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Efficient heat-energy conversion from a non-thermal Tomonaga-Luttinger liquid
Authors:
Hikaru Yamazaki,
Masashi Uemura,
Haruhi Tanaka,
Tokuro Hata,
Chaojing Lin,
Takafumi Akiho,
Koji Muraki,
Toshimasa Fujisawa
Abstract:
Energy harvesting is a technique that generates useful work from waste heat. Conventional energy harvesters acting on local thermal equilibrium states are constrained by thermodynamic limits, such as the Carnot efficiency. Quantum heat engines with non-thermal reservoirs are expected to exceed such limits. Here, we demonstrate energy harvesting from a nonthermal Tomonaga-Luttinger (TL) liquid in q…
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Energy harvesting is a technique that generates useful work from waste heat. Conventional energy harvesters acting on local thermal equilibrium states are constrained by thermodynamic limits, such as the Carnot efficiency. Quantum heat engines with non-thermal reservoirs are expected to exceed such limits. Here, we demonstrate energy harvesting from a nonthermal Tomonaga-Luttinger (TL) liquid in quantum Hall edge channels, where the non-thermal state is naturally formed due to the absence of thermalization. The scheme is tested with a quantum-dot energy harvester working on a non-thermal TL liquid supplied with waste heat from a quantum-point-contact transistor. Compared to the quasi-thermalized TL liquid, the non-thermal state prepared under the same heat is capable of a larger electromotive force and higher conversion efficiency. These characteristics can be understood by considering a binary Fermi distribution function of the non-thermal state induced by entropy-conserving equilibration. TL liquids are attractive non-thermal carriers for excellent energy harvesting.
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Submitted 30 September, 2025;
originally announced September 2025.
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Effects of GaAs Buffer Layer on Structural, Magnetic, and Transport Properties of Magnetic Topological Insulators Cr$_y$(Bi$_x$Sb$_{1-x}$)$_{2-y}$Te$_3$ and V$_y$(Bi$_x$Sb$_{1-x}$)$_{2-y}$Te$_3$ Films
Authors:
Yusuke Nakazawa,
Takafumi Akiho,
Kiyoshi Kanisawa,
Hiroshi Irie,
Norio Kumada,
Koji Muraki
Abstract:
Here, we study the effects of a GaAs buffer layer on the structural, magnetic, and transport properties of Cr$_y$(Bi$_x$Sb$_{1-x}$)$_{2-y}$Te$_3$ magnetic topological insulator thin films and compare them with those of V$_y$(Bi$_x$Sb$_{1-x}$)$_{2-y}$Te$_3$, which we recently reported. Similar to the case of V$_y$(Bi$_x$Sb$_{1-x}$)$_{2-y}$Te$_3$, growth on a GaAs buffer layer leads to some distinct…
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Here, we study the effects of a GaAs buffer layer on the structural, magnetic, and transport properties of Cr$_y$(Bi$_x$Sb$_{1-x}$)$_{2-y}$Te$_3$ magnetic topological insulator thin films and compare them with those of V$_y$(Bi$_x$Sb$_{1-x}$)$_{2-y}$Te$_3$, which we recently reported. Similar to the case of V$_y$(Bi$_x$Sb$_{1-x}$)$_{2-y}$Te$_3$, growth on a GaAs buffer layer leads to some distinctly different properties than direct growth on InP substrates. These include improved interface quality confirmed by transmission electron microscopy, enhanced magnetic coercive fields, and smaller resistivity peaks at the magnetization reversals. Furthermore, the Bi-ratio dependence of the carrier density reveals that the interface property also affects the Fermi level. These results demonstrate the importance of the buffer layer in controlling the electronic properties of the magnetic topological insulator films.
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Submitted 31 January, 2025;
originally announced January 2025.
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Coulomb oscillations of a quantum antidot formed by an airbridged pillar gate in the integer and fractional quantum Hall regime
Authors:
Tokuro Hata,
Hiroki Mitani,
Hidetaka Uchiyama,
Takafumi Akiho,
Koji Muraki,
Toshimasa Fujisawa
Abstract:
Quantum antidots (QAD) are attractive for manipulating quasiparticles in quantum Hall (QH) systems. Here, we form a QAD in the integer and fractional QH regimes at nominal Landau-level filling factor $ν$ = 2, 1, and 2/3 using a submicron pillar gate with an airbridge connection. After confirming the required conditions for a fully depleted QAD, we analyze the observed Coulomb oscillations in terms…
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Quantum antidots (QAD) are attractive for manipulating quasiparticles in quantum Hall (QH) systems. Here, we form a QAD in the integer and fractional QH regimes at nominal Landau-level filling factor $ν$ = 2, 1, and 2/3 using a submicron pillar gate with an airbridge connection. After confirming the required conditions for a fully depleted QAD, we analyze the observed Coulomb oscillations in terms of the area of the QAD and the effective charge for the oscillation period in an identical gate voltage range. The area at $ν$ = 2/3 is significantly smaller than that at $ν$ = 2 and 1, in qualitative agreement with the previous report. By assuming a constant gate capacitance, the effective charge at $ν$ = 2/3 is about 2/3 of that at $ν$ = 2 and 1. The QAD device can be used to capture and emit charges in the unit of 2e/3.
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Submitted 15 December, 2024;
originally announced December 2024.
