Probing the Superfluid Velocity with a Superconducting Tip: The Doppler Shift Effect

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 (QP) 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.

Imaging superconducting vortex cores and lattices with a scanning tunneling microscope

“…[115,192,193], a tip of Pb, and in Ref. [126] a tip of Nb is used. The superconducting gap of both elements is of same order than the superconducting gap of the sample.…”

Determination of the magnetic penetration depth in a superconducting Pb film

“…Bulk techniques [1][2][3][4][5][6] average the response of the whole sample and therefore are not sensitive to the local details such as possible material inhomogeneities. Local techniques [7][8][9][10][11][12] not only allow one to gain insights at the microscopic level but can be also considered as a more direct way to assess k. Within the local probe category, the most popular approach for estimating k consists of mapping the field profile of an isolated flux quantum and track its temperature evolution. Unfortunately, irrespective of whether the flux quantum is an Abrikosov vortex present in a thick sample, or a Pearl vortex 13 characteristic of thin films, the vertical component of the magnetic field picked up by the sensor depends on a single variable, z 0 þk, forming an indissociable additive combination of the vertical separation between the sensor and the surface of the superconductor, z 0 , and k. Therefore, the precision of the extracted k is directly linked to the quality of the calibration of the z-positioners and relies on perfect knowledge of the geometrical configuration and the sensor mounting.…”

Supercurrent on a vortex core in 2H-NbSe2: Current-driven scanning tunneling spectroscopy measurements

“…A simple, yet useful understanding of J d can be obtained by taking into account that the current Doppler-shifts the superconducting density of states. [32][33][34][35][36][37][38] At the depairing current density J d the Doppler shift is of the order of the gap and superconductivity is lost. Unfortunately, the flow of a current through the cross section of a bulk superconducting sample is a difficult problem involving electrodynamics and vortex matter which is not fully solved.…”