Linear Magnetic Field Suppression of the Superconducting Gap in NbTi and MgB$$_2$$ Wires

Abstract Superconducting vortices have a normal core and are pinned at imperfections, facilitating large current flow. Applications such as high-field solenoids or superconducting motors rarely use pure materials, as these are brittle, and instead employ superconductors embedded in ductile matrices (e.g., Cu or Ag). Processing superconductors into grains and then embedding in wires can significantly affect their properties, which remain less explored than in pure materials. In particular, the superconducting gap, relevant for vortex pinning, has been little studied in wires. Here, we determine the gap as a function of temperature and magnetic field in NbTi and MgB $$_2$$ 2 wires using scanning tunneling microscopy. We find strong gap inhomogeneity, with $$\Delta _{NbTi}=0.9\pm 0.6$$ Δ NbTi = 0.9 ± 0.6 mV and $$\Delta _{MgB_2}=1.8\pm 0.2$$ Δ M g B 2 = 1.8 ± 0.2 mV. The temperature dependence follows BCS theory. Under magnetic field, the gap decreases approximately linearly, deviating from the usual $$\left( 1-\frac{H}{H_{c2}}\right) ^2$$ 1 - H H c 2 2 behavior. We attribute this deviation to gap inhomogeneity arising possibly due to processing the materials into a wire. Our work shows that gap structure studies could complement efforts to improve superconducting properties of wires.