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Static quantum dot on a potential hilltop for generating and analyzing hot electrons in the quantum Hall regime
Authors:
Ryo Oishi,
Yuto Hongu,
Tokuro Hata,
Chaojing Lin,
Takafumi Akiho,
Koji Muraki,
Toshimasa Fujisawa
Abstract:
We propose and demonstrate a static quantum dot on a potential hilltop to generate and analyze ballistic hot electrons along a quantum Hall edge channel well above the chemical potential. High energy resolution associated with discrete energy levels is attractive for studying hot-electron dynamics. Particularly, the energy distribution function of hot electrons weakly coupled to cold electrons is…
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We propose and demonstrate a static quantum dot on a potential hilltop to generate and analyze ballistic hot electrons along a quantum Hall edge channel well above the chemical potential. High energy resolution associated with discrete energy levels is attractive for studying hot-electron dynamics. Particularly, the energy distribution function of hot electrons weakly coupled to cold electrons is investigated to reveal spectral diffusion with energy relaxation. The analysis allows us to estimate the maximum energy exchange per scattering, which is an important parameter to describe interacting electrons in the edge channel.
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Submitted 11 September, 2024;
originally announced September 2024.
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Andreev Reflection in the Quantum Hall Regime at an Al/InAs Junction on a Cleaved Edge
Authors:
Takafumi Akiho,
Hiroshi Irie,
Yusuke Nakazawa,
Satoshi Sasaki,
Norio Kumada,
Koji Muraki
Abstract:
We have fabricated a superconductor/semiconductor (S/Sm) junction composed of Al and InAs using cleaved edge overgrowth. By exploiting the unique geometry with a thin Al/Pt/Al trilayer formed on the side surface of an in-situ cleaved heterostructure wafer containing an InAs quantum well, we achieve a superconducting critical field of 5 T, allowing superconductivity and quantum Hall (QH) effects to…
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We have fabricated a superconductor/semiconductor (S/Sm) junction composed of Al and InAs using cleaved edge overgrowth. By exploiting the unique geometry with a thin Al/Pt/Al trilayer formed on the side surface of an in-situ cleaved heterostructure wafer containing an InAs quantum well, we achieve a superconducting critical field of 5 T, allowing superconductivity and quantum Hall (QH) effects to coexist down to Landau-level filling factor nu = 3. Andreev reflection at zero magnetic field shows a conductance enhancement that is limited solely by the Fermi velocity mismatch, demonstrating a virtually barrier-free, high-quality S/Sm junction. Bias spectroscopy in the QH regime reveals the opening of a superconducting gap, with the reduced downstream resistance demonstrating that the electron-hole Andreev conversion probability consistently exceeds 50%. Our results, obtained in a new experimental regime characterized by a clean edge-contacted junction with a superconducting electrode narrower than the coherence length, open new avenues for both theoretical and experimental studies of the interplay between superconductivity and QH effects and the engineering of exotic quasiparticles.
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Submitted 29 August, 2024; v1 submitted 31 July, 2024;
originally announced July 2024.
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Effects of GaAs buffer layer on quantum anomalous Hall insulator Vy(BixSb1-x)2-yTe3
Authors:
Yusuke Nakazawa,
Takafumi Akiho,
Kiyoshi Kanisawa,
Hiroshi Irie,
Norio Kumada,
Koji Muraki
Abstract:
We report the growth, structural characterization, and transport properties of the quantum anomalous Hall insulator Vy(BixSb1-x)2-yTe3 (VBST) grown on a GaAs buffer layer by molecular beam epitaxy on a GaAs(111)A substrate. X-ray diffraction and transmission electron microscopy show that the implementation of a GaAs buffer layer improves the crystal and interface quality compared to the control sa…
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We report the growth, structural characterization, and transport properties of the quantum anomalous Hall insulator Vy(BixSb1-x)2-yTe3 (VBST) grown on a GaAs buffer layer by molecular beam epitaxy on a GaAs(111)A substrate. X-ray diffraction and transmission electron microscopy show that the implementation of a GaAs buffer layer improves the crystal and interface quality compared to the control sample grown directly on an InP substrate. Both samples exhibit the quantum anomalous Hall effect (QAHE), but with similar thermal stability despite the different structural properties. Notably, the QAHE in the sample grown on a GaAs buffer layer displays a significantly larger (almost double) coercive field with a much smaller resistivity peak at the magnetization reversal. Possible effects of the interface quality on the magnetic properties of VBST and the QAHE are discussed.
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Submitted 16 July, 2024;
originally announced July 2024.
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Resonant plasmon-assisted tunneling in a double quantum dot coupled to a quantum-Hall plasmon resonator
Authors:
Chaojing Lin,
Ko Futamata,
Takafumi Akiho,
Koji Muraki,
Toshimasa Fujisawa
Abstract:
Edge magnetoplasmon is an emergent chiral bosonic mode promising for studying electronic quantum optics. While the plasmon transport has been investigated with various techniques for decades,its coupling to a mesoscopic device remained unexplored. Here, we demonstrate the coupling between a single plasmon mode in a quantum Hall plasmon resonator and a double quantum dot (DQD). Resonant plasmon-ass…
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Edge magnetoplasmon is an emergent chiral bosonic mode promising for studying electronic quantum optics. While the plasmon transport has been investigated with various techniques for decades,its coupling to a mesoscopic device remained unexplored. Here, we demonstrate the coupling between a single plasmon mode in a quantum Hall plasmon resonator and a double quantum dot (DQD). Resonant plasmon-assisted tunneling is observed in the DQD through absorbing or emitting plasmons stored in the resonator. By using the DQD as a spectrometer, the plasmon energy and the coupling strength are evaluated, which can be controlled by changing the electrostatic environment of the quantum Hall edge. The observed plasmon-electron coupling encourages us for studying strong coupling regimes of plasmonic cavity quantum electrodynamics.
