Superconductivity with high upper critical field in Ta-Hf Alloys

High upper-critical field superconducting alloys are required for superconducting device applications. In this study, we extensively characterized the structure and superconducting properties of alloys Ta${}_x$ Hf${}_{1-x}$ (x = 0.2, 0.4, 0.5, 0.6 and 0.8). The substitution of Hf (T${}_C$ = 0.12 K, type-I superconductor) with Ta (T${}_C$ = 4.4 K, type-I superconductor) shows an anomalous enhancement of T${}_C$ with variation of composition. Interestingly, all compositions exhibited strongly coupled bulk type-II superconductivity with a high upper critical field. In particular, for compositions x = 0.2, and 0.4, the upper critical field (H${}_{C2}$) approached the Pauli limiting field.