Dimensional crossover and symmetry transformation of the charge density waves in VSe2

TL;DR

This study investigates how the charge density wave (CDW) in VSe2 evolves from single-layer to bulk, revealing a dimensional crossover driven by phonon instability and competition of nesting vectors, affecting symmetry and transition temperature.

Contribution

It provides new insights into the dimensional crossover of CDWs in VSe2 and the underlying phonon instability mechanisms through ARPES measurements.

Findings

Single-layer VSe2 exhibits a (r7 x r3) CDW with broken three-fold symmetry.

Transition temperature for the bulk-like (4 x 4) CDW is higher than in the bulk.

Electronic structure evolution shows nearly bulk-like behavior at two layers.

Abstract

Collective phenomena in solids can be sensitive to the dimensionality of the system; a case of special interest is VSe2, which shows a (r7 x r3) charge density wave (CDW) in the single layer with the three-fold symmetry in the normal phase spontaneously broken, in contrast to the (4 x 4) in-plane CDW in the bulk. Angle-resolved photoemission spectroscopy (ARPES) from VSe2 ranging from a single layer to the bulk reveals the evolution of the electronic structure including the Fermi surface contours and the CDW gap. At a thickness of two layers, the ARPES maps are already nearly bulklike, but the transition temperature TC for the (4 x 4) CDW is much higher than the bulk value of 110 K. These results can be understood as a result of dimensional crossover of phonon instability driven by a competition of nesting vectors. Our study provides key insights into the CDW mechanisms and offers a perspective in the search and control of emergent phases in quantum materials.