Coexistence of induced superconductivity and quantum Hall states in InSb nanosheets

Jinhua Zhi, Ning Kang, Feifan Su, Dingxun Fan, Sen Li, Dong Pan, S. P. Zhao, Jianhua Zhao, and H. Q. Xu

Phys. Rev. B 99, 245302 – Published 14 June 2019

Abstract

Hybrid superconducting devices based on high-mobility two-dimensional electron gases with strong spin-orbit coupling are considered to offer a flexible and scalable platform for topological quantum computation. Here, we report the realization and electrical characterization of hybrid devices based on high-quality InSb nanosheets and superconducting niobium (Nb) electrodes. In these hybrid devices, we observe gate-tunable proximity-induced supercurrent and multiple Andreev reflections, indicating a transparent Nb-InSb nanosheet interface. The high critical magnetic field of Nb combined with high-mobility InSb nanosheets allows us to exploit the transport properties in the exotic regime where the superconducting proximity effect coexists with the quantum Hall effect. Transport spectroscopy measurements in such a regime reveal an enhancement of the conductance at the quantum Hall plateaus, accompanied by a pronounced zero-bias peak in the differential conductance. We discuss that these features originate from the formation of Andreev edge states at the superconductor-InSb nanosheet interface in the quantum Hall regime. In addition to shedding light on the interplay between superconductivity and quantum Hall effect, our work opens a new possibility to develop hybrid superconducting devices based on 2D semiconductor nanosheets with strong spin-orbit coupling.

Received 30 October 2018
Revised 19 May 2019

DOI:https://doi.org/10.1103/PhysRevB.99.245302

Physics Subject Headings (PhySH)

Andreev reflection Josephson effect Landau levels Spin-orbit coupling Superconductivity Transport phenomena

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jinhua Zhi¹, Ning Kang^1,*, Feifan Su², Dingxun Fan¹, Sen Li¹, Dong Pan³, S. P. Zhao², Jianhua Zhao^3,†, and H. Q. Xu^1,4,‡

¹Beijing Key Laboratory of Quantum Devices, Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, China
²Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
³State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China
⁴Beijing Academy of Quantum Information Sciences, Beijing 100193, China

^*Corresponding author: nkang@pku.edu.cn
^†Corresponding author: jhzhao@red.semi.ac.cn
^‡Corresponding author: hqxu@pku.edu.cn

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Issue

Vol. 99, Iss. 24 — 15 June 2019

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Images

Figure 1
Characterization of InSb nanosheets. (a) SEM and TEM images of synthesized InSb nanosheets. The 2D InSb nanosheets were epitaxially grown on freestanding 1D InAs nanowires on a $p$ -type Si(111) substrate. (b) Schematic of the Josephson junction geometry based on InSb nanosheet with global back gate and the measurement setup. (c) Two-terminal conductance vs gate voltage $V_{g}$ . Data taken at $V_{bias} = 1 mV$ and $T = 60 mK$ . From the transfer characteristics of the device, we can extract a field-effect electron mobility of $μ ≃ 6000 {cm}^{2} V^{- 1} s^{- 1}$ . Inset: false-color SEM image of a fabricated Nb-InSb nanosheet hybrid device: brown–Nb contact, green–InSb nanosheet. (d) Low-field Hall coefficients $R_{H}$ (red dots) and carrier densities $n$ (blue dots) as a function of the back-gate voltage $V_{g}$ . The blue line is a linear fit to the $n - V_{g}$ data.
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Figure 2
Gate-modulated supercurrent in an InSb nanosheet Josephson junction. (a) $V_{s d} - I_{bias}$ characteristics at $T = 10 mK$ and various back gate voltages $V_{g}$ showing the modulation of the critical current. (b) Color-scale plot of differential resistance $d V_{s d} / d I_{bias}$ vs $V_{g}$ and $I_{bias}$ at $T = 10$ mK and $B = 0 T$ . (c) Differential conductance $d I_{s d} / d V_{bias}$ vs $V_{bias}$ measured at $V_{g} = 1$ V and $T = 10$ mK. The arrows and vertical dotted lines indicate conductance peaks at voltage positions $e V_{n} = 2 Δ / n (n = 1, 2, 3 \dots)$ , corresponding to multiple Andreev reflections. (d) Current-voltage characteristics with the same condition as in (c). The linear fit of the current-voltage curve at $V_{bias} > 2 Δ / e$ extends to a finite value of the current at zero-bias voltage, i.e., excess current $I_{e x c}$ . The inset shows a typical evolution of the transmission coefficient as a function of gate voltage.
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Figure 3
The coexistence of superconductivity and quantum Hall states. (a) Right panel: color-scale plot of differential conductance for Nb-InSb nanosheet hybrid device as a function of gate voltage $V_{g}$ and magnetic field $B$ , measured at $T = 60 mK$ . The typical Landau fan diagram with conductance plateaus of filling factor $ν = n h / e B = 1, 2, 3 \dots$ is observed. The dashed black lines track the same quantum Hall states in 2D InSb nanosheet. Left panel: resistance of the niobium electrodes as a function of magnetic field $B$ at $T = 60$ mK. The horizontal dashed line indicates the critical field of niobium ( $H_{c 2} \sim 7$ T), below which an increase of conductance for the Nb-InSb nanosheet hybrid device is observed due to the superconducting proximity effect. (b) Gate-voltage dependence of differential conductance at different magnetic fields (2.7 T, 3.6 T, 6.3 T, 10.2 T). At $B = 10.2$ T (blue trace) above $H_{c 2}$ , fully developed conductance plateaus are evident at $ν = 1, 3$ . Below $H_{c 2}$ , the conductance in the plateau region shows an enhancement, deviating from the expected quantized value.
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Figure 4
Bias spectroscopy of Andreev reflection in the quantum Hall regime. (a), (b) Differential conductance $G = d I_{s d} / d V_{bias}$ in units of $e^{2} / h$ as a function of bias voltage and back-gate voltage, taken at 10 and 2.3 T, respectively. At $B = 10$ T, the niobium contacts are driven into the normal state. The conductance map shows clear diamond-shaped structures highlighted by the magenta dashed lines, demonstrating the energy gap of Landau levels (LLs). At $B = 2.3$ T, well below $H_{c 2}$ , the spectroscopy exhibits two symmetric ridges located around $V_{bias} ≃ \pm 0.6$ mV (white dashed lines), corresponding to the superconducting gap of the Nb film for $B = 2.3 T$ . (c) Vertical line cut from (a) at the gate voltage indicated by the red dashed line. The conductance shows a suppression around zero bias, consistent with an opening gap between LLs. Inset: Schematic illustration demonstrating quasiclassical picture of an alternating electron-hole cyclotron motion due to Andreev reflection along the 2DEG-S interfaces (left) and coherent Andreev edge-state formation in the quantum Hall regime (right). (d) Vertical line cut from (b) at the gate voltage indicated by the green dashed line. It clearly displays a sharp zero-bias conductance peak when the Nb electrodes are superconducting.
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Figure 5
Pronounced magnetoconductance oscillations in Nb-InSb nanosheet hybrid devices. (a) Magnetoconductance at $V_{bias} = 0.1 mV$ (subgap) and $V_{bias} = 2.5 mV$ (above gap). The subgap conductance clearly exhibits a pronounced oscillations under 4 T relative to above-gap conductance. (b) Conductance oscillation $Δ G_{s}$ as a function of inverse magnetic field $1 / B$ . Vertical dashed line indicates equal periods of oscillations. The inset shows the Fourier transform of the oscillations, which exhibits a single peak.
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