Planar graphene- NbSe2 Josephson junctions in a parallel magnetic field

Abstract

Thin transition metal dichalcogenides (TMDs) 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 NbSe2 as superconducting electrodes laterally coupled to graphene -making a planar, all van der Waals (vdW) 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 EZ 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 a 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. 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.By coupling graphene to exfoliated superconductors such as NbSe 2 [1-3] it is possible to realize Josephson junctions where both the normal and superconductor materials are two-dimensional (2D).Such junctions should sustain high in-plane magnetic fields. Thin NbSe 2 retains superconductivity at very high in-plane fields due to a combination of suppressed orbital depairing and Ising protection against pair-breaking [4, 5], and can sustain magnetic fields above 8 T without any measureable effect on the gap * Equal contribution 1

Superconducting 2D NbS2 Grown Epitaxially by Chemical Vapor Deposition

“…Additionally, these metallic TMDCs hold interesting low-temperature properties such as superconductivity, 8 , 9 magnetism, 10 and charge density waves 11 , 12 (CDW), where superconducting TMDCs could be used to observe Andreev reflection in 2D systems 13 , 14 and realize Josephson junctions. 15 Among metallic TMDCs, 2H-NbS 2 is a peculiar case in that it is the only superconductor with no CDW instabilities observed experimentally. 8 This is unusual if compared with other isostructural and isoelectronic compounds such as 2H-NbSe 2 and 2H-TaS 2 , where superconductivity coexists with CDW order.…”

Nanomaterials for Quantum Information Science and Engineering

“…Despite limited success with QD-based spin qubits, 1D In-V NWs are highly attractive for topological qubits and are an excellent platform to explore superconductor-semiconductor hybrid devices such as gatemon qubits [323] and Andreev spin qubits [373] . 2D semiconductors and graphene are similarly interesting for gatemon qubit types [333,360] .…”

High Magnetic Field Stability in a Planar Graphene-NbSe2 SQUID

“…The transition toward narrow supercurrent channels has already been hinted at in our previous work�indeed, multiple diffusive MLG−NbSe 2 junctions also undergo a transition to SQUID-like interference patterns where all lobes are of similar height at high B ∥ . 8 In that work, the patterns were too disordered to fit to eq 1, and we could not rule out the role of ripples due to the SiO 2 substrate. 8 In the present work, the device is flat due to the use of an hBN substrate.…”

“…5,6 Devices consisting of NbSe 2 flakes coupled on both sides of a narrow graphene channel (Figure 1a) are wellbehaved Josephson junctions (JJs). 7,8 Our two-dimensional planar Josephson junctions (2DJJs), constructed exclusively from van der Waals (vdW) materials by transferring a cracked NbSe 2 flake on top of graphene, are unique in retaining a Josephson effect at high parallel magnetic fields.…”