# Exciton-phonon interaction in the strong coupling regime in hexagonal boron nitride

## TL;DR

This paper investigates the exciton-phonon interaction in hexagonal boron nitride, revealing strong coupling effects that alter the optical line shape and temperature dependence, supporting Toyozawa's theoretical predictions.

## Contribution

It reports the first observation of Gaussian excitonic lineshapes with sublinear temperature dependence in hexagonal boron nitride, confirming strong exciton-phonon coupling.

## Key findings

- Gaussian lineshape observed at high coupling
- Linewidth scales with √T indicating strong coupling
- Supports Toyozawa's theoretical model

## Abstract

The temperature-dependent optical response of excitons in semiconductors is controlled by the exciton-phonon interaction. When the exciton-lattice coupling is weak, the excitonic line has a Lorentzian profile resulting from motional narrowing, with a width increasing linearly with the lattice temperature $T$. In contrast, when the exciton-lattice coupling is strong, the lineshape is Gaussian with a width increasing sublinearly with the lattice temperature, proportional to $\sqrt{T}$. While the former case is commonly reported in the literature, here the latter is reported for the first time, for hexagonal boron nitride. Thus the theoretical predictions of Toyozawa [Progr. Theor. Phys. 20, 53 (1958)] are supported by demonstrating that the exciton-phonon interaction is in the strong coupling regime in this Van der Waals crystal.