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Submitted 15 July, 2024; v1 submitted 22 June, 2024;
originally announced June 2024.
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Tunable tunnel coupling in a double quantum antidot with cotunneling via localized state
Authors:
Tokuro Hata,
Kazuhiro Sada,
Tomoki Uchino,
Daisuke Endo,
Takafumi Akiho,
Koji Muraki,
Toshimasa Fujisawa
Abstract:
Controlling tunnel coupling between quantum antidots (QADs) in the quantum Hall (QH) regime is problematic. We propose and demonstrate a scheme for tunable tunnel coupling between two QADs by utilizing a cotunneling process via a localized state as a third QAD. The effective tunnel coupling can be tuned by changing the localized level even with constant nearest-neighbor tunnel couplings. We system…
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Controlling tunnel coupling between quantum antidots (QADs) in the quantum Hall (QH) regime is problematic. We propose and demonstrate a scheme for tunable tunnel coupling between two QADs by utilizing a cotunneling process via a localized state as a third QAD. The effective tunnel coupling can be tuned by changing the localized level even with constant nearest-neighbor tunnel couplings. We systematically study the variation of transport characteristics in the effectively triple QAD system at the Landau level filling factor $ν=2$. The tunable tunnel coupling is clarified by analyzing the anti-crossing of Coulomb blockade peaks in the charge stability diagram, in agreement with numerical simulations based on the master equation. The scheme is attractive for studying coherence and interaction in QH systems.
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Submitted 28 August, 2023;
originally announced August 2023.
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Non-thermal Tomonaga-Luttinger liquid eventually emerging from hot electrons in the quantum Hall regime
Authors:
Kotaro Suzuki,
Tokuro Hata,
Yuya Sato,
Takafumi Akiho,
Koji Muraki,
Toshimasa Fujisawa
Abstract:
Dynamics of integrable systems, such as Tomonaga-Luttinger (TL) liquids, is deterministic, and the absence of stochastic thermalization processes provides unique characteristics, such as long-lived non-thermal metastable states with many conserved quantities. Here, we show such non-thermal states can emerge even when the TL liquid is excited with extremely high-energy hot electrons in chiral quant…
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Dynamics of integrable systems, such as Tomonaga-Luttinger (TL) liquids, is deterministic, and the absence of stochastic thermalization processes provides unique characteristics, such as long-lived non-thermal metastable states with many conserved quantities. Here, we show such non-thermal states can emerge even when the TL liquid is excited with extremely high-energy hot electrons in chiral quantum-Hall edge channels. This demonstrates the robustness of the integrable model against the excitation energy. Crossover from the single-particle hot electrons to the many-body TL liquid is investigated by using on-chip detectors with a quantum point contact and a quantum dot. The charge dynamics can be understood with a single-particle picture only for hot electrons. The resulting electron-hole plasma in the TL liquid shows a non-thermal metastable state, in which warm and cold electrons coexist without further thermalization. The multi-temperature constituents are attractive for transporting information with conserved quantities along the channels.
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Submitted 15 May, 2023;
originally announced May 2023.
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Coherent-Incoherent Crossover of Charge and Neutral Mode Transport as Evidence for the Disorder-Dominated Fractional Edge Phase
Authors:
Masayuki Hashisaka,
Takuya Ito,
Takafumi Akiho,
Satoshi Sasaki,
Norio Kumada,
Naokazu Shibata,
Koji Muraki
Abstract:
Couplings between topological edge channels open electronic phases possessing nontrivial eigenmodes far beyond the noninteracting-edge picture. However, inelastic scatterings mask the eigenmodes' inherent features, often preventing us from identifying the phases, as is the case for the quintessential Landau-level filling factor v = 2/3 edge composed of the counter-propagating v = 1/3 and 1 (1/3-1)…
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Couplings between topological edge channels open electronic phases possessing nontrivial eigenmodes far beyond the noninteracting-edge picture. However, inelastic scatterings mask the eigenmodes' inherent features, often preventing us from identifying the phases, as is the case for the quintessential Landau-level filling factor v = 2/3 edge composed of the counter-propagating v = 1/3 and 1 (1/3-1) channels. Here, we study the coherent-incoherent crossover of the 1/3-1 channels by tuning the channel length in-situ using a new device architecture comprising a junction of v = 1/3 and 1 systems, the particle-hole conjugate of the 2/3 edge. We successfully observed the concurrence of the fluctuating electrical conductance and the quantized thermal conductance in the crossover regime, the definitive hallmark of the eigenmodes in the disorder-dominated edge phase left experimentally unverified.
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Submitted 27 December, 2022;
originally announced December 2022.
