Electronic structure and anharmonic phonon mode in BaIr2Ge7 with two-dimensional Ba-Ge networks studied by photoemission spectroscopy

We report the electronic structure of BaIr2Ge7 with two types of cage structure by means of angle-resolved photoemission spectroscopy (ARPES) and hard x-ray photoemission spectroscopy. ARPES spectra reveal the three-dimensional and multiband Fermi surfaces (FSs) originating from the hybridized Ir 5d and Ge 4p orbitals. The observed FSs show C2 symmetry, reflecting the orthorhombic Ammm crystal structure of BaIr2Ge7. The temperature dependence of the ARPES spectra exhibits the thermal spectral broadening, and the width of the spectral peak shows a concave-downward behavior with temperature. Considering the effect of anharmonic phonon modes, we have reproduced the temperature dependence of the electrical resistivity as well as the thermal spectral broadening. The resultant renormalized phonon frequencies mr0(1) = 146.9 K and m(2) r0 = 70.6 K are comparable to the Einstein temperatures estimated from the previous specific heat measurement. Our results suggest the existence of the weak anharmonic phonon modes in BaIr2Ge7.