Transition from band insulator to excitonic insulator via alloying Se into Monolayer TiS$_3$: A Computational Study

TL;DR

This computational study explores how alloying Se into monolayer TiS3 affects its electronic and excitonic properties, revealing a transition towards an excitonic insulator state with potential for material engineering.

Contribution

It demonstrates the impact of Se alloying on bandgap and exciton binding energy, and predicts spontaneous exciton condensation in TiS3 alloys, offering new insights into excitonic insulator engineering.

Findings

Bandgap decreases significantly with Se alloying.

Exciton binding energy remains relatively stable despite alloying.

Negative exciton formation energy indicates potential excitonic insulator state.

Abstract

First-principles density functional theory plus Bethe-Salpeter equation calculations are employed to investigate the electronic and excitonic properties of monolayer titanium trichalcogenide alloys TiS$_{3-x}$Se$_x$ ($x$=1 and 2). It is found that bandgap and exciton binding energy display asymmetric dependence on the substitution of Se for S. While the bandgap can be significantly decreased as compared to that of pristine TiS$_3$, the exciton binding energy just varies a little, regardless of position and concentration of the Se substitution. A negative exciton formation energy is found when the central S atoms are replaced by Se atoms, suggesting a many-body ground state with the spontaneous exciton condensation. Our work thus offers a new insight for engineering an excitonic insulator.