Structural Evolution and Onset of the Density Wave Transition in the CDW Superconductor LaPt$_2$Si$_2$ Clarified with Synchrotron XRD

TL;DR

This study uses high-resolution synchrotron XRD to clarify the complex temperature-dependent structural evolution and density wave transitions in LaPt$_2$Si$_2$, resolving controversies from previous limited studies.

Contribution

It provides detailed experimental insights into the sequence of structural transitions and CDW behavior in LaPt$_2$Si$_2$, which were previously unclear due to reliance on theoretical and preliminary data.

Findings

Identified a series of structural transitions including tetragonal to incommensurate and orthorhombic phases.

Observed superstructural lattice distortions associated with CDW formation.

Revealed discommensuration of the CDW in LaPt$_2$Si$_2$.

Abstract

The quasi-2D Pt-based rare earth intermetallic material LaPt$_2$Si$_2$ has attracted attention as it exhibits strong interplay between charge density wave (CDW) and and superconductivity (SC). However, the most of the results reported on this material come from theoretical calculations, preliminary bulk investigations and powder samples, which makes it difficult to uniquely determine the temperature evolution of its crystal structure and, consequently, of its CDW transition. Therefore, the published literature around LaPt$_2$Si$_2$ is often controversial. In this paper, we clarify the complex evolution of the crystal structure, and the temperature dependence of the development of density wave transitions, in good quality LaPt$_2$Si$_2$ single crystals, with high resolution synchrotron X-ray diffraction data. According to our findings, on cooling from room temperature LaPt$_2$Si$_2$ undergoes a series of subtle structural transitions which can be summarised as follows: second order commensurate tetragonal ($P4/nmm$)-to-incommensurate structure followed by a first order incommensurate-to-commensurate orthorhombic ($Pmmn$) transition and then a first order commensurate orthorhombic ($Pmmn$)-to-commensurate tetragonal ($P4/nmm$). The structural transitions are accompanied by both incommensurate and commensurate superstructural distortions of the lattice. The observed behavior is compatible with discommensuration of the CDW in this material.