arXiv subjects

Hiroshi Kamata

Publications and source records attributed to Hiroshi Kamata.

15 recordsLinked to original sources

arXiv2023

Velocity and confinement of edge plasmons in HgTe-based 2D topological insulators

High-frequency transport in the edge states of the quantum spin Hall (QSH) effect has to date rarely been explored, though it could cast light on the scattering mechanisms taking place therein. We here report on the measurement of the plasmon velocity in topological HgTe quantum wells both in the QSH and quantum Hall (QH) regimes, using harmonic GHz excitations and phase-resolved detection. We observe low plasmon velocities corresponding to large transverse widths, which we ascribe to the prominent influence of charge puddles forming in the vicinity of edge channels. Together with other recent works, it suggests that puddles play an essential role in the edge state physics and probably constitute a main hurdle on the way to clean and robust edge transport.

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arXiv2022

Time-resolved measurement of ambipolar edge magnetoplasmon transport in InAs/InGaSb composite quantum wells

Time-resolved charge transport measurement for one-dimensional edge states is a powerful means for investigating nonequilibrium charge dynamics and underlying interaction effects therein. Here, we report a versatile on-chip time-resolved transport measurement scheme that does not require a quantum point contact and is therefore applicable to narrow-gap systems. We apply the technique to non-inverted InAs/In$_{x}$Ga$_{1-x}$Sb composite quantum wells, where its ambipolar character enables us to demonstrate the scheme in both the electron and hole regimes separately using a single device. Time-resolved measurements in the quantum Hall regimes clearly exhibit the chirality of each carrier, with pulsed charge waveforms observed only for one magnetic field direction opposite for electrons and holes. Waveform analysis in the time domain reveals reduced group velocity and broadening of edge magnetoplasmon pulses in both the electron and hole regimes, suggesting the influence of charge puddles in the bulk. Our time-resolved measurement scheme, applicable to various systems, will pave the way for investigations of dynamical properties of exotic topological edge states.

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arXiv2022

Quasiparticle trapping at vortices producing Josephson supercurrent enhancement

The Josephson junction of a strong spin-orbit material under a magnetic field is a promising Majorana fermion candidate. Supercurrent enhancement by a magnetic field has been observed in the InAs nanowire Josephson junctions and assigned to a topological transition. In this work we observe a similar phenomenon but discuss the non-topological origin by considering trapping of quasiparticles by vortices that penetrate the superconductor under a finite magnetic field. This assignment is supported by the observed hysteresis of the switching current when sweeping up and down the magnetic field. Our experiment shows the importance of quasiparticles in superconducting devices with a magnetic field, which can provide important insights for the design of quantum qubits using superconductors.

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arXiv2020

Characterization of helical Luttinger liquids in microwave stepped-impedance edge resonators

Coulomb interaction has important consequences on the physics of quantum spin Hall edge states, weakening the topological protection via two-particle scattering and renormalizing both the velocity and charge of collective plasmon modes compared to that of free electrons. Despite these effects, interactions remain difficult to quantify. We propose here simple and robust edge resonator geometries to characterize Coulomb interaction by means of high-frequency measurements. They rely on a transmission line approach, and take advantage of the impedance mismatch between the edge states and their microwave environment.

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arXiv2020

Half-Integer Shapiro Steps in a Short Ballistic InAs Nanowire Josephson Junction

We report on half-integer Shapiro steps observed in an InAs nanowire Josephson junction. We observed the Shapiro steps of the short ballistic InAs nanowire Josephson junction and found anomalous half-integer steps in addition to the conventional integer steps. The half-integer steps disappear as the temperature increases or transmission of the junction decreases. These experimental results agree closely with numerical calculation of the Shapiro response for the skewed current phase relation in a short ballistic Josephson junction.

