The coexistence of charge density wave and superconductivity 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 SrPt2As2 and LaPt2Si2 has been investigated based on the ab initio density functional theory band structure and phonon calculations. We have found that the local split distortions Pt atoms in the [As-PtAs] layers play an essential role in driving the five-fold supercell CDW instability as well as the phonon softening instability in SrPt2As2. By contrast, the CDW and phonon softening instabilities in LaPt2Si2 occur without split distortions of Pt atoms, indicating that the driving mechanisms of the CDW in SrPt2As2 and LaPt2Si2 are different. We have found that the CDW instability for the former arises from the Fermi surface nesting, while, for the latter, from the saddle point scattering. The phonon calculations, however, suggest that the CDW and the superconductivity coexist in [X-Pt-X] layers (X = As or Si) for both cases.