Probing the superfluid velocity with a superconducting tip: the Doppler shift effect

TL;DR

This paper demonstrates that scanning tunneling spectroscopy with a superconducting tip can locally probe superfluid velocities in superconductors by detecting Doppler shifts in quasiparticle spectra, revealing vortex structures and supercurrents.

Contribution

It introduces a method using superconducting tips in STS to directly measure local superfluid velocities and supercurrents in superconductors, including vortex imaging.

Findings

STS with SC tips is sensitive to Doppler shifts in quasiparticle spectra.

Vortex cores and supercurrents are mapped at different length scales.

Interplay between pair potential and supercurrents at vortex edges is observed.

Abstract

We address the question of probing the supercurrents in superconducting (SC) samples on a local scale by performing Scanning Tunneling Spectroscopy (STS) experiments with a SC tip. In this configuration, we show that the tunneling conductance is highly sensitive to the Doppler shift term in the SC quasiparticle spectrum of the sample, thus allowing the local study of the superfluid velocity. Intrinsic screening currents, such as those surrounding the vortex cores in a type II SC in a magnetic field, are directly probed. With Nb tips, the STS mapping of the vortices, in single crystal 2H-NbSe_2, reveals both the vortex cores, on the scale of the SC coherence length $\xi$, and the supercurrents, on the scale of the London penetration length $\lambda$. A subtle interplay between the SC pair potential and the supercurrents at the vortex edge is observed. Our results open interesting prospects for the study of screening currents in any superconductor.