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arXiv2019

Dominant non-local superconducting proximity effect due to electron-electron interaction in a ballistic double nanowire

Cooper pair splitting (CPS) can induce non-local correlation between two normal conductors coupling to a superconductor. CPS into a double one-dimensional electron gas is an appropriate platform for extracting large amount of entangled electron pairs and one of the key ingredients for engineering Majorana Fermions with no magnetic field. Here we study CPS using a Josephson junction of a gate-tunable ballistic InAs double nanowire. The measured switching current into the two nanowires significantly larger than sum of that into the respective nanowires, indicating the inter-wire superconductivity dominant compared to the intra-wire superconductivity. From dependence on the number of propagating channels in the nanowires, the observed CPS is assigned to one-dimensional electron-electron interaction. Our results will pave the way for utilizing one-dimensional electron-electron interaction to reveal physics of high-efficient CPS and engineer Majorana Fermions in double nanowire systems via CPS.

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arXiv2019

Dynamical separation of bulk and edge transport in HgTe-based 2D topological insulators

Topological effects in edge states are clearly visible on short lengths only, thus largely impeding their studies. On larger distances, one may be able to dynamically enhance topological signatures by exploiting the high mobility of edge states with respect to bulk carriers. Our work on microwave spectroscopy highlights the responses of the edges which host very mobile carriers, while bulk carriers are drastically slowed down in the gap. Though the edges are denser than expected, we establish that charge relaxation occurs on short timescales, and suggests that edge states can be addressed selectively on timescales over which bulk carriers are frozen.

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arXiv2018

Strong Electron-Electron Interactions of a Tomonaga--Luttinger Liquid Observed in InAs Quantum Wires

We report strong electron-electron interactions in quantum wires etched from an InAs quantum well, a material known to have strong spin-orbit interactions. We find that the current through the wires as a function of the bias voltage and temperature follows the universal scaling behavior of a Tomonaga--Luttinger liquid. Using a universal scaling formula, we extract the interaction parameter and find strong electron-electron interactions, increasing as the wires become more depleted. We establish theoretically that spin-orbit interactions cause only minor modifications of the interaction parameter in this regime, indicating that genuinely strong electron-electron interactions are indeed achieved in the device. Our results suggest that etched InAs wires provide a platform with both strong electron-electron and strong spin-orbit interactions.

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arXiv2018

Anomalous modulation of Josephson radiation in nanowire-based Josephson junctions

We investigate the Josephson radiation of nanowire (NW)-based Josephson junctions in a parallel magnetic field. The Josephson junction made of an InAs NW with superconducting Al leads shows the emission spectra which follow the Josephson frequency $f_{J}$ over the range 4-8 GHz at zero magnetic field. We observe an apparent deviation of the emission spectra from the Josephson frequency which is accompanied by a strong enhancement of the switching current above a magnetic field of $\sim 300$ mT. The observed modulations can be understood to reflect trivial changes in the superconducting circuit surrounding the device which is strongly affected by the applied magnetic field. Our findings will provide a way to accurately investigate topological properties in NW-based systems.

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arXiv2018

Cooper-pair splitting in two parallel InAs nanowires

We report on the fabrication and electrical characterization of an InAs double - nanowire (NW) device consisting of two closely placed parallel NWs coupled to a common superconducting electrode on one side and individual normal metal leads on the other. In this new type of device we detect Cooper-pair splitting (CPS) with a sizeable efficiency of correlated currents in both NWs. In contrast to earlier experiments, where CPS was realized in a single NW, demonstrating an intrawire electron pairing mediated by the superconductor (SC), our experiment demonstrates an inter- wire interaction mediated by the common SC. The latter is the key for the realization of zero-magnetic field Majorana bound states, or Parafermions; in NWs and therefore constitutes a milestone towards topological superconductivity. In addition, we observe transport resonances that occur only in the superconducting state, which we tentatively attribute to Andreev Bound states and/or Yu-Shiba resonances that form in the proximitized section of one NW.

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arXiv2017

Magnetic Field Inducing Zeeman Splitting and Anomalous Conductance Reduction of Half-integer Quantized Plateaus in InAs Quantum Wires

We report on magnetic field dependence of half-integer quantized conductance plateaus (HQPs) in InAs quantum wires. We observed HQPs at zero applied magnetic field in InAs quantum wires fabricated from a high-quality InAs quantum well. The application of in-plane magnetic field causes Zeeman splitting of the HQP features, indicating that the origin of the observed HQP is not spontaneous spin polarization. Additionally we observe that conductance of the split HQPs decreases gradually as the in-plane magnetic field increases. We finally assume electron-electron interaction as a possible mechanism to account for the zero-field HQPs and the anomalous field dependence.