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Fast time-domain current measurement for quantum dot charge sensing using a homemade cryogenic transimpedance amplifier
Authors:
Heorhii Bohuslavskyi,
Masayuki Hashisaka,
Takase Shimizu,
Takafumi Akiho,
Koji Muraki,
Norio Kumada
Abstract:
We developed a high-speed and low-noise time-domain current measurement scheme using a homemade GaAs high-electron-mobility-transistor-based cryogenic transimpedance amplifier (TIA). The scheme is versatile for broad cryogenic current measurements, including semiconductor spin-qubit readout, owing to the TIA's having low input impedance comparable to that of commercial room-temperature TIAs. The T…
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We developed a high-speed and low-noise time-domain current measurement scheme using a homemade GaAs high-electron-mobility-transistor-based cryogenic transimpedance amplifier (TIA). The scheme is versatile for broad cryogenic current measurements, including semiconductor spin-qubit readout, owing to the TIA's having low input impedance comparable to that of commercial room-temperature TIAs. The TIA has a broad frequency bandwidth and a low noise floor, with a trade-off between them governed by the feedback resistance $R_{FB}$. A lower $R_{FB}$ of 50 k$Ω$ enables high-speed current measurement with a -3dB cutoff frequency $f_{-3dB}$ = 28 MHz and noise-floor $NF = 8.5 \times 10^{-27}$ A$^{2}$/Hz, while a larger $R_{FB}$ of 400 k$Ω$ provides low-noise measurement with $NF = 1.0 \times 10^{-27}$ A$^{2}$/Hz and $f_{-3dB}$ = 4.5 MHz. Time-domain measurement of a 2-nA peak-to-peak square wave, which mimics the output of the standard spin-qubit readout technique via charge sensing, demonstrates a signal-to-noise ratio (SNR) of 12.7, with the time resolution of 48 ns, for $R_{FB}$ = 200 k$Ω$, which compares favorably with the best-reported values for the radio-frequency (RF) reflectometry technique. The time resolution can be further improved at the cost of the SNR (or vice versa) by using an even smaller (larger) $R_{FB}$, with a further reduction in the noise figure possible by limiting the frequency band with a low-pass filter. Our scheme is best suited for readout electronics for cryogenic sensors that require a high time resolution and current sensitivity and thus provides a solution for various fundamental research and industrial applications.
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Submitted 7 October, 2022;
originally announced October 2022.
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Non-uniform heat redistribution among multiple channels in the integer quantum Hall regime
Authors:
Ryota Konuma,
Chaojing Lin,
Tokuro Hata,
Taichi Hirasawa,
Takafumi Akiho,
Koji Muraki,
Toshimasa Fujisawa
Abstract:
Heat transport in multiple quantum-Hall edge channels at Landau-level filling factor nu = 2, 4, and 8 is investigated with a quantum point contact as a heat generator and a quantum dot as a local thermometer. Heat distribution among the channels remains highly non-uniform, which can be understood with the plasmon eigenmodes of the multiple channels. The heat transport can be controlled with anothe…
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Heat transport in multiple quantum-Hall edge channels at Landau-level filling factor nu = 2, 4, and 8 is investigated with a quantum point contact as a heat generator and a quantum dot as a local thermometer. Heat distribution among the channels remains highly non-uniform, which can be understood with the plasmon eigenmodes of the multiple channels. The heat transport can be controlled with another quantum point contact as a quantized heat valve, as manifested by stepwise increases of heat current at the thermometer. This encourages developing integrated heat circuits with quantum-Hall edge channels.
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Submitted 21 June, 2022;
originally announced June 2022.
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Impact of in-situ controlled disorder screening on fractional quantum Hall effects and composite-fermion transport
Authors:
T. Akiho,
K. Muraki
Abstract:
We examine the impact of random potential due to remote impurites (RIs) and its in-situ controlled screening on fractional quantum Hall effects (FQHEs) around Landau-level filling factor $ν= 1/2$. The experiment is made possible by using a dual-gate GaAs quantum well (QW) that allows for the independent control of the density $n_{e}$ of the two-dimensional electron system in the QW and that (…
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We examine the impact of random potential due to remote impurites (RIs) and its in-situ controlled screening on fractional quantum Hall effects (FQHEs) around Landau-level filling factor $ν= 1/2$. The experiment is made possible by using a dual-gate GaAs quantum well (QW) that allows for the independent control of the density $n_{e}$ of the two-dimensional electron system in the QW and that ($n_\text{SL}$) of excess electrons in the modulation-doping superlattice. As the screening is reduced by decreasing $n_\text{SL}$ at a fixed $n_{e}$, we observe a decrease in the apparent energy gap of the FQHEs deduced from thermal activation, which signifies a corresponding increase in the disorder broadening $Γ$ of composite fermions (CFs). Interestingly, the increase in $Γ$ is accompanied by a noticeable increase in the longitudinal resistivity at $ν= 1/2$ ($ρ_{1/2}$), with a much stronger correlation with $Γ$ than electron mobility $μ$ has. The in-situ control of RI screening enables us to disentangle the contributions of RIs and background impurities (BIs) to $ρ_{1/2}$, with the latter in good agreement with the CF theory. We construct a scaling plot that helps in estimating the BI contribution to $ρ_{1/2}$ for a given set of $n_{e}$ and $μ$.
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Submitted 28 March, 2022;
originally announced March 2022.
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Gate tuning of fractional quantum Hall states in InAs two-dimensional electron gas
Authors:
S. Komatsu,
H. Irie,
T. Akiho,
T. Nojima,
T. Akazaki,
K. Muraki
Abstract:
We report the observation of fractional quantum Hall (FQH) effects in a two-dimensional electron gas (2DEG) confined to an InAs/AlGaSb quantum well, using a dual-gated Hall-bar device allowing for the independent control of the vertical electric field and electron density. At a magnetic field of 24 T, we observe FQH states at several filling factors, namely $ν= 5/3$, $2/3$, and $1/3$, in addition…
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We report the observation of fractional quantum Hall (FQH) effects in a two-dimensional electron gas (2DEG) confined to an InAs/AlGaSb quantum well, using a dual-gated Hall-bar device allowing for the independent control of the vertical electric field and electron density. At a magnetic field of 24 T, we observe FQH states at several filling factors, namely $ν= 5/3$, $2/3$, and $1/3$, in addition to the $ν= 4/3$ previously reported for an InAs 2DEG. The $ν= 4/3$ and $5/3$ states, which are absent at zero back-gate voltage, emerge as the quantum well is made more symmetric by applying a positive back-gate voltage. The dependence of zero-field electron mobility on the quantum-well asymmetry reveals a significant contribution of interface-roughness scattering, with much stronger scattering at the lower InAs/AlGaSb interface. However, the dependence of the visibility of the FQH effects on the quantum-well asymmetry is not entirely consistent with that of mobility, suggesting that a different source of disorder is also at work.
