CDW-Exciton Condensate Competition and a Condensate Driven Force

TL;DR

This paper investigates the competition between charge-density wave and excitonic condensate in layered materials, revealing a new force driven by excitonic condensation that may influence lattice structures.

Contribution

It introduces a microscopic mean field theory modeling the coupled CDW and EC order parameters and predicts a new condensate-driven force affecting material structure.

Findings

Excitonic energy gap varies with layer separation.

A new force ${\\cal F}_{EC}$ driven by EC is proposed.

Potential explanation for lattice deformation in TMDCs.

Abstract

We examine the competition between the charge-density wave (CDW) instability and the excitonic condensate (EC) in spatially separated layers of electrons and holes. The CDW and the EC order parameters (OPs), described by two different mechanisms and hence two different transition temperatures $T^{CDW}_c$ and $T^{EC}_c$, are self-consistently coupled by a microscopic mean field theory. We discuss the results in our model specifically focusing on the transition-metal dichalcogenides which are considered as the most typical examples of strongly coupled CDW/EC systems with atomic layer separations where the electronic energy scales are large with the critical temperatures in the range $T^{EC}_c \sim T^{CDW}_c \sim 100-200 K$. An important consequence of this is that the excitonic energy gap, hence the condensed free energy, vary with the layer separation resulting in a new type of force ${\cal F}_{EC}$. We discuss the possibility of this force as the possible driver of the structural lattice deformation observed in some TMDCs with a particular attention on the $ 1 {\it T}$-$TiSe_2$ below $200 K$.