Strong electron-Phonon coupling superconductivity induced by a low-Lying phonon in irGe

The physical properties of the previously reported superconductor IrGe and the Rh_1-xIr_xGe solid solution are investigated. IrGe has an exceptionally high superconducting transition temperature (T_c = 4.7 K) among the isostructural 1 : 1 late-metal germanides MGe (M 1/4 Rh, Pd, Ir, and Pt). Specific-heat measurements reveal that IrGe has an anomalously low Debye temperature, originating from a low-lying phonon, compared to the other MGe phases. A large jump at Tc in the specific-heat data clearly indicates that IrGe is a strong coupling superconductor. In the Rh_1-xIrxGe solid solution, a relationship between an anomalous change in lattice constants and the Debye temperature is observed. We conclude that the unusually high Tc for IrGe is likely due to strong electron-phonon coupling derived from the presence of a low-lying phonon.