Two-dimensional charge density wave TaX$_2$ (X=S, Se, Te) from first principles

TL;DR

This study uses first-principles calculations to explore the structural, electronic, and magnetic properties of two-dimensional TaX2 materials, revealing phase transitions, magnetic behaviors, and stacking-dependent phenomena in charge density wave states.

Contribution

It provides a comprehensive first-principles analysis of 2D TaX2 compounds, highlighting the effects of CDW reconstruction on electronic and magnetic properties, including interlayer stacking influences.

Findings

CDW induces metal-insulator transition in 1T-TaS2 and 1T-TaSe2.

Magnetic moments emerge within Star of David clusters due to lattice reconstruction.

Stacking order affects magnetic properties in bilayer CDW systems.

Abstract

Transition metal dichalcogenides are rich in their structural phases, e.g. 1T-TaS2 and 1T-TaSe2 form charge density wave (CDW) under low temperature with interesting and exotic properties. Here, we present a systematic study of different structures in two-dimensional TaX2 (X=S, Se, Te) using density functional theory calculations with consideration of van der Waals interaction. All the normal phases present metal characteristics with various ground state and magnetic properties. The lattice reconstruction of CDW drastically affects the electronic and structural characteristics of 1T-TaS2 and 1T-TaSe2, leading to a transition from metal to insulator and an emergence of magnetic moment within periodic atomic clusters called the Star of David. The evaluated Heisenberg couplings indicate the weak ferromagnetic coupling between the clusters in monolayer. Furthermore, in bilayer commensurate CDW cases, we find intriguing phenomenon of the varying magnetic properties with different stacking orders. The magnetic moment in each layer disappears when two layers are coupled, but may sustain in certain stackings of interlayer antiferromagnetic configurations.