can be tuned by a combination of the bias current and $Γ$. Tunneling spectroscopy can test the theory and also give insight into how to engineer $Γ$ via materials processing. Implications of the theory would be useful to improve performances of various superconducting quantum devices. △ Less

Submitted 2 April, 2020; originally announced April 2020.

Comments: 16 pages, 16 figures

Journal ref: Phys. Rev. Research 2, 033203 (2020)

Dissipative conductivity of a dirty superconductor with Dynes subgap states under a dc bias current up to the depairing current density

Authors: Takayuki Kubo

Abstract: We study the dissipative conductivity $σ_1$ of a dirty superconductor with a finite Dynes parameter $Γ$ under a dc-biased weak time-dependent field. The Usadel equation for the current-carrying state is solved to calculate the pair potential, penetration depth, supercurrent density, and quasiparticle spectrum. It is shown that, while the depairing current density $j_d$ for $Γ=0$ is coincident with… ▽ More We study the dissipative conductivity $σ_1$ of a dirty superconductor with a finite Dynes parameter $Γ$ under a dc-biased weak time-dependent field. The Usadel equation for the current-carrying state is solved to calculate the pair potential, penetration depth, supercurrent density, and quasiparticle spectrum. It is shown that, while the depairing current density $j_d$ for $Γ=0$ is coincident with the Kupriyanov-Lukichev theory, a finite $Γ$ decreases the superfluid density, resulting in a reduction of $j_d$. The broadening of the peaks of the quasiparticle density of states induced by a combination of a finite $Γ$ and a dc bias can reduce $σ_1$ below that for the ideal dirty BCS superconductor with $Γ=0$, while subgap states at Fermi level proportional to $Γ$ results in a residual conductivity at $T\to 0$. We find the optimum combination of $Γ$ and the dc bias to minimize $σ_1$ by scanning all $Γ$ and all currents up to $j_d$. By using the results, it is possible to improve $j_d$ and reduce electromagnetic dissipation in various superconducting quantum devices. △ Less

Submitted 4 December, 2019; originally announced December 2019.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. Research 2, 013302 (2020)

Field-dependent nonlinear surface resistance and its optimization by surface nano-structuring in superconductors

Authors: Takayuki Kubo, Alex Gurevich

Abstract: We propose a theory of nonlinear surface resistance of a dirty superconductor in a strong radio-frequency (RF) field, taking into account magnetic and nonmagnetic impurities, finite quasiparticle lifetimes, and a thin proximity-coupled normal layer characteristic of the oxide surface of many materials. The Usadel equations were solved to obtain the quasiparticle density of states (DOS) and the low… ▽ More We propose a theory of nonlinear surface resistance of a dirty superconductor in a strong radio-frequency (RF) field, taking into account magnetic and nonmagnetic impurities, finite quasiparticle lifetimes, and a thin proximity-coupled normal layer characteristic of the oxide surface of many materials. The Usadel equations were solved to obtain the quasiparticle density of states (DOS) and the low-frequency surface resistance $R_s$ as functions of the RF field amplitude $H_0$. It is shown that the interplay of the broadening of the DOS peaks and a decrease of a quasiparticle gap caused by the RF currents produces a minimum in $R_s(H_0)$ and an extended rise of the quality factor $Q(H_0)$ with the RF field. Paramagnetic impurities shift the minimum in $R_s(H_0)$ to lower fields and can reduce $R_s(H_0)$ in a wide range of $H_0$. Subgap states in the DOS can give rise to a residual surface resistance while reducing $R_s$ at higher temperatures. A proximity-coupled normal layer at the surface can shift the minimum in $R_s(H_0)$ to either low and high fields and can reduce $R_s$ below that of an ideal surface. The theory shows that the behavior of $R_s(H_0)$ changes as the temperature and the RF frequency are increased, and the field dependence of $Q(H_0)$ can be very sensitive to the materials processing. Our results suggest that the nonlinear RF losses can be minimized by tuning pairbreaking effects at the surface using impurity management or surface nanostructuring. △ Less

Submitted 28 July, 2019; originally announced July 2019.