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Submitted 10 December, 2021;
originally announced December 2021.
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Time-resolved investigation of plasmon mode along interface channels in integer and fractional quantum Hall regimes
Authors:
Chaojing Lin,
Masayuki Hashisaka,
Takafumi Akiho,
Koji Muraki,
Toshimasa Fujisawa
Abstract:
Quantum Hall (QH) edge channels appear not only along the edge of the electron gas but also along an interface between two QH regions with different filling factors. However, the fundamental transport characteristics of such interface channels are not well understood, particularly in the high-frequency regime. In this study, we investigate the interface plasmon mode along the edge of a metal gate…
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Quantum Hall (QH) edge channels appear not only along the edge of the electron gas but also along an interface between two QH regions with different filling factors. However, the fundamental transport characteristics of such interface channels are not well understood, particularly in the high-frequency regime. In this study, we investigate the interface plasmon mode along the edge of a metal gate electrode with ungated and gated QH regions in both integer and fractional QH regimes using a time-resolved measurement scheme. The observed plasmon waveform was delayed and broadened due to the influence of the charge puddles formed around the channel. The charge velocity and diffusion constant of the plasmon mode were evaluated by analyzing the waveform using a distributed circuit model. We found that the conductive puddles in the gated region induce significant dissipation in plasmon transport. For instance, a fractional interface channel with a reasonably fast velocity was obtained by preparing a fractional state in the ungated region and an integer state in the gated region, whereas a channel in the swapped configuration was quite dissipative. This reveals a high-quality interface channel that provides a clean path to transport fractional charges for studying various fractional QH phenomena.
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Submitted 6 September, 2021;
originally announced September 2021.
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Andreev reflection of fractional quantum Hall quasiparticles
Authors:
Masayuki Hashisaka,
Thibaut Jonckheere,
Takafumi Akiho,
Satoshi Sasaki,
Jerome Rech,
Thierry Martin,
Koji Muraki
Abstract:
Electron correlation in a quantum many-body state appears as peculiar scattering behaviour at its boundary, symbolic of which is Andreev reflection at a metal-superconductor interface. Despite being fundamental in nature, dictated by the charge conservation law, however, the process has had no analogues outside the realm of superconductivity so far. Here, we report the observation of an Andreev-li…
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Electron correlation in a quantum many-body state appears as peculiar scattering behaviour at its boundary, symbolic of which is Andreev reflection at a metal-superconductor interface. Despite being fundamental in nature, dictated by the charge conservation law, however, the process has had no analogues outside the realm of superconductivity so far. Here, we report the observation of an Andreev-like process originating from a topological quantum many-body effect instead of superconductivity. A narrow junction between fractional and integer quantum Hall states shows a two-terminal conductance exceeding that of the constituent fractional state. This remarkable behaviour, while theoretically predicted more than two decades ago but not detected to date, can be interpreted as Andreev reflection of fractionally charged quasiparticles. The observed fractional quantum Hall Andreev reflection provides a fundamental picture that captures microscopic charge dynamics at the boundaries of topological quantum many-body states.
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Submitted 19 May, 2021;
originally announced May 2021.
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Cryogenic GaAs high-electron-mobility-transistor amplifier for current noise measurements
Authors:
Sanghyun Lee,
Masayuki Hashisaka,
Takafumi Akiho,
Kensuke Kobayashi,
Koji Muraki
Abstract:
We show that a cryogenic amplifier composed of a homemade GaAs high-electron-mobility transistor (HEMT) is suitable for current-noise measurements in a mesoscopic device at dilution-refrigerator temperatures. The lower noise characteristics of our homemade HEMT leads to a lower noise floor in the experimental setup and enables more efficient current-noise measurement than is available with a comme…
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We show that a cryogenic amplifier composed of a homemade GaAs high-electron-mobility transistor (HEMT) is suitable for current-noise measurements in a mesoscopic device at dilution-refrigerator temperatures. The lower noise characteristics of our homemade HEMT leads to a lower noise floor in the experimental setup and enables more efficient current-noise measurement than is available with a commercial HEMT. We present the dc transport properties of the HEMT and the gain and noise characteristics of the amplifier. With the amplifier employed for current-noise measurements in a quantum point contact, we demonstrate the high resolution of the measurement setup by comparing it with that of the conventional one using a commercial HEMT.
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Submitted 23 February, 2021;
originally announced February 2021.
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Screening effects of superlattice doping on the mobility of GaAs two-dimensional electron system revealed by in-situ gate control
Authors:
Takafumi Akiho,
Koji Muraki
Abstract:
We investigate the screening effects of excess electrons in the doped layer on the mobility of a GaAs two-dimensional electron system (2DES) with a modern architecture using short-period superlattice (SL) doping. By controlling the density of excess electrons in the SL with a top gate while keeping the 2DES density constant with a back gate, we are able to compare 2DESs with the same density but d…
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We investigate the screening effects of excess electrons in the doped layer on the mobility of a GaAs two-dimensional electron system (2DES) with a modern architecture using short-period superlattice (SL) doping. By controlling the density of excess electrons in the SL with a top gate while keeping the 2DES density constant with a back gate, we are able to compare 2DESs with the same density but different degrees of screening using one sample. Using a field-penetration technique and circuit-model analysis, we determine the density of states and excess electron density in the SL, quantities directly linked to the screening capability. The obtained relation between mobility and excess electron density is consistent with the theory taking into account the screening by the excess electrons in the SL. The quantum lifetime determined from Shubnikov-de Haas oscillations is much lower than expected from theory and did not show a discernible change with excess electron density.
