Superconductivity Above H_{c2} as a Probe for Niobium RF-Cavity Surfaces

TL;DR

This study investigates the magnetic response and AC conductivity of niobium cylinders above the upper critical field to understand surface superconductivity, revealing a percolation transition and effects of surface treatments on superconducting properties.

Contribution

It provides new insights into the nucleation and coherence of surface superconductivity in niobium, highlighting the impact of surface treatments and the dimensional nature of the percolation transition.

Findings

Identification of incoherent and coherent surface superconductivity nucleation points.

Surface treatments significantly affect the critical field H_{c3}.

Percolation transition in surface superconductivity is 2D for smooth surfaces and crossover to 3D for rougher surfaces.

Abstract

From the magnetic response of Nb-cylinders to time-varying fields the AC-conductivity, $\sigma'-i\sigma''$, and the critical current density $J_c^s$ are determined above the upper critical field $H_{c2}$. The field dependencies of $\sigma'$ and $\sigma''$ allow us to identify the nucleation of incoherent surface superconductivity (SSC) at $H_{c3}$ and of coherent SSC at $H_{c3}^c\approx 0.81H_{c3}$. The latter result turns out to be independent on different surface roughnesses and surface impurities obtained by surface treatments like chemical (BCP) and electrolytical polishing (EP), and low temperature baking (LTB). They all have a large impact on $H_{c3}$ itself, which is associated with a change of electron mean free path $\ell$ due to impurities in a surface layer. A detailed analysis of $\sigma'$, $\sigma''$ and $J_c^s$ near $H_{c3}^c$ reveals that the coherence of the SSC results from a percolation transition. For EP cylinders, this transition appears to be strictly two dimensional (2D), while the analysis for BCP cylinders with a rougher surface reveal much smaller $J_c^s$ and indicate a crossover to 3D. As a rather surprising feature, we detected large concentrations of paramagnetic moments, which increased under LTB and were reduced by EP.