Superconductivity in Ba_(n_2)Ir_(4n)Ge_(12n+4) (n = 1,2) with cage structure and softening of low-lying localized mode

摘要

We report on new superconductors Ba_(n+2)Ir_(4n)Ge_(12n+4) (n = 1, 2) with critical temperatures T_c = 6.1 and 3.2 K, respectively, along with their crystal structures, electron transport, and specific heat. The compounds are composed of alternating Ba@Ir_8Ge_(16) and Ba@Ir_2Ge_(16) cages, both of which are larger in the n = 1 sample than in the w = 2 sample. The normal-state heat capacity reveals two low-lying vibration modes associated with guest Ba cations, and both characteristic temperatures in Ba_3Ir_4Ge_(16) are smaller than those in Ba_4Ir_8Ge_(28). Meanwhile, the density functional theory calculations reveal that the Ge-4p bands dominated the Fermi level in both samples. We propose that the softening of localized phonons due to expansion of the cage strengthens the electron-phonon coupling between Ba cations and Ge anions, leading to the higher T_c in Ba_3Ir_4Ge_(16).