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Submitted 5 February, 2021;
originally announced February 2021.
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Quantized charge fractionalization at quantum Hall Y junctions in the disorder-dominated regime
Authors:
Chaojing Lin,
Masayuki Hashisaka,
Takafumi Akiho,
Koji Muraki,
Toshimasa Fujisawa
Abstract:
Fractionalization is a phenomenon where an elementary excitation partitions into several pieces. This picture explains non-trivial transport through a junction of one-dimensional edge channels defined by topologically distinct quantum Hall states, for example, a hole-conjugate state at Landau-level filling factor $ν$ = 2/3. Here we employ a time-resolved scheme to identify an elementary fractional…
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Fractionalization is a phenomenon where an elementary excitation partitions into several pieces. This picture explains non-trivial transport through a junction of one-dimensional edge channels defined by topologically distinct quantum Hall states, for example, a hole-conjugate state at Landau-level filling factor $ν$ = 2/3. Here we employ a time-resolved scheme to identify an elementary fractionalization process; injection of charge q from a non-interaction region into an interacting and scattering region of one-dimensional channels results in the formation of a collective excitation with charge $(1-\textit{r})\textit{q}$ by reflecting fractionalized charge $\textit{rq}$. The fractionalization factors, $\textit{r}$ = 0.34$\pm$0.03 for $ν$ = 2/3 and $\textit{r}$ = 0.49$\pm$0.03 for $ν$ = 2, are consistent with the quantized values of 1/3 and 1/2, respectively, which are expected in the disorder dominated regime. The scheme can be used for generating and transporting fractionalized charges with a well-defined time course along a well-defined path.
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Submitted 2 December, 2020;
originally announced December 2020.
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Energy gap tuning and gate-controlled topological phase transition in InAs/In$_{x}$Ga$_{1-x}$Sb composite quantum wells
Authors:
H. Irie,
T. Akiho,
F. Couëdo,
K. Suzuki,
K. Onomitsu,
K. Muraki
Abstract:
We report transport measurements of strained InAs/In$_{x}$Ga$_{1-x}$Sb composite quantum wells (CQWs) in the quantum spin Hall phase, focusing on the control of the energy gap through structural parameters and an external electric field. For highly strained CQWs with $x = 0.4$, we obtain a gap of 35 meV, an order of magnitude larger than that reported for binary InAs/GaSb CQWs. Using a dual-gate c…
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We report transport measurements of strained InAs/In$_{x}$Ga$_{1-x}$Sb composite quantum wells (CQWs) in the quantum spin Hall phase, focusing on the control of the energy gap through structural parameters and an external electric field. For highly strained CQWs with $x = 0.4$, we obtain a gap of 35 meV, an order of magnitude larger than that reported for binary InAs/GaSb CQWs. Using a dual-gate configuration, we demonstrate an electrical-field-driven topological phase transition, which manifests itself as a re-entrant behavior of the energy gap. The sizeable energy gap and high bulk resistivity obtained in both the topological and normal phases of a single device open the possibility of electrical switching of the edge transport.
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Submitted 3 August, 2020;
originally announced August 2020.
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Determination of $g$-factor in InAs two-dimensional electron system by capacitance spectroscopy
Authors:
Hiroshi Irie,
Takafumi Akiho,
Koji Muraki
Abstract:
We determine the effective $g$-factor ($|g^\ast|$) of a two-dimensional electron gas (2DEG) using a new method based on capacitance spectroscopy. The capacitance-voltage profile of a 2DEG in an InAs/AlGaSb quantum well measured in an in-plane magnetic field shows a double-step feature that indicates the Zeeman splitting of the subband edge. The method allows for simultaneous and independent determ…
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We determine the effective $g$-factor ($|g^\ast|$) of a two-dimensional electron gas (2DEG) using a new method based on capacitance spectroscopy. The capacitance-voltage profile of a 2DEG in an InAs/AlGaSb quantum well measured in an in-plane magnetic field shows a double-step feature that indicates the Zeeman splitting of the subband edge. The method allows for simultaneous and independent determination of $|g^\ast|$ and effective mass $m^\ast$. Data suggest that the biaxial tensile strain in the InAs layer has considerable impacts on both $m^\ast$ and $g^\ast$. Our method provides a means to determine $|g^\ast|$ that is complementary to the commonly used coincidence technique.
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Submitted 19 May, 2020;
originally announced May 2020.
