Abstract
Using muon spin rotation it is shown that the field of first flux penetration in Nb is enhanced by about 30% if coated with an overlayer of Nb3Sn or MgB2. This is consistent with an increase from the lower critical magnetic field up to the superheating field of the Nb substrate. In the experiments presented here coatings of Nb3Sn and MgB2 with a thickness between 50 and 2000 nm have been tested. 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 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 was found, also pushing beyond .
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