In-plane anisotropy of charge density wave fluctuations in 1$T$-TiSe$_2$

TL;DR

This study uses X-ray diffuse scattering to reveal the anisotropic nature of charge density wave fluctuations in 1$T$-TiSe$_2$, showing decoupled layers and independent phase fluctuations above the transition temperature.

Contribution

It provides detailed characterization of the anisotropic CDW fluctuations and their evolution across the transition, highlighting the independence of phase fluctuations in different CDW components.

Findings

Out-of-plane diffuse scattering forms rod-like structures indicating decoupled layers.

In-plane diffuse scattering shows anisotropic ellipses.

Long-range coherence develops at the transition temperature.

Abstract

We report measurements of anisotropic triple-$q$ charge density wave (CDW) fluctuations in the transition metal dichalcogenide 1$T$-TiSe$_2$ over a large volume of reciprocal space with X-ray diffuse scattering. Above the transition temperature, $T_{\text{CDW}}$, the out-of-plane diffuse scattering is characterized by rod-like structures which indicate that the CDW fluctuations in neighboring layers are largely decoupled. In addition, the in-plane diffuse scattering is marked by ellipses which reveal that the in-plane fluctuations are anisotropic. Our analysis of the diffuse scattering line shapes and orientations suggests that the three charge density wave components contain independent phase fluctuations. At $T_{\text{CDW}}$, long range coherence is established in both the in-plane and out-of-plane directions, consistent with the large observed value of the CDW gap compared to $T_{\text{CDW}}$, and the predicted presence of a hierarchy of energy scales.