Probing superconducting ground states in VEC-optimized Hf-Ta-Nb-Mo-W high entropy alloys

Structural characterization, along with detailed analysis of the superconducting and normal-state properties, was performed on Hf-Ta-Nb-Mo-W high entropy alloys (HEAs) with an optimized valence electron concentration (VEC) of ∼4.85. The results demonstrate that, despite their intrinsic chemical complexity, these HEAs maintain a highly stable single-phase body-centered cubic (bcc) structure. The Hf-Nb-Ta-Mo-W compositions exhibit bulk superconductivity with unconventional characteristics, including a maximum superconducting T_c of 6.05 K and the coexistence of fully gapped superconductivity with strong-coupling behavior. Thermally activated flux flow analysis reveals that collective vortex pinning dominates over single-vortex pinning in these superconducting HEAs. The findings confirm s-wave superconductivity in the presence of strong electronic correlations, exhibiting features reminiscent of nodal gap structures observed in heavy fermion and cuprate superconductors. The elemental composition and optimized VEC critically enhance superconductivity, primarily by modifying the density of states and electron-phonon coupling through the unique electronic and structural characteristics of these materials.