Dynamical Josephson effects in NbSe2

S Tran,J Sell,J R Williams

doi:10.1103/physrevresearch.2.043204

S Tran, J Sell + Show 1 more

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https://doi.org/10.1103/physrevresearch.2.043204

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Abstract

The study of superconducting materials that also possess nontrivial correlations or interactions remains an active frontier of condensed matter physics. NbSe$_2$ belongs to this class of superconductors and recent research has focused on the two-dimensional properties of this layered material. Here an investigation of the superconducting-to-normal-state transition in NbSe$_2$ is detailed, and found to be driven by dynamically-created vortices. Under the application of RF radiation, these vortices allow for two novel Josephson effects to be observed. The first is a coupling between Josephson currents and charge density waves in phase-slip junctions. The second is the Josephson detection of multi-band superconductivity, which is revealed in an anomalous magnetic field and RF frequency response of the AC Josephson effect. Our results shed light on the nature of superconductivity in this material, unearthing exotic phenomena by exploiting nonequilibrium superconducting effects in atomically-thin materials.

Highlights

The Bardeen–Cooper–Schrieffer (BCS) description of a superconductor provides a mean-field description of Copper pairs in a material in equilibrium
Should there be a junction created by the phase slip line (PSL), radio frequency (RF) radiation mixed with the internal Josephson frequency should produce voltage steps in the I–V curve—called Shapiro steps—of height related to the applied frequency: V = h f /2e [16]
Motion of a sliding charge density wave (CDW) is described by an equation that is very similar to Josephson junctions—each being described by a differential equation isomorphic to that of a driven pendulum

Summary

INTRODUCTION

The Bardeen–Cooper–Schrieffer (BCS) description of a superconductor provides a mean-field description of Copper pairs in a material in equilibrium. This produces a chain of fast (kinematic) vortices, resulting in a region of suppressed superconductivity that spans the sample width [Fig. 1(f), Regime II] This is called a phase slip line (PSL) and it forms a Josephson junction with a length of ∼ξ. Should there be a junction created by the PSL, RF radiation mixed with the internal Josephson frequency should produce voltage steps in the I–V curve (or peaks in r)—called Shapiro steps—of height related to the applied frequency: V = h f /2e [16] This is known as the AC Josephson effect. It follows a different power dependence than the PSLs and the transition to the normal state This feature is not captured in the simulation of the Shapiro step diagrams arising solely from PSLs [13]. This value is consistent with sub-10 K measurements for NbSe3 nanowires [22]

Findings

REGIME I

CONCLUSION