Competing charge density wave phases in YNiC2

TL;DR

This study combines experimental and computational methods to analyze competing charge density wave phases in YNiC2, revealing their electronic structures, transition temperatures, and effects on physical properties.

Contribution

It provides a detailed characterization of incommensurate and commensurate CDW phases in YNiC2 using combined experimental and DFT approaches, highlighting their electronic and phononic distinctions.

Findings

Incommensurate CDW appears below 305 K with minimal Fermi surface impact.

Commensurate CDW emerges below 272 K, significantly altering electronic structure.

CDW phases are energetically close, driven by anisotropic electron-phonon coupling.

Abstract

Charge density wave (CDW) orders in YNiC2 are studied by means of combined experimental and computational techniques. On the experimental side, single crystals grown by the floating-zone method were examined by means of X-ray diffraction, as well as transport and thermal techniques. Density functional theory (DFT) calculations founded on the experimentally determined parent and CDW-modified crystal structures provide details of electronic and phononic structures as well as electron-phonon coupling and resolve changes inflicted upon entering the different CDW phases. Thereby, contrasting effects of subsequently emerging CDW states characterized by incommensurate q_{1ic} and commensurate q_{2c} modulation vectors are revealed. The former state, on-setting below T_{1ic}~ 305 K, weakly modifies the electronic structure by opening an almost isotropic gap on a minor part of the Fermi surface (FS). The latter phase, which takes over below T_{2c}~ 272 K has a more pronounced impact on physical properties via a decomposition of larger parts of the FS. These dissimilar behaviors are directly reflected in the electronic transport anisotropy, which is significantly weakened in the q_{2c}-type CDW state. As revealed by our DFT studies, CDW phases are very close in energy and their origin is directly related to the anisotropy of electron-phonon coupling, which is linked to a specific orbital character of related FS sheets. Specific heat and thermal expansion studies reveal a nearly reversible first-order phase transition at around T_{2c}~ 272 K, where both CDW phases co-exist within a T-interval of about 10 K.