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Impact of epitaxial strain on the topological-nontopological phase diagram and semimetallic behavior of InAs/GaSb composite quantum wells
Authors:
H. Irie,
T. Akiho,
F. Couëdo,
R. Ohana,
K. Suzuki,
K. Onomitsu,
K. Muraki
Abstract:
We study the influence of epitaxial strain on the electronic properties of InAs/GaSb composite quantum wells (CQWs), host structures for quantum spin Hall insulators, by transport measurements and eight-band $\mathbf{k\cdot p}$ calculations. Using different substrates and buffer layer structures for crystal growth, we prepare two types of samples with vastly different strain conditions. CQWs with…
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We study the influence of epitaxial strain on the electronic properties of InAs/GaSb composite quantum wells (CQWs), host structures for quantum spin Hall insulators, by transport measurements and eight-band $\mathbf{k\cdot p}$ calculations. Using different substrates and buffer layer structures for crystal growth, we prepare two types of samples with vastly different strain conditions. CQWs with a nearly strain-free GaSb layer exhibit a resistance peak at the charge neutrality point that reflects the opening of a topological gap in the band-inverted regime. In contrast, for CQWs with 0.50\% biaxial tensile strain in the GaSb layer, semimetallic behavior indicating a gap closure is found for the same degree of band inversion. Additionally, with the tensile strain, the boundary between the topological and nontopological regimes is located at a larger InAs thickness. Eight-band $\mathbf{k\cdot p}$ calculations reveal that tensile strain in GaSb not only shifts the phase boundary but also significantly modifies the band structure, which can result in the closure of an indirect gap and make the system semimetallic even in the topological regime. Our results thus provide a global picture of the topological-nontopological phase diagram as a function of layer thicknesses and strain.
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Submitted 27 February, 2020;
originally announced February 2020.
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Ballistic hot-electron transport in a quantum Hall edge channel defined by a double gate
Authors:
Shunya Akiyama,
Taichi Hirasawa,
Yuya Sato,
Takafumi Akiho,
Koji Muraki,
Toshimasa Fujisawa
Abstract:
Ballistic transport of hot electrons in a quantum Hall edge channel is attractive for studying electronic analog of quantum optics, where the edge potential profile is an important parameter that governs the charge velocity and scattering by longitudinal-optical (LO) phonons. Here we use a parallel double gate to control the electric field of the edge potential, and investigate the ballistic lengt…
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Ballistic transport of hot electrons in a quantum Hall edge channel is attractive for studying electronic analog of quantum optics, where the edge potential profile is an important parameter that governs the charge velocity and scattering by longitudinal-optical (LO) phonons. Here we use a parallel double gate to control the electric field of the edge potential, and investigate the ballistic length of the channel by using hot-electron spectroscopy. The ballistic length is significantly enhanced by reducing the LO phonon scattering rate in the tailored potential.
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Submitted 7 December, 2019;
originally announced December 2019.
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Charge equilibration in integer and fractional quantum Hall edge channels in a generalized Hall-bar device
Authors:
Chaojing Lin,
Ryota Eguchi,
Masayuki Hashisaka,
Takafumi Akiho,
Koji Muraki,
Toshimasa Fujisawa
Abstract:
Charge equilibration between quantum-Hall edge states can be studied to reveal geometric structure of edge channels not only in the integer quantum Hall (IQH) regime but also in the fractional quantum Hall (FQH) regime particularly for hole-conjugate states. Here we report on a systematic study of charge equilibration in both IQH and FQH regimes by using a generalized Hall bar, in which a quantum…
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Charge equilibration between quantum-Hall edge states can be studied to reveal geometric structure of edge channels not only in the integer quantum Hall (IQH) regime but also in the fractional quantum Hall (FQH) regime particularly for hole-conjugate states. Here we report on a systematic study of charge equilibration in both IQH and FQH regimes by using a generalized Hall bar, in which a quantum Hall state is nested in another quantum Hall state with different Landau filling factors. This provides a feasible way to evaluate equilibration in various conditions even in the presence of scattering in the bulk region. The validity of the analysis is tested in the IQH regime by confirming consistency with previous works. In the FQH regime, we find that the equilibration length for counter-propagating $δν$ = 1 and $δν$ = -1/3 channels along a hole-conjugate state at Landau filling factor $ν$ = 2/3 is much shorter than that for co-propagating $δν$ = 1 and $δν$ = 1/3 channels along a particle state at $ν$ = 4/3. The difference can be associated to the distinct geometric structures of the edge channels. Our analysis with generalized Hall bar devices would be useful in studying edge equilibration and edge structures.
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Submitted 3 May, 2019;
originally announced May 2019.
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Pseudospin Berry phase as a signature of nontrivial band topology in a two-dimensional system
Authors:
F. Couëdo,
H. Irie,
T. Akiho,
K. Suzuki,
K. Onomitsu,
K. Muraki
Abstract:
Electron motion in crystals is governed by the coupling between crystal momentum and internal degrees of freedom such as spin implicit in the band structure. The description of this coupling in terms of a momentum-dependent effective field and the resultant Berry phase has greatly advanced our understanding of diverse phenomena including various Hall effects and has lead to the discovery of new st…
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Electron motion in crystals is governed by the coupling between crystal momentum and internal degrees of freedom such as spin implicit in the band structure. The description of this coupling in terms of a momentum-dependent effective field and the resultant Berry phase has greatly advanced our understanding of diverse phenomena including various Hall effects and has lead to the discovery of new states of matter exemplified by topological insulators. While experimental studies on topological systems have focused on the gapless states that emerge at the surfaces or edges, the underlying nontrivial topology in the bulk has not been manifested. Here we report the observation of Berry's phase in magneto-oscillations and quantum Hall effects of a coupled electron-hole system hosted in quantum wells with inverted bands. In contrast to massless Dirac fermions in graphene, for which the Berry phase $Γ$ is quantized at $π$, we observe that $Γ$ varies with the Fermi level $E_\mathrm{F}$, passing through $π$ as $E_\mathrm{F}$ traverses the energy gap that opens due to electron-hole hybridization. We show that the evolution of $Γ$ is a manifestation of the pseudospin texture that encodes the momentum-dependent electron-hole coupling and is therefore a bulk signature of the nontrivial band topology.
