Abstract
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 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 SrPt2As2. By contrast, the CDW and phonon softening instabilities in LaPt2Si2 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.
Highlights
Low-dimensional systems often suffer from intrinsic instabilities, revealing diverse interesting phase transitions upon cooling, such as charge density wave (CDW), spin density wave, superconductivity (SC), and so on
To examine the mechanism of CDW instability in SrPt2As2, we focused on the role of split distortions of Pt, and so considered two structures
The no-split-SPA structure is close to the tetragonal CaBe2Ge2-type structure above TCDW
Summary
Low-dimensional systems often suffer from intrinsic instabilities, revealing diverse interesting phase transitions upon cooling, such as charge density wave (CDW), spin density wave, superconductivity (SC), and so on. They reported that CaBe2Ge2-type is more stable than ThCr2Si2-type polymorphs None of these studies explored the electronic structures of the low T phase of SrPt2As2 having the split-off positions of Pt and the CDW modulated structure. Another Pt-based layered system LaPt2Si2 has similar physical properties to SrPt2As2. At high T, LaPt2Si2 crystallizes in the tetragonal structure of CaBe2Ge2-type, which is similar to Fig. 1a (Sr and As are replaced by La and Si) Upon cooling, it undergoes the CDW transition at 112 K with the CDW vector of qCDW = (n/3, 0, 0) (n = 1 or 2)[8]. The unique feature of the split-off positions of Pt1 and As2 in SrPt2As2 has not been observed in LaPt2Si2
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