Superconductivity and Thermal Conduction in Multicomponent Alloys

Abstract

Superconducting high-entropy alloys have recently emerged as a new platform for exploring quantum and transport phenomena in highly disordered metallic systems, while also offering potential advantages for applications requiring mechanical robustness and tolerance to extreme environments. However, the mechanisms governing their superconducting and thermal transport properties – particularly the roles of lattice distortion, electronic structure, and disorder – remain poorly understood. This thesis presents a two-fold investigation of niobium-based body-centered cubic multicomponent alloys, focusing on (i) superconductivity and (ii) thermal transport in both the normal and superconducting states. In the first part, a systematic series of Nb-based alloys, ranging from binary to quinary compositions, is examined to identify the dominant factors controlling superconductivity. Superconducting critical properties are found to evolve non-monotonically with increasing alloy complexity. Notably, alloys with higher lattice distortion can still exhibit enhanced critical temperature and upper critical field. Combined experimental measurements and first-principles Eliashberg analysis reveal that the position of the Nb d-band relative to the Fermi level is the primary electronic descriptor governing electron–phonon coupling and superconducting performance, while lattice distortion acts as a secondary modifier. These results establish a mechanism-based framework for designing superconducting high-entropy alloys. In the second part, the temperature-dependent thermal conductivity of the same alloy series is investigated from 2 to 300 K, spanning both normal and superconducting regimes. Increasing compositional disorder strongly suppresses electronic heat transport through enhanced scattering processes, driving the system into the dirty limit. As a result, the lattice contribution becomes increasingly significant with increasing alloy complexity. The obtained results demonstrate that multicomponent alloying enables the simultaneous tuning of superconducting and thermal transport properties through controlled disorder, providing new design strategies for functional materials operating under extreme conditions.