  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.