Comments: 20 pages, 24 figures

Journal ref: Phys. Rev. B 100, 064522 (2019)

Lower critical field measurement of NbN multilayer thin film superconductor at KEK

Authors: H. Ito, H. Hayano, T. Kubo, T. Saeki, R. Katayama, Y. Iwashita, H. Tongu, R. Ito, T. Nagata, C. Z. Antoine

Abstract: The multilayer thin film structure of the superconductor has been proposed by A. Gurevich to enhance the maximum gradient of SRF cavities. The lower critical field Hc1 at which the vortex starts penetrating the superconducting material will be improved by coating Nb with thin film superconductor such as NbN. It is expected that the enhancement of Hc1 depends on the thickness of each layer. In orde… ▽ More The multilayer thin film structure of the superconductor has been proposed by A. Gurevich to enhance the maximum gradient of SRF cavities. The lower critical field Hc1 at which the vortex starts penetrating the superconducting material will be improved by coating Nb with thin film superconductor such as NbN. It is expected that the enhancement of Hc1 depends on the thickness of each layer. In order to determine the optimum thickness of each layer and to compare the measurement results with the theoretical prediction proposed by T. Kubo, we developed the Hc1 measurement system using the third harmonic response of the applied AC magnetic field at KEK. For the Hc1 measurement without the influence of the edge or the shape effects, the AC magnetic field can be applied locally by the solenoid coil of 5mm diameter in our measurement system. ULVAC made the NbN-SiO2 multilayer thin film samples of various NbN thicknesses. In this report, the measurement result of the bulk Nb sample and NbN-SiO2 multilayer thin film samples of different thickness of NbN layer will be discussed. △ Less

Submitted 8 July, 2019; originally announced July 2019.

Comments: 5 pages, 7 figures, contribution to the 19th International Conference on RF Superconductivity (SRF2019)

Lower Critical Field Measurement System based on Third-Harmonic Method for Superconducting RF Materials

Authors: Hayato Ito, Hitoshi Hayano, Takayuki Kubo, Takayuki Saeki

Abstract: We develop a lower critical field (Hc1) measurement system using the third-harmonic response of an applied AC magnetic field from a solenoid coil positioned above a superconducting sample. Parameter Hc1 is measured via detection of the third-harmonic component, which drastically changes when a vortex begins to penetrate the superconductor with temperature increase. The magnetic field locally appli… ▽ More We develop a lower critical field (Hc1) measurement system using the third-harmonic response of an applied AC magnetic field from a solenoid coil positioned above a superconducting sample. Parameter Hc1 is measured via detection of the third-harmonic component, which drastically changes when a vortex begins to penetrate the superconductor with temperature increase. The magnetic field locally applied to one side of the sample mimics the magnetic field within superconducting radio-frequency (SRF) cavities and prevents edge effects of the superconducting sample. With this approach, our measurement system can potentially characterize surface-engineered SRF materials such as Superconductor-Insulator-Superconductor multilayer structure (S-I-S structure). As a validation test, we measure the temperature dependence of Hc1 of two high-RRR bulk Nb samples and obtain results consistent with the literature. We also confirm that our system can apply magnetic fields of at least 120 mT at 4-5 K without any problem of heat generation of the coil. This field value is higher than those reported in previous works and makes it possible to more accurately estimate Hc1 at lower temperatures. △ Less

Submitted 20 June, 2019; originally announced June 2019.

Comments: 17 pages, 11 figures

Terahertz Raman spectroscopy probe of intermolecular vibration in high-mobility organic semiconductors under uniaxial strain

Authors: Junto Tsurumi, Takayoshi Kubo, Hiroyuki Ishii, Masato Mitani, Toshihiro Okamoto, Shun Watanabe, Jun Takeya

Abstract: Terahertz Raman spectroscopy was performed on high-mobility organic single-crystal semiconductors, by which the phonon energy at the Gamma point was qualified as a function of external uniaxial strain. The observation of peak shifts in Raman modes under uniaxial strain revealed that application of an external strain can effectively tune the intermolecular vibration that particularly correlates ele… ▽ More Terahertz Raman spectroscopy was performed on high-mobility organic single-crystal semiconductors, by which the phonon energy at the Gamma point was qualified as a function of external uniaxial strain. The observation of peak shifts in Raman modes under uniaxial strain revealed that application of an external strain can effectively tune the intermolecular vibration that particularly correlates electron-phonon coupling. Terahertz Raman spectroscopy conducted in conjunction with molecular dynamics simulation provides an in-depth understanding of the recently discovered very large strain effect on charge conduction in high-mobility organic semiconductors. △ Less

Submitted 3 May, 2019; originally announced May 2019.

