Dimensional cross-over of the charge density wave order parameter in thin exfoliated 1T-VSe$_2$

TL;DR

This study investigates how the charge density wave order parameter in exfoliated VSe₂ varies with thickness, revealing a non-monotonic behavior linked to a dimensional crossover from three to two dimensions.

Contribution

It provides a detailed real-space analysis of the CDW in VSe₂ across different thicknesses using STM, introducing a new method to derive the local order parameter and transition temperature.

Findings

Non-monotonic thickness dependence of CDW transition temperature

Identification of a 3D to 2D dimensional crossover in FS topology

Reconciliation of previous conflicting transport studies

Abstract

The capability to isolate one to few unit-cell thin layers from the bulk matrix of layered compounds opens fascinating prospects to engineer novel electronic phases. However, a comprehensive study of the thickness dependence and of potential extrinsic effects are paramount to harness the electronic properties of such atomic foils. One striking example is the charge density wave (CDW) transition temperature in layered dichalcogenides whose thickness dependence remains unclear in the ultrathin limit. Here we present a detailed study of the thickness and temperature dependences of the CDW in VSe$_2$ by scanning tunnelling microscopy (STM). We show that mapping the real-space CDW periodicity over a broad thickness range unique to STM provides essential insight. We introduce a robust derivation of the local order parameter and transition temperature based on the real space charge modulation amplitude. Both quantities exhibit a striking non-monotonic thickness dependence that we explain in terms of a 3D to 2D dimensional crossover in the FS topology. This finding highlights thickness as a true tuning parameter of the electronic ground state and reconciles seemingly contradicting thickness dependencies determined in independent transport studies.