Effect of strain on the critical current and critical field of B1 structure NbN superconductors

Abstract
The effect of uniaxial strain on the critical current of NbN superconductors has been measured at 4.2 K in magnetic fields from 6 to 22 T. Unlike A15 superconductors, the critical current of B1 structure NbN shows no measurable dependence on elastic strain, even at fields as high as 22 T. Furthermore, the bulk upper critical field determined from the high-field critical current data is independent of uniaxial strain within experimental error (±0.5%). The irreversible strain limit εirrev where the superconductor is permanently damaged was measured to be greater than 0.7% (0.5% intrinsic strain) in 0.2-μm-thick film samples. Values of εirrev were observed to correlate with the microstructure of the NbN films as well as with the onset of substrate yielding.