Comments: 11 pages, 4 figures

Surface impedance and optimum surface resistance of a superconductor with imperfect surface

Authors: Alex Gurevich, Takayuki Kubo

Abstract: We calculate a low-frequency surface impedance of a dirty, s-wave superconductor with an imperfect surface incorporating either a thin layer with a reduced pairing constant or a thin, proximity-coupled normal layer. Such structures model realistic surfaces of superconducting materials which can contain oxide layers, absorbed impurities or nonstoichiometric composition. We solved the Usadel equatio… ▽ More We calculate a low-frequency surface impedance of a dirty, s-wave superconductor with an imperfect surface incorporating either a thin layer with a reduced pairing constant or a thin, proximity-coupled normal layer. Such structures model realistic surfaces of superconducting materials which can contain oxide layers, absorbed impurities or nonstoichiometric composition. We solved the Usadel equations self-consistently and obtained spatial distributions of the order parameter and the quasiparticle density of states which then were used to calculate a low-frequency surface resistance $R_s(T)$ and the magnetic penetration depth $λ(T)$ as functions of temperature in the limit of local London electrodynamics. It is shown that the imperfect surface in a single-band s-wave superconductor results in a non-exponential temperature dependence of $Z(T)$ at $T\ll T_c$ which can mimic the behavior of multiband or d-wave superconductors. The imperfect surface and the broadening of the gap peaks in the quasiparticle density of states $N(ε)$ in the bulk give rise to a weakly temperature-dependent residual surface resistance. We show that the surface resistance can be optimized and even reduced below its value for an ideal surface by engineering $N(ε)$ at the surface using pairbreaking mechanisms, particularly, by incorporating a small density of magnetic impurities or by tuning the thickness and conductivity of the normal layer and its contact resistance. The results of this work address the limit of $R_s$ in superconductors at $T\ll T_c$, and the ways of engineering the optimal density of states by surface nano-structuring and impurities to reduce losses in superconducting micro-resonators, thin film strip lines, and radio frequency cavities for particle accelerators. △ Less

Submitted 8 November, 2017; originally announced November 2017.

Large-gap magnetic topological heterostructure formed by subsurface incorporation of a ferromagnetic layer

Authors: Toru Hirahara, Sergey V. Eremeev, Tetsuroh Shirasawa, Yuma Okuyama, Takayuki Kubo, Ryosuke Nakanishi, Ryota Akiyama, Akari Takayama, Tetsuya Hajiri, Shin-ichiro Ideta, Masaharu Matsunami, Kazuki Sumida, Koji Miyamoto, Yasumasa Takagi, Kiyohisa Tanaka, Taichi Okuda, Toshihiko Yokoyama, Shin-ichi Kimura, Shuji Hasegawa, Evgueni V. Chulkov

Abstract: Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the… ▽ More Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi2Se4 /Bi2Se3 heterostructure which was fabricated by self-assembling a MnBi2Se4 layer on top of the Bi2Se3 surface as a result of the co-deposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi2Se4 /Bi2Se3 heterostructure shows ferromagnetism up to room temperature and a clear Dirac-cone gap opening of ~100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = -1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future " topotronics" devices. △ Less

Submitted 6 September, 2017; originally announced September 2017.

Comments: 13pages, 4 figures

Journal ref: Nano Letters 17, 3493 (2017)

Emergent Weyl fermion excitations in TaP explored by 181-Ta quadrupole resonance

Authors: H. Yasuoka, T. Kubo, Y. Kishimoto, D. Kasinathan, M. Schmidt, B. Yan, Y. Zhang, H. Tou, C. Felser, A. P. Mackenzie, M. Baenitz

Abstract: The $^{181}$Ta quadrupole resonance (NQR) technique has been utilized to investigate the microscopic magnetic properties of the Weyl semi-metal TaP. We found three zero-field NQR signals associated with the transition between the quadrupole split levels for Ta with $I$=7/2 nuclear spin. A quadrupole coupling constant, $ν_{\mathrm{Q}}$ =19.250 MHz, and an asymmetric parameter of the electric field… ▽ More The $^{181}$Ta quadrupole resonance (NQR) technique has been utilized to investigate the microscopic magnetic properties of the Weyl semi-metal TaP. We found three zero-field NQR signals associated with the transition between the quadrupole split levels for Ta with $I$=7/2 nuclear spin. A quadrupole coupling constant, $ν_{\mathrm{Q}}$ =19.250 MHz, and an asymmetric parameter of the electric field gradient, $η$ = 0.423 were extracted, in good agreement with band structure calculations. In order to examine the magnetic excitations, the temperature dependence of the spin lattice relaxation rate (1/$T_{\mathrm{1}}T$) has been measured for the $f_{\mathrm{2}}$-line ($\pm$5/2 $\leftrightarrow$ $\pm$3/2 transition). We found that there exists two regimes with quite different relaxation processes. Above $T\text{*}$ $\approx$ 30 K, a pronounced (1/$T_{\mathrm{1}}T$) $\propto$ $T^{2}$ behavior was found, which is attributed to the magnetic excitations at the Weyl nodes with temperature dependent orbital hyperfine coupling. Below $T\text{*}$, the relaxation is mainly governed by Korringa process with 1/$T_{\mathrm{1}}T$ = constant, accompanied by an additional $T^{-1/2}$ type dependence to fit our experimental data. We show that Ta-NQR is a novel probe for the bulk Weyl fermions and their excitations. △ Less

Submitted 22 November, 2016; originally announced November 2016.

