La2Rh$_{3+\delta}$Sb4: a new ternary superconducting rhodium-antimonide

Rhodium-containing compounds offer a fertile playground to explore novel materials with superconductivity (SC) and other fantastic electronic correlation effects. A new ternary rhodium-antimonide La2Rh_{3+\delta}Sb4 (\delta\approx 1/8) has been synthesized by a Bi-flux method. It crystallizes in the orthorhombic Pr2Ir3Sb4-like structure, with the space group Pnma (No. 62). The crystalline structure appears as stacking the two-dimensional RhSb4- and RhSb5-polyhedra networks along b axis, and the La atoms embed in the cavities of these networks. Band structure calculations confirm it as a multi-band metal with a van-Hove singularity like feature at the Fermi level, whose density of states are mainly of Rh-4d and Sb-5p characters. The calculations also imply that the redundant Rh acts as charge dopant. SC is observed in this material with onset transition at T_c^\mathrm{on}\approx 0.8 K. Ultra-low temperature magnetic susceptibility and specific heat measurements suggest that it is an s-wave type-II superconductor. Our work may also imply that the broad Ln_2Tm_{3+\delta}Sb4 (Ln = rare earth, Tm = Rh, Ir) family may host new material bases where new superconductors, quantum magnetism and other electronic correlation effects could be found.