Experimental determination of the bare energy gap of GaAs without the zero-point renormalization

TL;DR

This study experimentally determines the unrenormalized energy gap of GaAs at zero temperature, revealing a significant zero-point phonon contribution to the bandgap that has been challenging to measure directly.

Contribution

It provides the first experimental estimate of the bare energy gap of GaAs without zero-point renormalization, using the Urbach tail as a key indicator.

Findings

Zero temperature bare energy gap of GaAs is 1.581 eV.

Zero-point phonon renormalization of the gap is approximately 66 meV.

Supports the hypothesis that Urbach tail focus indicates the unrenormalized gap.

Abstract

The energy gap of simple band insulators like GaAs is a strong function of temperature due to the electron-phonon interactions. Interestingly, the perturbation from zero-point phonons is also predicted to cause significant (a few percent) renormalization of the energy gap at absolute zero temperature but its value has been difficult to estimate both theoretically and, of course, experimentally. Given the experimental evidence [Bhattacharya, et al., Phys. Rev. Lett. 114, 047402 (2015)] that strongly supports that the exponential broadening (Urbach tail) of the excitonic absorption edge at low temperatures is the manifestation of this zero temperature electron-phonon scattering, we argue that the location of the Urbach focus is the zero temperature unrenormalized gap. Experiments on GaAs yield the zero temperature bare energy gap to be 1.581 eV and thus the renormalization is estimated to be 66 meV.