Comments: Paper + Supplement, 5 figures

Journal ref: Phys. Rev. Lett. 118, 236403 (2017)

Model of Flux Trapping in Cooling Down Process

Authors: Takayuki Kubo

Abstract: The flux trapping that occurs in the process of cooling down of the superconducting cavity is studied. The critical fields $B_{c2}$ and $B_{c1}$ depend on a position when a material temperature is not uniform. In a region with $T\simeq T_c$, $B_{c2}$ and $B_{c1}$ are strongly suppressed and can be smaller than the ambient magnetic field, $B_a$. A region with $B_{c2}\le B_a$ is normal conducting, t… ▽ More The flux trapping that occurs in the process of cooling down of the superconducting cavity is studied. The critical fields $B_{c2}$ and $B_{c1}$ depend on a position when a material temperature is not uniform. In a region with $T\simeq T_c$, $B_{c2}$ and $B_{c1}$ are strongly suppressed and can be smaller than the ambient magnetic field, $B_a$. A region with $B_{c2}\le B_a$ is normal conducting, that with $B_{c1}\le B_a < B_{c2}$ is in the vortex state, and that with $B_{c1}> B_a$ is in the Meissner state. As a material is cooled down, these three domains including the vortex state domain sweep and pass through the material. In this process, vortices contained in the vortex state domain are trapped by pinning centers distributing in the material. A number of trapped fluxes can be evaluated by using the analogy with the beam-target collision event, where beams and a target correspond to pinning centers and the vortex state domain, respectively. We find a number of trapped fluxes and thus the residual resistance are proportional to the ambient magnetic field and the inverse of the temperature gradient. The obtained formula for the residual resistance is consistent with experimental results. The present model focuses on what happens at the phase transition fronts during a cooling down, reveals why and how the residual resistance depends on the temperature gradient, and naturally explains how the fast cooling works. △ Less

Submitted 10 September, 2015; originally announced September 2015.

Comments: 5 pages, 5 figures, proceedings of the 17th International Conference on RF Superconductivity (SRF 2015), Whistler, Canada, 13-18 September, 2015

Coulomb interaction-induced Aharonov-Bohm oscillations

Authors: Toshihiro Kubo, Yasuhiro Tokura

Abstract: 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… ▽ More 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. △ Less

Submitted 2 August, 2015; originally announced August 2015.

Comments: 21 pages, 5 figures

Quantum adiabatic pumping by modulating tunnel phase in quantum dots

Authors: Masahiko Taguchi, Satoshi Nakajima, Toshihiro Kubo, Yasuhiro Tokura

Abstract: 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 stepli… ▽ More 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. △ Less

Submitted 20 September, 2016; v1 submitted 31 March, 2015; originally announced April 2015.

Comments: 16 pages, 7 figures

Journal ref: Journal of the Physical Society of Japan 85, 084704 (2016)

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

Authors: Satoshi Nakajima, Masahiko Taguchi, Toshihiro Kubo, Yasuhiro Tokura

Abstract: 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 paramete… ▽ More 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. △ Less

Submitted 27 June, 2016; v1 submitted 25 January, 2015; originally announced January 2015.

Comments: 18 pages, 3 figures, corrected typos

Journal ref: Phys. Rev. B 92, 195420 (2015)

Field limit and nano-scale surface topography of superconducting radio-frequency cavity made of extreme type II superconductor

Authors: Takayuki Kubo

Abstract: The field limit of superconducting radio-frequency cavity made of type II superconductor with a large Ginzburg-Landau parameter is studied with taking effects of nano-scale surface topography into account. If the surface is ideally flat, the field limit is imposed by the superheating field. On the surface of cavity, however, nano-defects almost continuously distribute and suppress the superheating… ▽ More The field limit of superconducting radio-frequency cavity made of type II superconductor with a large Ginzburg-Landau parameter is studied with taking effects of nano-scale surface topography into account. If the surface is ideally flat, the field limit is imposed by the superheating field. On the surface of cavity, however, nano-defects almost continuously distribute and suppress the superheating field everywhere. The field limit is imposed by an effective superheating field given by the product of the superheating field for ideal flat surface and a suppression factor that contains effects of nano-defects. A nano-defect is modeled by a triangular groove with a depth smaller than the penetration depth. An analytical formula for the suppression factor of bulk and multilayer superconductors are derived in the framework of the London theory. As an immediate application, the suppression factor of the dirty Nb processed by the electropolishing is evaluated by using results of surface topographic study. The estimated field limit is consistent with the present record field of nitrogen-doped Nb cavities. Suppression factors of surfaces of other bulk and multilayer superconductors, and those after various surface processing technologies can also be evaluated by using the formula. △ Less

Submitted 14 May, 2015; v1 submitted 15 December, 2014; originally announced December 2014.

