Electronic band structure and lattice dynamics of 5d-electron potential superconductors SrM2Ge2 (M = Ir, Pt)

Abstract

Motivated by current interest in the materials with heavy elements, we perform theoretical calculations on two 5d-electron compounds, SrM2Ge2 (M = Ir, Pt), which are observed superconductivity by former experiments. The electronic and phonon properties are obtained by ab initio calculations. The results show that the Pt/Ir-5d orbital are strongly hybridized with Ge-4p orbital, which contribute mainly to electronic states near the Fermi level (EF). Two (four) bands in SrIr2Ge2 (SrPt2Ge2) cross EF to construct rather three-dimensional Fermi surfaces. In SrPt2Ge2, novel Dirac cones around Σ point are observed, which are related to the band inversion between Ge-4p and Pt-5d orbital. When spin-orbit coupling (SOC) is considered, the Dirac cone along Z-Σ becomes fully gapped, but the cone along Γ − Σ line is stable. The lattice dynamics and electron-phonon coupling (EPC) are also studied with and without SOC. We find EPC is quite weak in SrIr2Ge2 but strong in SrPt2Ge2. Unstable phonon modes are observed in SrPt2Ge2 with ThCr2Si2-type structure, corresponding to metastable phase observed by experiments.