Strong Electron-Phonon Coupling Superconductivity Induced by a Low-Lying Phonon in IrGe

The physical properties of the previously reported superconductor IrGe and the Rh1-xIrxGe 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 = 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 T-c in the specific-heat data clearly indicates that IrGe is a strong coupling superconductor. In the Rh1-xIrxGe solid solution, a relationship between an anomalous change in lattice constants and the Debye temperature is observed. We conclude that the unusually high T-c for IrGe is likely due to strong electron-phonon coupling derived from the presence of a low-lying phonon.