# Tightly bound excitons in two-dimensional semiconductors with a flat valence band

## TL;DR

This paper derives an explicit formula for exciton binding energy in 2D semiconductors with flat valence bands, highlighting the impact of band flatness on exciton properties and phase transitions.

## Contribution

It provides a quasiclassical expression for exciton binding energy in flat-band 2D semiconductors, relevant to recently discovered materials like 1T-TiSe2.

## Key findings

- Flat valence bands reduce exciton binding energy.
- The model applies to materials with unstable electronic ground states.
- Implications for phase transitions to excitonic insulators.

## Abstract

This theoretical paper offers an explicit expression for the binding energy of excitons in a two-dimensional semiconductor with a flat valence band. The formula has been derived quasiclassically assuming that the exciton is tightly bound; i.e., its ground-state radius is determined by the intrinsic polarizability of the semiconductor rather than by the dielectric properties of the environment. The model is relevant to a few two-dimensional semiconductors discovered recently, including distorted 1T-TiSe$_2$, with a supposedly unstable electronic ground state. The valence band flatness reduces the exciton binding energy, which also may have an effect on the phase transition to an excitonic insulator.