Comments: 17 pages, 6 figures

Journal ref: Prog. Theor. Exp. Phys. 2015, 063G01 (2015)

Vortex-penetration field at a groove with a depth smaller than the penetration depth

Authors: Takayuki Kubo

Abstract: Analytical formula to evaluate the vortex-penetration field at a groove with a depth smaller than penetration depth is derived, which can be applied to surfaces of cavities or test pieces made from extreme type II superconductors such as nitrogen-doped Nb or alternative materials like Nb3Sn or NbN. Analytical formula to evaluate the vortex-penetration field at a groove with a depth smaller than penetration depth is derived, which can be applied to surfaces of cavities or test pieces made from extreme type II superconductors such as nitrogen-doped Nb or alternative materials like Nb3Sn or NbN. △ Less

Submitted 21 October, 2014; originally announced October 2014.

Comments: 3 pages, 1 figure, the 27th Linear Accelerator Conference, LINAC14, Geneva, Switzerland, 31 August - 5 September, 2014

Journal ref: Proceedings of the 27th Linear Accelerator Conference, LINAC14, Geneva, Switzerland (2014), p. 590, TUPP071

Superconducting nano-layer coating without insulator

Authors: Takayuki Kubo

Abstract: The superconducting nano-layer coating without insulator layer is studied. The magnetic-field distribution and the forces acting on a vortex are derived. Using the derived forces, the vortex-penetration field and the lower critical magnetic field can be discussed. The vortex-penetration field is identical with the multilayer coating, but the lower critical magnetic field is not. Forces acting on a… ▽ More The superconducting nano-layer coating without insulator layer is studied. The magnetic-field distribution and the forces acting on a vortex are derived. Using the derived forces, the vortex-penetration field and the lower critical magnetic field can be discussed. The vortex-penetration field is identical with the multilayer coating, but the lower critical magnetic field is not. Forces acting on a vortex from the boundary of two superconductors play an important role in evaluations of the free energy. △ Less

Submitted 5 October, 2014; originally announced October 2014.

Comments: 4 pages, 4 figures, the 27th Linear Accelerator Conference, LINAC14, Geneva, Switzerland, 31 August - 5 September, 2014

Journal ref: Proceedings of the 27th Linear Accelerator Conference, LINAC14, Geneva, Switzerland (2014), p. 1026, THPP074

Review of the multilayer coating model

Authors: Takayuki Kubo, Yoshihisa Iwashita, Takayuki Saeki

Abstract: The recent theoretical study on the multilayer-coating model published in Applied Physics Letters [1] is reviewed. Magnetic-field attenuation behavior in a multilayer coating model is different from a semi-infinite superconductor and a superconducting thin film. This difference causes that of the vortex-penetration field at which the Bean-Livingston surface barrier disappears. A material with smal… ▽ More The recent theoretical study on the multilayer-coating model published in Applied Physics Letters [1] is reviewed. Magnetic-field attenuation behavior in a multilayer coating model is different from a semi-infinite superconductor and a superconducting thin film. This difference causes that of the vortex-penetration field at which the Bean-Livingston surface barrier disappears. A material with smaller penetration depth, such as a pure Nb, is preferable as the substrate for pushing up the vortex-penetration field of the superconductor layer. The field limit of the whole structure of the multilayer coating model is limited not only by the vortex-penetration field of the superconductor layer, but also by that of the substrate. Appropriate thicknesses of superconductor and insulator layers can be extracted from contour plots of the field limit of the multilayer coating model given in Ref.[1]. △ Less

Submitted 2 July, 2014; originally announced July 2014.

Comments: 3 pages, 3 figures, the 5th International Particle Accelerator Conference (IPAC14), Dresden, Germany, 15-20 June, 2014

Journal ref: Proceedings of IPAC14, Dresden, Germany (2014), p. 2522, WEPRI023

Two-dimensional models of the magnetic-field enhancement at pit and bump

Authors: Takayuki Kubo

Abstract: Formulae of magnetic field enhancement at a two-dimensional semi-elliptical bump and a two-dimensional pit with chamfered edges are derived by using the method of conformal mapping. The latter can be regarded as an approximated model of the two-dimensional pit with round edges. Formulae of magnetic field enhancement at a two-dimensional semi-elliptical bump and a two-dimensional pit with chamfered edges are derived by using the method of conformal mapping. The latter can be regarded as an approximated model of the two-dimensional pit with round edges. △ Less

Submitted 2 July, 2014; originally announced July 2014.

