Investigating the Impact of Van Hove Singularities in Exciton Dispersions

Optimising the transport properties of excitons is a crucial step in improving many semiconductor materials for advanced optoelectronic applications. Since excitonic properties are largely determined by the exciton bandstructure, precise tuning of the excitonic dispersion is key to enhancing materials performance, and complex dispersions beyond the parabolic effective-mass approximation must be characterised and investigated thoroughly.

When present in electronic dispersions, VHSs are known to cause remarkable electronic phenomena such as magnetic phases and unconventional superconductivity. Mirroring the concept in exciton bands could lead to similarly useful enhancements in excitonic properties. Here, we present a novel excitonic bandstructure that incorporates a VHS, and using microscopic many-particle theory, we investigate its effect on exciton dynamics, phonon-mediated scattering, and exciton diffusion coefficients. The resulting excitonic behaviour is highly tuneable, featuring enhanced anisotropic diffusion that strongly depends on the VHS shape and the relative energy of optical phonons within the material.

This work introduces a new method for tailoring exciton behaviour using a VHS, and highlights the importance of exploring complex exciton dispersions. It also demonstrates how microscopic theory can complement first-principles calculations, to enable the deterministic design of novel exciton bandstructures.