Superconductivity with high upper critical field in Ta-Hf alloys

High upper-critical field superconducting alloys are required for superconducting device applications. In this paper, we extensively characterized the structure and superconducting properties of alloys ${\mathrm{Ta}}_{x}{\mathrm{Hf}}_{1\ensuremath{-}x}$ ($x$ = 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 a variation of composition. Interestingly, all compositions exhibited strongly coupled bulk type-II superconductivity with a high upper critical field.