Comments: 3 pages, 3 figures, the 5th International Particle Accelerator Conference (IPAC14), Dresden, Germany, 15-20 June, 2014

Journal ref: Proceedings of IPAC14, Dresden, Germany (2014), p. 2525, WEPRI024

Fluctuation theorem for heat transport probed by a thermal electrode

Authors: Y. Utsumi, O. Entin-Wohlman, A. Aharony, T. Kubo, Y. Tokura

Abstract: We analyze the full-counting statistics of the electric heat current flowing in a two-terminal quantum conductor whose temperature is probed by a third electrode ("probe electrode"). In particular we demonstrate that the cumulant-generating function obeys the fluctuation theorem in the presence of a constant magnetic field. The analysis is based on the scattering matrix of the three-terminal junct… ▽ More We analyze the full-counting statistics of the electric heat current flowing in a two-terminal quantum conductor whose temperature is probed by a third electrode ("probe electrode"). In particular we demonstrate that the cumulant-generating function obeys the fluctuation theorem in the presence of a constant magnetic field. The analysis is based on the scattering matrix of the three-terminal junction (comprising of the two electronic terminals and the probe electrode), and a separation of time scales: it is assumed that the rapid charge transfer across the conductor and the rapid relaxation of the electrons inside the probe electrode give rise to much slower energy fluctuations in the latter. This separation allows for a stochastic treatment of the probe dynamics, and the reduction of the three-terminal setup to an effective two-terminal one. Expressions for the lowest nonlinear transport coefficients, e.g., the linear-response heat-current noise and the second nonlinear thermal conductance, are obtained and explicitly shown to preserve the symmetry of the fluctuation theorem for the two-terminal conductor. The derivation of our expressions which is based on the transport coefficients of the three-terminal system explicitly satisfying the fluctuation theorem, requires the full calculations of vertex corrections. △ Less

Submitted 21 March, 2014; originally announced March 2014.

Comments: 16 pages, 7 figures

Journal ref: Physical Review B 89, 205314 (2014)

Power dependence of electric dipole spin resonance

Authors: Yasuhiro Tokura, Toshihiro Kubo, William John Munro

Abstract: 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… ▽ More 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. △ Less

Submitted 31 July, 2013; originally announced August 2013.

Comments: 4 pages, conference paper of APPC12

Vortex penetration field of the multilayer coating model

Authors: Takayuki Kubo, Yoshihisa Iwashita, Takayuki Saeki

Abstract: The vortex penetration field of the multilayer coating model with a single superconductor layer and a single insulator layer formed on a bulk superconductor are derived. The same formula can be applied to a model with a superconductor layer formed on a bulk superconductor without an insulator layer. The vortex penetration field of the multilayer coating model with a single superconductor layer and a single insulator layer formed on a bulk superconductor are derived. The same formula can be applied to a model with a superconductor layer formed on a bulk superconductor without an insulator layer. △ Less

Submitted 1 July, 2013; originally announced July 2013.

Comments: 3 pages, 2 figures, the 16th International Conference on RF Superconductivity (SRF 2013), Paris, France, 22-27 September, 2013

Excitation spectroscopy of few-electron states in artificial diatomic molecules

Authors: T. Hatano, Y. Tokura, S. Amaha, T. Kubo, S. Teraoka, S. Tarucha

Abstract: We study the excitation spectroscopy of few-electron, parallel coupled double quantum dots (QDs). By applying a finite source drain voltage to a double QD (DQD), the first excited states observed in nonequilibrium charging diagrams can be classified into two kinds in terms of the total effective electron number in the DQD, assuming a core filling. When there are an odd (even) number of electrons,… ▽ More We study the excitation spectroscopy of few-electron, parallel coupled double quantum dots (QDs). By applying a finite source drain voltage to a double QD (DQD), the first excited states observed in nonequilibrium charging diagrams can be classified into two kinds in terms of the total effective electron number in the DQD, assuming a core filling. When there are an odd (even) number of electrons, one (two)-electron antibonding (triplet) state is observed as the first excited state. On the other hand, at a larger source drain voltage we observe higher excited states, where additional single-particle excited levels are involved. Eventually, we identify the excited states with a calculation using the Hubbard model and, in particular, we elucidate the quadruplet state, which is normally forbidden by the spin blockade caused by the selection rule. △ Less

Submitted 28 June, 2013; v1 submitted 17 June, 2013; originally announced June 2013.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B 87, 241414(R)(2013)

Radio-frequency electromagnetic field and vortex penetration in multilayered superconductors

