Measurement of the ac conductivity of a niobium single crystal in a swept magnetic field

TL;DR

This study investigates how swept magnetic fields influence the ac conductivity of a niobium single crystal, revealing increased dissipation, a conductive surface layer, and effects on surface superconducting states.

Contribution

It provides new experimental insights into the impact of swept magnetic fields on vortex and surface superconducting properties in niobium.

Findings

Swept magnetic fields significantly increase vortex dissipation.

A surface layer with enhanced conductivity was observed.

The ac response of surface superconducting states is affected by the swept field.

Abstract

We report results of experimental studies of ac susceptibility of Nb single crystal at low frequencies in swept magnetic fields applied parallel to the surface. Analysis of the experimental data shows that the swept magnetic field significantly changes the vortex conductivity in the bulk. It becomes dissipative with unexpected large frequency dispersion. At the surface, one observes a layer with enhanced conductivity in comparison to the bulk. This layer provides a considerably large contribution to the shielding and absorption of an ac field even in dc fields below $H_{c2}$. We have also demonstrated that the swept magnetic field apparently affects an ac response of the surface superconducting states.