Evidence of a new low field cross-over in the vortex critical velocity of type-II superconducting thin films

TL;DR

This paper reports the discovery of a new low-field cross-over in vortex critical velocity in type-II superconducting thin films, linked to vortex channeling and inhomogeneous magnetic states, expanding understanding of vortex dynamics.

Contribution

It introduces the observation of a novel low-field cross-over in vortex critical velocity, supported by experimental evidence and magneto-optic imaging, revealing a new vortex motion regime.

Findings

Identification of a new cross-over field Bcr1 below which v* decreases with decreasing B

Confirmation of vortex channeling due to fan-like magnetic penetration

Evidence of inhomogeneous magnetic states causing channel-like vortex motion

Abstract

We measure current-voltage characteristics as function of magnetic field and temperature in Nb strips of different thickness and width. The instability voltage of the flux flow state related to the vortex critical velocity v* is studied and compared with the Larkin-Ovchinnikov theory. Beside the usual power-law dependence v* ~ B^-1/2, in the low field range a new cross-over field, Bcr1, is observed below which v* decreases by further lowering the external magnetic field B. We ascribe this unexpected cross-over to vortex channeling due to a fan-like penetration of the applied magnetic field as confirmed by magneto-optic imaging. The observation of Bcr1 becomes a direct evidence of a general feature in type-II superconducting films at low fields, that is a channel-like vortex motion induced by the inhomogeneous magnetic state caused by the relatively strong pinning.