Authors: Takayuki Kubo, Yoshihisa Iwashita, Takayuki Saeki

Abstract: A multilayered structure with a single superconductor layer and a single insulator layer formed on a bulk superconductor is studied. General formulae for the vortex-penetration field of the superconductor layer and the magnetic field on the bulk superconductor, which is shielded by the superconductor and insulator layers, are derived with a rigorous calculation of the magnetic field attenuation in… ▽ More A multilayered structure with a single superconductor layer and a single insulator layer formed on a bulk superconductor is studied. General formulae for the vortex-penetration field of the superconductor layer and the magnetic field on the bulk superconductor, which is shielded by the superconductor and insulator layers, are derived with a rigorous calculation of the magnetic field attenuation in the multilayered structure. The achievable peak surface field depends on the thickness and its material of the superconductor layer, the thickness of the insulator layer and material of the bulk superconductor. The calculation shows a good agreement with an experimental result. A combination of the thicknesses of superconductor and insulator layers to enhance the field limit can be given by the formulae for any given materials. △ Less

Submitted 9 January, 2014; v1 submitted 25 April, 2013; originally announced April 2013.

Comments: 4 pages, 4 figures; figure and table are added, discussions extended, references added, to appear in Applied Physics Letters

Journal ref: Appl. Phys. Lett. 104, 032603 (2014)

Backaction Dephasing by a Quantum Dot Detector

Authors: Toshihiro Kubo, Yasuhiro Tokura

Abstract: 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 rel… ▽ More 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. △ Less

Submitted 10 October, 2012; originally announced October 2012.

Comments: 10 pages, 2 figures

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

Authors: Toshihiro Kubo, Yuki Ichigo, Yasuhiro Tokura

Abstract: 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 p… ▽ More 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. △ Less

Submitted 6 May, 2011; originally announced May 2011.

Comments: 17 pages, 9 figures, accepted for publication in Physical Review B

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

Authors: Toshihiro Kubo, Yasuhiro Tokura, Seigo Tarucha

Abstract: 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 cohe… ▽ More 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. △ Less

Submitted 21 December, 2010; originally announced December 2010.

Comments: 11 pages, 5 figures, accepted for publication in Physical Review B

Journal ref: Physical Review B 83, 115310 (2011)

Aharonov-Bohm Oscillations Changed by Indirect Interdot Tunneling via Electrodes in Parallel-Coupled Vertical Double Quantum Dots

Authors: T. Hatano, T. Kubo, Y. Tokura, S. Amaha, S. Teraoka, S. Tarucha

Abstract: Aharonov-Bohm (AB) oscillations are studied for a parallel coupled vertical double quantum dot with a common source and drain electrode. We observe AB oscillations of current via a one-electron bonding state as the ground state and an anti-bonding state as the excited state. As the center gate voltage becomes more negative, the oscillation period is clearly halved for both the bonding and antibond… ▽ More Aharonov-Bohm (AB) oscillations are studied for a parallel coupled vertical double quantum dot with a common source and drain electrode. We observe AB oscillations of current via a one-electron bonding state as the ground state and an anti-bonding state as the excited state. As the center gate voltage becomes more negative, the oscillation period is clearly halved for both the bonding and antibonding states, and the phase changes by half a period for the antibonding state. This result can be explained by a calculation that takes account of the indirect interdot coupling via the two electrodes. △ Less

Submitted 16 February, 2011; v1 submitted 19 December, 2010; originally announced December 2010.

Comments: 5 pages main text + 6 pages supplementary material

Journal ref: Physical Review Letters 106, 076801 (2011)

Transport properties of two laterally coupled vertical quantum dots in series with tunable inter-dot coupling

Authors: T. Hatano, S. Amaha, T. Kubo, S. Teraoka, Y. Tokura, J. A. Gupta, D. G. Austing, S. Tarucha

Abstract: We describe the electronic properties of a double dot for which the lateral coupling between the two vertical dots can be controlled in-situ with a center gate voltage (Vc) and the current flows through the two dots in series. When Vc is large and positive, the two dots merge. As Vc is made less positive, two dots are formed whose coupling is reduced. We measure charging diagrams for positive and… ▽ More We describe the electronic properties of a double dot for which the lateral coupling between the two vertical dots can be controlled in-situ with a center gate voltage (Vc) and the current flows through the two dots in series. When Vc is large and positive, the two dots merge. As Vc is made less positive, two dots are formed whose coupling is reduced. We measure charging diagrams for positive and negative source-drain voltages in the weak coupling regime and observe current rectification due to the Pauli spin blockade when the hyperfine interaction between the electrons and the nuclei is suppressed. △ Less

Submitted 31 July, 2010; originally announced August 2010.

