Planar graphene-${\mathrm{NbSe}}_{2}$ Josephson junctions in a parallel magnetic field

Planar graphene- ${NbSe}_{2}$ Josephson junctions in a parallel magnetic field

Tom Dvir, Ayelet Zalic, Eirik Holm Fyhn, Morten Amundsen, Takashi Taniguchi, Kenji Watanabe, Jacob Linder, and Hadar Steinberg

Phys. Rev. B 103, 115401 – Published 2 March 2021

Abstract

Thin transition metal dichalcogenides sustain superconductivity at large in-plane magnetic fields due to Ising spin-orbit protection, which locks their spins in an out-of-plane orientation. Here we use thin $Nb {Se}_{2}$ as superconducting electrodes laterally coupled to graphene, making a planar, all van der Waals two-dimensional Josephson junction (2DJJ). We map out the behavior of these novel devices with respect to temperature, gate voltage, and both out-of-plane and in-plane magnetic fields. Notably, the 2DJJs sustain supercurrent up to parallel fields as high as 8.5 T, where the Zeeman energy $E_{Z}$ rivals the Thouless energy $E_{T h}$ , a regime hitherto inaccessible in graphene. As the parallel magnetic field $H_{∥}$ increases, the 2DJJ's critical current is suppressed and in a few cases undergoes suppression and recovery. We explore the behavior in $H_{∥}$ by considering theoretically two effects: a 0- $π$ transition induced by tuning of the Zeeman energy and the unique effect of ripples in an atomically thin layer which create a small spatially varying perpendicular component of the field. The 2DJJs have potential utility as flexible probes for two-dimensional superconductivity in a variety of materials and introduce high $H_{∥}$ as a newly accessible experimental knob.

Received 9 August 2020
Revised 26 January 2021
Accepted 28 January 2021

DOI:https://doi.org/10.1103/PhysRevB.103.115401

Physics Subject Headings (PhySH)

Josephson effect Superconducting phase transition

Graphene Junctions Low-temperature superconductors Multilayer thin films

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Tom Dvir ^1,*, Ayelet Zalic^1,*, Eirik Holm Fyhn ², Morten Amundsen ², Takashi Taniguchi³, Kenji Watanabe ⁴, Jacob Linder², and Hadar Steinberg ¹

¹The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
²Center for Quantum Spintronics, Department of Physics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway
³International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
⁴Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

^*These authors contributed equally to this work.

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Vol. 103, Iss. 11 — 15 March 2021

