Spin-Orbit-Enhanced Robustness of Supercurrent in $\mathrm{Graphene}/{\mathrm{WS}}_{2}$ Josephson Junctions

Spin-Orbit-Enhanced Robustness of Supercurrent in $Graphene / {WS}_{2}$ Josephson Junctions

T. Wakamura, N. J. Wu, A. D. Chepelianskii, S. Guéron, M. Och, M. Ferrier, T. Taniguchi, K. Watanabe, C. Mattevi, and H. Bouchiat

Phys. Rev. Lett. 125, 266801 – Published 21 December 2020

Abstract

We demonstrate the enhanced robustness of the supercurrent through graphene-based Josephson junctions in which strong spin-orbit interactions (SOIs) are induced. We compare the persistence of a supercurrent at high out-of-plane magnetic fields between Josephson junctions with graphene on hexagonal boron-nitride and graphene on ${WS}_{2}$ , where strong SOIs are induced via the proximity effect. We find that in the shortest junctions both systems display signatures of induced superconductivity, characterized by a suppressed differential resistance at a low current, in magnetic fields up to 1 T. In longer junctions, however, only graphene on ${WS}_{2}$ exhibits induced superconductivity features in such high magnetic fields, and they even persist up to 7 T. We argue that these robust superconducting signatures arise from quasiballistic edge states stabilized by the strong SOIs induced in graphene by ${WS}_{2}$ .

Received 3 August 2020
Accepted 16 November 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.266801

Physics Subject Headings (PhySH)

Andreev reflection Ballistic transport Edge states Josephson effect Proximity effect Quantum interference effects Quantum transport Spin-orbit coupling Superconducting fluctuations Superconductivity

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

T. Wakamura¹, N. J. Wu ^1,2, A. D. Chepelianskii¹, S. Guéron¹, M. Och³, M. Ferrier¹, T. Taniguchi⁴, K. Watanabe⁵, C. Mattevi³, and H. Bouchiat^1,*

¹Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, 91405, Orsay, France
²Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d’Orsay, 91405, Orsay, France
³Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom
⁴International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
⁵Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

^*Corresponding author. helene.bouchiat@u-psud.fr

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Vol. 125, Iss. 26 — 31 December 2020

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Figure 1
Device structure and transport properties around zero magnetic field. (a) Schematic illustration of a graphene-based Josephson junction device of length $L$ and width $W$ employed in this study. (b) $V_{g}$ dependence of $R$ for the $Gr / {WS}_{2}$ and Gr/hBN junctions with MoRe in the normal state ( $L = 500 nm$ ). The contact resistance is subtracted in the data. (c),(d) Color-coded $d V / d I$ , plotted as a function of $I_{dc}$ and $B$ for $Gr / {WS}_{2}$ [(c)] and Gr/hBN junctions [(d)] with $L = 500 nm$ measured at $V_{g} = 60 V$ .
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Figure 2
Color-coded $d V / d I$ as a function $I_{dc}$ and $B$ around $B = 10000 G$ at $V_{g} = 60 V$ for all samples. For $L = 100 nm$ [(a),(d)], superconducting pockets are clearly visible, in the form of field regions of low $d V / d I$ at low $I_{dc}$ , for both the $Gr / {WS}_{2}$ and Gr/hBN junctions around $B = 8000 G$ . For $L = 300 nm$ [(b),(e)] and $L = 500 nm$ [(c),(f)], superconducting pockets are visible only for the $Gr / {WS}_{2}$ junction. (g)–(i) Cross-sectional image along the light blue line shown in (a)–(f) of $d V / d I$ as a function of $I_{dc}$ for $Gr / {WS}_{2}$ and Gr/hBN junctions with different $L$ . Red and light green curves are from $Gr / {WS}_{2}$ and Gr/hBN junctions, respectively. The suppressed $d V / d I$ at low $I_{dc}$ , signature of an induced superconducting proximity effect, is clearly visible for the $Gr / {WS}_{2}$ junctions of every length but only for the shortest Gr/hBN junction. In (g), the peak (or bump) of $d V / d I$ for Gr/hBN is located out of the range of $I_{dc}$ in the measurement, and in (h) [(i)], the $d V / d I$ for Gr/hBN ( $Gr / {WS}_{2}$ ) is vertically shifted to compare to that for $Gr / {WS}_{2}$ (Gr/hBN). The residual resistance around $50 Ω$ at $I_{dc} = 0$ for (a)–(i) arises from the measurement wires. The dashed line in (g) represents the value of $I_{dc}$ , which defines $I_{c}$ .
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Figure 3
Monitoring the superconducting proximity effect over a wide field range, via the zero bias differential resistance variations with $B$ , for the three junction lengths and both $Gr / {WS}_{2}$ and Gr/hBN systems, at $V_{g} = 60 V$ . (a) For $L = 100 nm$ , both $Gr / {WS}_{2}$ and Gr/hBN junctions display comparable oscillation amplitudes up to $B \sim 20000 G$ . (b) For $L = 300 nm$ , whereas the Gr/hBN junction displays larger amplitude oscillations near $B = 0$ , they are rapidly suppressed as $B$ increases. By contrast, the $Gr / {WS}_{2}$ junction displays a relatively large amplitude of resistance oscillations that persists even around 20 000 G. (c) The difference between $Gr / {WS}_{2}$ and Gr/hBN is the most striking for $L = 500 nm$ junctions. The relative oscillation amplitude of the $Gr / {WS}_{2}$ junction’s differential resistance is around 50 times greater than that of the Gr/hBN junction over the entire field range. The inset in (b) displays a magnified view of the oscillations for $Gr / {WS}_{2}$ around $B = 13000 G$ .
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Physical Review Letters

Spin-Orbit-Enhanced Robustness of Supercurrent in Graphene/WS2 Josephson Junctions

T. Wakamura, N. J. Wu, A. D. Chepelianskii, S. Guéron, M. Och, M. Ferrier, T. Taniguchi, K. Watanabe, C. Mattevi, and H. Bouchiat

Phys. Rev. Lett. 125, 266801 – Published 21 December 2020

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Spin-Orbit-Enhanced Robustness of Supercurrent in $Graphene / {WS}_{2}$ Josephson Junctions