Comments: 16 pages, 3 figures, accepted for Applied Physics Letters

Dephasing in an Aharonov-Bohm interferometer containing a lateral double quantum dot induced by coupling with a quantum dot charge sensor

Authors: T. Kubo, Y. Tokura, S. Tarucha

Abstract: We theoretically investigated the dephasing in an Aharonov-Bohm interferometer containing a lateral double quantum dot induced by coupling with a quantum dot charge sensor. We employed the interpolative 2nd-order perturbation theory to include the charge sensing Coulomb interaction. It is shown that the visibility of the Aharonov-Bohm oscillation of the linear conductance decreases monotonically a… ▽ More We theoretically investigated the dephasing in an Aharonov-Bohm interferometer containing a lateral double quantum dot induced by coupling with a quantum dot charge sensor. We employed the interpolative 2nd-order perturbation theory to include the charge sensing Coulomb interaction. It is shown that the visibility of the Aharonov-Bohm oscillation of the linear conductance decreases monotonically as the sensing Coulomb interaction increases. In particular, for a weak sensing interaction regime, the visibility decreases parabolically, and it behaves linearly for a strong sensing interaction regime. △ Less

Submitted 12 May, 2010; originally announced May 2010.

Comments: 13 pages, 5 figures, to be published in Journal of Physics A

Journal ref: Journal of Physics A 43, 354020 (2010)

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

Authors: Toshiaki Obata, Michel Pioro-Ladriere, Yasuhiro Tokura, Yun-Sok Shin, Toshihiro Kubo, Katsuharu Yoshida, Tomoyasu Taniyama, Seigo Tarucha

Abstract: 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… ▽ More 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. △ Less

Submitted 3 February, 2010; originally announced February 2010.

Comments: 5 pages, 3 figures

Static Friction as a Function of Waiting Time Probed by Dynamics of Driven Vortices in La2-xSrxCuO4 Thin Films

Authors: D. Nakamura, T. Kubo, S. Kitamura, A. Maeda

Abstract: We investigated the dynamics of driven vortices in high-$T_c$ superconductor as an ideal model system to study the physics of friction. The waiting-time dependence of the maximum static friction force, $F_s(t_w)$, was measured in La$_{2-x}$Sr$_x$CuO$_4$ thin films with different structures, sample sizes and pinning force. We found various kinds of $F_s(t_w)$ in the $B$-$T$ phase diagram and in d… ▽ More We investigated the dynamics of driven vortices in high-$T_c$ superconductor as an ideal model system to study the physics of friction. The waiting-time dependence of the maximum static friction force, $F_s(t_w)$, was measured in La$_{2-x}$Sr$_x$CuO$_4$ thin films with different structures, sample sizes and pinning force. We found various kinds of $F_s(t_w)$ in the $B$-$T$ phase diagram and in different types of samples. The results suggest that the relaxation by thermal fluctuation is strongly affected by the pinning strength, the vortex bundle size and the system size. Based on these results, we found crucial conditions to determine the validity of the Amontons-Coulomb's law, and proposed a criterion. △ Less

Submitted 17 June, 2009; originally announced June 2009.

Comments: 5 pages, 1 table, 3 figures, submitted to Phys. Rev. Lett

Electrically driven single electron spin resonance in a slanting Zeeman field

Authors: M. Pioro-Ladriere, T. Obata, Y. Tokura, Y. -S. Shin, T. Kubo, K. Yoshida, T. Taniyama, S. Tarucha

Abstract: The rapidly rising fields of spintronics and quantum information science have led to a strong interest in developing the ability to coherently manipulate electron spins. Electron spin resonance (ESR) is a powerful technique to manipulate spins that is commonly achieved by applying an oscillating magnetic field. However, the technique has proven very challenging when addressing individual spins.… ▽ More The rapidly rising fields of spintronics and quantum information science have led to a strong interest in developing the ability to coherently manipulate electron spins. Electron spin resonance (ESR) is a powerful technique to manipulate spins that is commonly achieved by applying an oscillating magnetic field. However, the technique has proven very challenging when addressing individual spins. In contrast, by mixing the spin and charge degrees of freedom in a controlled way through engineered non-uniform magnetic fields, electron spin can be manipulated electrically without the need of high-frequency magnetic fields. Here we realize electrically-driven addressable spin rotations on two individual electrons by integrating a micron-size ferromagnet to a double quantum dot device. We find that the electrical control and spin selectivity is enabled by the micro-magnet's stray magnetic field which can be tailored to multi-dots architecture. Our results demonstrate the feasibility of manipulating electron spins electrically in a scalable way. △ Less

Submitted 7 May, 2008; originally announced May 2008.

Comments: 25 pages, 6 figures