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Submitted 23 December, 2019; v1 submitted 14 February, 2019;
originally announced February 2019.
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Counterflowing edge current and its equilibration in quantum Hall devices with sharp edge potential: Roles of incompressible strips and contact configuration
Authors:
T. Akiho,
H. Irie,
K. Onomitsu,
K. Muraki
Abstract:
We report the observation of counterflowing edge current in InAs quantum wells which leads to the breakdown of quantum Hall (QH) effects at high magnetic fields. Counterflowing edge channels arise from the Fermi-level pinning of InAs and the resultant sharp edge potential with downward bending. By measuring the counterflow conductance for varying edge lengths, we determine the effective number…
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We report the observation of counterflowing edge current in InAs quantum wells which leads to the breakdown of quantum Hall (QH) effects at high magnetic fields. Counterflowing edge channels arise from the Fermi-level pinning of InAs and the resultant sharp edge potential with downward bending. By measuring the counterflow conductance for varying edge lengths, we determine the effective number $\langle N_\text{C} \rangle$ of counterflowing modes and their equilibration length $λ_\text{eq}$ at bulk integer filling factor $ν= 1$--$4$. $λ_\text{eq}$ increased exponentially with magnetic field $B$, reaching $200~μ$m for $ν= 4$ at $B \geq 7.6$~T. Our data reveal important roles of the innermost incompressible strip with even filling in determining $\langle N_\text{C} \rangle$ and $λ_\text{eq}$ and the impact of the contact configuration on the QH effect breakdown. Our results show that counterflowing edge channels manifest as transport anomalies only at high fields and in short edges. This in turn suggests that, even in the integer QH regime, the actual microscopic structure of edge states can differ from that anticipated from macroscopic transport measurements, which is relevant to various systems including atomic-layer materials.
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Submitted 12 February, 2019;
originally announced February 2019.
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Waveform measurement of charge- and spin-density wave packets in a Tomonaga-Luttinger liquid
Authors:
Masayuki Hashisaka,
Naoaki Hiyama,
Takafumi Akiho,
Koji Muraki,
Toshimasa Fujisawa
Abstract:
In contrast to a free electron system, a Tomonaga-Luttinger (TL) liquid in a one dimensional (1D) electron system hosts charge and spin excitations as independent entities. When an electron wave packet is injected into a TL liquid, it transforms into wave packets carrying either charge or spin that propagate at different group velocities and move away from each other. This process, known as spin-c…
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In contrast to a free electron system, a Tomonaga-Luttinger (TL) liquid in a one dimensional (1D) electron system hosts charge and spin excitations as independent entities. When an electron wave packet is injected into a TL liquid, it transforms into wave packets carrying either charge or spin that propagate at different group velocities and move away from each other. This process, known as spin-charge separation, is the hallmark of TL physics. While the existence of these TL eigenmodes has been identified in momentum- or frequency-resolved measurements, their waveforms, which are a direct manifestation of 1D electron dynamics, have been long awaited to be measured. In this study, we present a time domain measurement for the spin-charge-separation process in an asymmetric chiral TL liquid comprising quantum Hall (QH) edge channels. We measure the waveforms of both charge and spin excitations by combining a spin filter with a time-resolved charge detector. Spatial separation of charge- and spin-wave packets over a distance exceeding 200 um was confirmed. In addition, we found that the 1D electron dynamics can be controlled by tuning the electric environment. These experimental results provide fundamental information about non-equilibrium phenomena in actual 1D electron systems.
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Submitted 14 March, 2017;
originally announced March 2017.
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Engineering quantum spin Hall insulators by strained-layer heterostructures
Authors:
Takafumi Akiho,
François Couëdo,
Hiroshi Irie,
Kyoichi Suzuki,
Koji Onomitsu,
Koji Muraki
Abstract:
Quantum spin Hall insulators (QSHIs), also known as two-dimensional topological insulators, have emerged as an unconventional class of quantum states with insulating bulk and conducting edges originating from nontrivial inverted band structures, and have been proposed as a platform for exploring spintronics applications and exotic quasiparticles related to the spin-helical edge modes. Despite theo…
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Quantum spin Hall insulators (QSHIs), also known as two-dimensional topological insulators, have emerged as an unconventional class of quantum states with insulating bulk and conducting edges originating from nontrivial inverted band structures, and have been proposed as a platform for exploring spintronics applications and exotic quasiparticles related to the spin-helical edge modes. Despite theoretical proposals for various materials, however, experimental demonstrations of QSHIs have so far been limited to two systems--HgTe/CdTe and InAs/GaSb--both of which are lattice-matched semiconductor heterostructures. Here we report transport measurements in yet another realization of a band-inverted heterostructure as a QSHI candidate--InAs/In$_{x}$Ga$_{1-x}$Sb with lattice mismatch. We show that the compressive strain in the In$_{x}$Ga$_{1-x}$Sb layer enhances the band overlap and energy gap. Consequently, high bulk resistivity, two orders of magnitude higher than for InAs/GaSb, is obtained deep in the band-inverted regime. The strain also enhances bulk Rashba spin-orbit splitting, leading to an unusual situation where the Fermi level crosses only one spin branch for electronlike and holelike bands over a wide density range. These properties make this system a promising platform for robust QSHIs with unique spin properties and demonstrate strain to be an important ingredient for tuning spin-orbit interaction.
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Submitted 10 November, 2016; v1 submitted 24 August, 2016;
originally announced August 2016.