Internal dynamics and dielectric screening of confined multiexciton states

TL;DR

This study explains how biexcitons in large CsPbBr3 nanocrystals exhibit reduced dielectric screening compared to excitons and trions, due to differences in internal dynamics frequencies affecting dielectric response.

Contribution

The paper introduces a physical explanation for reduced dielectric screening of biexcitons in large nanocrystals based on internal dynamical frequencies.

Findings

Biexcitons have smaller internal dynamical frequencies than excitons and trions.

Reduced frequencies lead to lower dielectric constants for biexcitons.

The behavior contrasts with strongly confined nanocrystals.

Abstract

Recent experimental and computational studies suggest that biexcitons (BX) confined in large CsPbBr$_3$ nanocrystals experience reduced dielectric screening as compared to excitons (X) and trions (X$^*$). Here we provide a physical rationale to explain such a behavior. A characteristic frequency is introduced, which describes the internal dynamics of an exciton within the excitonic complex. By means of effective mass--variational Quantum Monte Carlo simulations, we show that, in large nanocrystals, the frequency is similar for X and X$^*$, but smaller for BX. Because the frequencies exceed that of the bulk longitudinal optical phonon, this leads to a reduced dielectric constant for BX, which is in contrast with the behavior of strongly confined nanocrystals.