Effect of C additives with 0.5 % in weight on structural, optical and superconducting properties of Ta–Nb–Hf–Zr–Ti high entropy alloy films

We investigated the superconducting (SC) properties of Ta–Nb–Hf–Zr–Ti high-entropy alloy (HEA) thin films with 0.5% weight C additives. The C additives stabilize the structural properties and enhance the SC critical properties, including μ0Hc2 (13.45 T) and Tc (7.5 K). The reflectance of the C-added HEA film is enhanced in the low-energy region, resulting in a higher optical conductivity, which is consistent with the lower electrical resistivity. In addition, we observed SC vortices in the C-added HEA film using magnetic force microscopy. The magnetic penetration depths (λ) of the pure HEA and C-added HEA films were estimated from their Meissner force curves by comparing them with those of a reference Nb film. At 4.2 K, the λ of the C-added film is 360 nm, shorter than that of the pure HEA film (560 nm), indicating stronger superconductivity against an applied magnetic field.