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 $\mu_0$Hc$_2$ (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 ($\lambda$) 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 {\lambda} 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.