Fundamental collapse of the exciton-exciton effective scattering

TL;DR

This paper investigates the conditions under which the effective exciton-exciton scattering collapses, revealing a fundamental phenomenon where scattering can vanish due to a delicate balance of Coulomb processes, depending on initial momenta and mass ratios.

Contribution

It uncovers the fundamental collapse of exciton-exciton effective scattering under specific energy and momentum configurations, highlighting the role of Coulomb process compensation.

Findings

Effective scattering collapses for large initial momentum differences.

Collapse occurs predominantly in the forward direction.

Near a mass ratio of 1/2, scattering remains near zero for specific momentum differences.

Abstract

The exciton-exciton effective scattering which rules the time evolution of two excitons is studied as a function of initial momentum difference, scattering angle and electron-to-hole mass ratio. We show that this effective scattering can collapse for energy-conserving configurations provided that the difference between the two initial exciton momenta is larger than a threshold value. Sizeable scatterings then exist in the forward direction only. We even find that, for an electron-to-hole mass ratio close to 1/2, the exciton-exciton effective scattering stays close to zero in all directions when the difference between the initial exciton momenta has a very specific value. This unexpected but quite remarkable collapse comes from tricky compensation between direct and exchange Coulomb processes which originates from the fundamental undistinguishability of the exciton fermionic components.