Unconventional bright ground-state excitons in monolayer TiI$_2$ from first-principles calculations

TL;DR

This study uses first-principles calculations to reveal that monolayer TiI$_2$ has a unique bright exciton ground state, driven by spin-orbit effects and weak exchange interactions, with stability under strain and in trion states.

Contribution

It identifies the mechanisms behind the bright exciton ground state in TiI$_2$, a novel feature among transition metal dichalcogenides, and demonstrates its stability and inheritance in charged excitons.

Findings

TiI$_2$ has a bright exciton ground state.

The bright exciton remains stable under mechanical strain.

Charged excitons inherit the bright ground state.

Abstract

Based on \textit{ab initio} screened configuration interaction calculations we find that TiI$_2$ has a bright exciton ground state and identify two key mechanisms that lead to this unprecedented feature among transition metal dichalcogenides. First, the spin-orbit induced conduction band splitting results in optically allowed spin-alignment for electrons and holes across a significant portion of the Brillouin zone around the $\mathbf{K}$-valley, avoiding band crossings seen in materials like monolayer MoSe$_2$. Second, a sufficiently weak exchange interaction ensures that the bright exciton remains energetically below the dark exciton state. We further show that the bright exciton ground state is stable under various mechanical strains and that trion states (charged excitons) inherit this bright ground state. Our findings are expected to spark further investigation into related materials that bring along the two key features mentioned, as bright ground-state excitons are crucial for applications requiring fast radiative recombination.