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Figure 1
(a) Fabrication of planar graphene- $Nb {Se}_{2}$ JJs involves (1) exfoliation of graphene on ${SiO}_{2}$ and $Nb {Se}_{2}$ on PDMS, (2) stamping a cracked $Nb {Se}_{2}$ flake onto graphene, (3) stamping a thin hBN flake for encapsulation of the crack, (4) and patterning of electrodes. Steps are illustrated from left to right. (b) Optical image of junction A with schematics of current flow. $Nb {Se}_{2}$ thickness is around 10 nm. Scale bar is 10 $μ m$ . (c) Illustration of a rectangular junction geometry. (d) A false-color SEM image of the region marked by a red square in (b), showing the actual junction geometry, with the graphene flake contour highlighted and the direction of $H_{∥}$ indicated. Scale bar is 2 $μ m$ . (e) Schematic illustration of the JJ in a four-probe electronic configuration. Current flows in plane from $Nb {Se}_{2}$ to graphene to $Nb {Se}_{2}$ . The crack is shielded from the top by hBN. Gate voltage is applied across the ${SiO}_{2}$ dielectric.
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Figure 2
(a) $I − V$ curves of junction A (monolayer graphene) taken at different gate voltages (see legend). (b) Differential resistance $d V / d I$ of junction A as a function of bias current and gate voltage. (c) Temperature dependence of the critical current of junction A (blue dots) and a fit to Eq. (1) (orange line), taken with a gate voltage of $- 20$ V. (d)–(f) Differential resistance of junction A as a function of bias current and external perpendicular magnetic field, taken with gate voltages of 12 V, $- 4$ V (Dirac point), and $- 20$ V, respectively. All the panels show data at $H_{∥} = 0$ T and $T = 30$ mK.
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Figure 3
(a)–(d) Differential resistance of junction A as a function of bias current and external perpendicular magnetic field, taken with applied in-plane magnetic field of 0, 1, 2, and 3 T, respectively. All measurements were conducted with $V_{G}$ = 20 V. (e) Parallel field dependence of the maximal critical current for junctions A, B, C, and D. $I_{C}$ is normalized to $I_{C} (H_{∥} = 0, H_{⊥} = 0)$ , and the in-plane field is normalized by a junction-specific decay field $H_{D}$ . Each value is extracted from a 2D scan of $R (V, H_{⊥})$ at a given $H_{∥}$ and is defined as the maximal $I_{C}$ obtained in each scan. $H_{D}$ = 0.6, 0.16, 0.4, and 0.9 T for junctions A, B, C, and D, respectively. The field at which exponential decay slows $H_{T}$ is indicated by a dotted line for junctions A, B, and C. (f) $I_{C}$ of junction A as a function of $H_{∥}$ and $H_{⊥}$ . The curves were shifted to correct for sample misalignment and were then aligned to be as continuous as possible. Logarithmic color scale. (g) Dependence of the maximal $I_{C}$ (blue) and of $I_{C}$ at $H_{⊥} = 0$ (orange), extracted from (f).
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Figure 4
High-field supercurrent. (a) The interference pattern of junction A at a parallel field of 8.5 T shows clear lobes of zero resistance (data are from a different cooldown than Fig. 3). (b) $I − V$ curve from (a) where the critical current is maximal (blue) and minimal (orange). The measurement was set to stop when the normal state transport was observed.
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Figure 5
(a)–(c) Calculated critical current $I_{C}$ with a logarithmic color scale as a function of $H_{⊥}$ and $H_{∥}$ . (a) Simulated Zeeman effect with $E_{T h} = 64 μ eV$ and a rectangular junction of dimensions $L = 214$ nm, $W = 4.69 μ m$ without ripples. (b) A rectangular junction with ripples, disregarding the Zeeman effect. (c) Our measured junction contour with varying $L$ , $E_{T h} = 64 μ eV$ , Zeeman effect, and ripples. (d) Top: ripple profile used to generate the maps in (b) and (c). Note that the actual aspect ratio is around $W / L = 20$ . Bottom: illustration of a long-wavelength ripple which could give rise to a zero in $I_{c}$ at low fields. (e) $I_{C}$ at $H_{⊥} = 0$ (red) and maximal $I_{C}$ for all $H_{⊥}$ (blue) vs $H_{∥}$ ; line cuts are taken from the simulation in (c). (f)–(h) $I_{C}$ vs $H_{⊥}$ for $H_{∥} = 0, 4.1$ , and $6.2 T$ ; line cuts are from the simulation in (c).
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Figure 6
(a)–(d) Differential resistance of junction A as a function of bias current and gate voltage, taken with applied in-plane magnetic field of 0, 1, 2, and 3 T, respectively. All measurements were conducted with $H_{⊥} = 0$ . (e) $I_{C}$ vs $H_{⊥}$ and $V_{G}$ for junction A, taken with parallel field $H_{∥}$ = 3 T.
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Physical Review B

covering condensed matter and materials physics

Planar graphene- ${NbSe}_{2}$ Josephson junctions in a parallel magnetic field

Tom Dvir, Ayelet Zalic, Eirik Holm Fyhn, Morten Amundsen, Takashi Taniguchi, Kenji Watanabe, Jacob Linder, and Hadar Steinberg

Phys. Rev. B 103, 115401 – Published 2 March 2021

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Physical Review B

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Planar graphene-NbSe2 Josephson junctions in a parallel magnetic field

Tom Dvir, Ayelet Zalic, Eirik Holm Fyhn, Morten Amundsen, Takashi Taniguchi, Kenji Watanabe, Jacob Linder, and Hadar Steinberg

Phys. Rev. B 103, 115401 – Published 2 March 2021

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Planar graphene- ${NbSe}_{2}$ Josephson junctions in a parallel magnetic field