Superconductivity in Single Crystals of Lu₃T₄Ge_13–x (T = Co, Rh, Os) and Y₃T₄Ge_13–x (T = Ir, Rh, Os)

Single crystals of Lu 3 T 4 Ge 13– x ( T = Co, Rh, Os) and Y 3 T 4 Ge 13– x ( T = Ir, Rh, Os) have been grown using the self-flux method and found to adopt the Yb 3 Rh 4 Sn 13– x structure type. Magnetization and specific heat measurements confirm that all compounds are bulk superconductors, some with reduced superconducting gaps determined from specific heat suggesting multiband superconductivity. However, an unusual increase of the electrical resistivity and a decrease of the charge carrier density on cooling in the normal state are observed for all six compounds even though density functional periodic calculations reveal a metallic ground state. We empirically show that large atomic displacement parameter ratios in the R 3 T 4 Ge 13– x compounds are correlated with the semiconductor-like behavior, which resolves the contradiction between the experiment and the calculations.