Scrutinizing the Debye plasma model: Rydberg excitons unravel the properties of low-density plasmas in semiconductors

TL;DR

This study uses high-principal quantum number Rydberg excitons in cuprous oxide to accurately probe low-density electron-hole plasmas, revealing that classical Debye theory is insufficient and quantum many-body effects are essential.

Contribution

It demonstrates that Rydberg excitons can precisely determine plasma properties and shows the necessity of quantum many-body theory over classical models in low-density plasmas.

Findings

Classical Debye model fails to describe plasma effects in semiconductors.

Quantum many-body effects are crucial for understanding low-density plasmas.

Discovery of a new exciton scattering mechanism involving coupled plasmon-phonon modes.

Abstract

For low-density plasmas, the classical limit described by the Debye-H\"uckel theory is still considered as an appropriate description even though a clear experimental proof of this paradigm is lacking due to the problems in determining the plasma-induced shift of single-particle energies in atomic systems. We show that Rydberg excitons in states with a high principal quantum number are highly sensitive probes for their surrounding making it possible to unravel accurately the basic properties of low-density non-degenerate electron-hole plasmas. To this end, we accurately measure the parameters of Rydberg excitons such as energies and linewidths in absorption spectra of bulk cuprous oxide crystals in which a tailored electron-hole plasma has been generated optically. Since from the absorption spectra exciton energies, as well as the shift of the single-particle energies given by the band edge, can be directly derived, the measurements allow us to determine the plasma density and temperature independently, which has been a notoriously hard problem in semiconductor physics. Our analysis shows unambiguously that the impact of the plasma cannot be described by the classical Debye model, but requires a quantum many-body theory, not only for the semiconductor plasma investigated here, but in general. Furthermore, it reveals a new exciton scattering mechanism with coupled plasmon-phonon modes becoming important even at very low plasma densities.