Ab-inito study of biexcitons using exciton product basis.

Biexcitons are bound states of two electrons and two holes. We employ a first-principles many-body theory that uses the exciton product basis to describe biexcitons. This basis offers an advantage over the conventional four-particle approach, as it inherently includes strong electron-hole interactions within the exciton, leaving only the weaker exciton-exciton interactions to be addressed. This leads to faster convergence of the biexcitonic energies. The interactions among the biexciton's constituent fermions are described using an effective many-body Hamiltonian [1], which incorporates screening. We have used the many-body theory of cobosons [2] to construct the biexciton Hamiltonian using the exciton product basis. Using this method, we have calculated biexciton energies for molecules and carbon nanotubes, and our results are in good agreement with experiments.

[1] A. Torche and G. Bester, Phys. Rev. B 100, 201403 (2019).

[2] M. Combescot, O. Betbeder-Matibet, and F. Dubin, Phys. Rep. 463, 215 (2008).