# Superheating in coated niobium

## TL;DR

This study demonstrates that coating niobium with Nb_3Sn or MgB_2 enhances its flux penetration threshold to near the superheating field, with coatings of 50-2000nm thickness, indicating a barrier effect that improves superconductor performance.

## Contribution

It provides experimental evidence that coatings of Nb_3Sn and MgB_2 increase the flux penetration field in niobium, suggesting a mechanism involving the proximity effect and boundary barriers.

## Key findings

- Coatings increase flux penetration field by about 30%.
- H_entry does not depend on coating material or thickness.
- Low temperature baking increases H_entry by 6%.

## Abstract

Using muon spin rotation it is shown that the field of first flux penetration H_entry in Nb is enhanced by about 30% if coated with an overlayer of Nb_3Sn or MgB_2. This is consistent with an increase from the lower critical magnetic field H_c1 up to the superheating field H_sh of the Nb substrate. In the experiments presented here coatings of Nb_3Sn and MgB_2 with a thickness between 50 and 2000nm have been tested. H_entry does not depend on material or thickness. This suggests that the energy barrier at the boundary between the two materials prevents flux entry up to H_sh of the substrate. A mechanism consistent with these findings is that the proximity effect recovers the stability of the energy barrier for flux penetration, which is suppressed by defects for uncoated samples. Additionally, a low temperature baked Nb sample has been tested. Here a 6% increase of H_entry was found, also pushing H_entry beyond H_c1.