The mechanism of charge density wave in Pt-based layered superconductors: SrPt 2 As 2 and LaPt 2 Si 2

The intriguing coexistence of the charge density wave (CDW) and superconductivity in SrPt 2 As 2 and LaPt 2 Si 2 has been investigated based on the ab initio density functional theory band structure and phonon calculations. We have found that the CDW instabilities for both cases arise from the q-dependent electron-phonon coupling with quasi-nesting feature of the Fermi surface. The band structure obtained by the band-unfolding technique reveals the sizable q-dependent electron-phonon coupling responsible for the CDW instability. The local split distortions of Pt atoms in the [As-Pt-As] layers play an essential role in driving the five-fold supercell CDW instability as well as the phonon softening instability in SrPt 2 As 2. By contrast, the CDW and phonon softening instabilities in LaPt 2 Si 2 occur without split distortions of Pt atoms. The phonon calculations suggest that the CDW and the superconductivity coexist in [X-Pt-X] layers (X = As or Si) for both cases.