Surface versus bulk superconductivity in NbN thin films under magnetic fields

Abstract

The enhancement of superconductivity at surfaces and interfaces, where the order parameter persists at higher temperatures or magnetic fields than in the bulk, is a subtle yet fundamentally important phenomenon in condensed matter physics. In this work, we experimentally investigate the difference between near-surface and bulk superconducting transitions in 160-nm-thick NbN films, a conventional s-wave superconductor, under magnetic fields up to 8 T. To this end, we developed a highly sensitive nonlocal four-probe measurement scheme capable of simultaneously detecting surface and bulk electrical responses. This approach revealed distinct critical temperatures and superconducting transition widths that vary with the orientation of the applied magnetic field, ranging from perpendicular to parallel to the film plane. Our findings are consistent with recent theoretical predictions concerning multiple phase transitions in superconductors with spin-imbalanced fermion populations. The results not only establish a robust methodology for probing boundary-induced superconducting phenomena, but also provide insights relevant to the design of superconducting devices with engineered surface properties.

Source-reported materials — not catalogue approval

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