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 5 d -electron compounds, Sr M 2 Ge 2 (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-5 d orbital are strongly hybridized with Ge-4 p orbital, which contribute mainly to electronic states near the Fermi level ( E F ). Two (four) bands in SrIr 2 Ge 2 (SrPt 2 Ge 2 ) cross E F to construct rather three-dimensional Fermi surfaces. In SrPt 2 Ge 2 , novel Dirac cones around Σ point are observed, which are related to the band inversion between Ge-4 p and Pt-5 d 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 SrIr 2 Ge 2 but strong in SrPt 2 Ge 2 . Unstable phonon modes are observed in SrPt 2 Ge 2 with ThCr 2 Si 2 -type structure, corresponding to metastable phase observed by experiments.