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arXiv2017

Equal-Spin Andreev Reflection in Junctions of Spin-Resolved Quantum Hall Bulk State and Spin-Singlet Superconductor

The recent development of superconducting spintronics has revealed the spin-triplet superconducting proximity effect from a spin-singlet superconductor into a spin-polarized normal metal. In addition recently superconducting junctions using semiconductors are in demand for highly controlled experiments to engineer topological superconductivity. Here we report experimental observation of Andreev reflection in junctions of spin-resolved quantum Hall (QH) states in an InAs quantum well and the spin-singlet superconductor NbTi. The measured conductance indicates a sub-gap feature and two peaks on the outer side of the sub-gap feature in the QH plateau-transition regime increases. The observed structures can be explained by considering transport with Andreev reflection from two channels, one originating from equal-spin Andreev reflection intermediated by spin-flip processes and second arising from normal Andreev reflection. This result indicates the possibility to induce the superconducting proximity gap in the the QH bulk state, and the possibility for the development of superconducting spintronics in semiconductor devices.

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arXiv2013

Fractionalized Wave Packets from an Artificial Tomonaga-Luttinger Liquid

The model of interacting fermion systems in one dimension known as Tomonaga-Luttinger liquid (TLL) provides a simple and exactly solvable theoretical framework, predicting various intriguing physical properties. Evidence of TLL has been observed as power-law behavior in the electronic transport and momentum-resolved spectroscopy on various types of one-dimensional (1D) conductors. However, these measurements, which rely on dc transport involving tunneling processes, cannot identify the eigenmodes of the TLL, i.e., collective excitations characterized by non-trivial effective charge e* and charge velocity v*. The elementary process of charge fractionalization, a phenomenon predicted to occur at the junction of a TLL and non-interacting leads, has not been observed. Here we report time-resolved transport measurements on an artificial TLL comprised of coupled integer quantum Hall edge channels, successfully identifying single charge fractionalization processes. An electron wave packet with charge e incident from a non-interacting region breaks up into several fractionalized charge wave packets at the edges of the artificial TLL region, from which e* and v* can be directly evaluated. These results are informative for elucidating the nature of TLLs and low-energy excitations in the edge channels.

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arXiv2013

Distributed-element circuit model of edge magnetoplasmon transport

We report experimental and theoretical studies of edge magnetoplasmon (EMP) transport in quantum Hall (QH) devices. We develop a model that allows us to calculate the transport coefficients of EMPs in QH devices with various geometries. In our model, a QH system is described as a chiral distributed-element (CDE) circuit, where the effects of Coulomb interaction are represented by an electrochemical capacitance distributed along unidirectional transmission lines. We measure the EMP transport coefficients through single- and coupled-edge channels, a quantum point contact, and single- and double-cavity structures. These measured transmission spectra can be reproduced well by simulations using the corresponding CDE circuits. By fitting the experimental results with the simulations, we deduce the circuit parameters that characterize the electrostatic environment around the edge channels in a realistic QH system. The observed gate-voltage dependences of the EMP transport properties in gate-defined structures are explained in terms of the gate tuning of the circuit parameters in CDE circuits.

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arXiv2010

Voltage-controlled Group Velocity of Edge Magnetoplasmon in the Quantum Hall Regime

We investigate the group velocity of edge magnetoplasmons (EMPs) in the quantum Hall regime by means of time-of-flight measurement. The EMPs are injected from an Ohmic contact by applying a voltage pulse, and detected at a quantum point contact by applying another voltage pulse to its gate. We find that the group velocity of the EMPs traveling along the edge channel defined by a metallic gate electrode strongly depends on the voltage applied to the gate. The observed variation of the velocity can be understood to reflect the degree of screening caused by the metallic gate, which damps the in-plane electric field and hence reduces the velocity. The degree of screening can be controlled by changing the distance between the gate and the edge channel with the gate voltage.

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