Superconductivity with high upper critical field in Ta-Hf alloys*

Recently, there has been considerable interest in exploring superconducting alloys for potential applications in superconducting devices [1-3]. In this study, I will present our findings on superconductivity in Ta _x Hf _1-x alloys, utilizing magnetization, electrical resistivity, and specific heat measurements on polycrystalline samples [3].

The crystal structures of these alloys are composition dependent. Notably, when we substituted Hf (a Type-I superconductor with a critical temperature, T_C, of 0.12 K) with Ta (also a Type-I superconductor with a T_C of 4.4 K), a remarkable enhancement in T_C was observed, along with a transition to Type-II superconductivity featuring high upper-critical fields. This transition is of particular interest from the perspective of materials science. Our specific heat measurements provided evidence of strong coupling superconductivity, aligning with conventional principles of superconductivity. These metallic alloys exhibit both metallic properties and a high upper-critical field, making them highly promising for practical superconducting devices like SQUIDs and qubits.