Endohedral Cluster Superconductors in the Mo-Ga-Sn System Explored by the Joint Flux Technique

Endohedral Ga cluster compounds feature nontrivial superconducting states including the two-gap superconductivity similar in nature to MgB₂. We use the joint flux synthetic technique to introduce Sn into the Ga matrix and tune the valence electron count in the two new endohedral cluster superconductors Mo₈Ga_41-Sn and Mo₄Ga_21-Sn with critical temperatures of T_c = 8.7 and 5.85 K, respectively. While the former compound is a derivative of the previously known Mo₈Ga₄₁ superconductor, where Sn atoms are enclosed inside the Sn@Ga₆ octahedral clusters, the latter is a new architecture built upon Mo@Ga₉Sn clusters, Ga@Ga₁₂ cuboctahedra, and Sn₄ squares. We show that this novel Mo₄Ga_21-Sn superconductor features strong electron-phonon coupling with the large ratio of 2Δ(0)/(k_BT_c) = 4.1 similar to that of the Mo₈Ga₄₁ superconductor with the closely related crystal structure.