Optical conductivity of an anharmonic large polaron gas at weak coupling

TL;DR

This paper investigates how anharmonic electron-phonon interactions in polar solids influence optical conductivity, revealing an additional absorption peak at twice the phonon energy as a signature of such interactions.

Contribution

It provides a semi-analytical calculation of optical conductivity including anharmonic effects, highlighting a new absorption peak due to 1-electron-2-phonon interactions.

Findings

Discovery of an additional absorption peak at twice the phonon energy.

Quantitative estimates of polaron effective mass and scattering times.

Proposal of the new peak as an experimental indicator of anharmonic interactions.

Abstract

In a polar solid, electrons or other charge carriers can interact with the phonons of the ionic lattice, leading to the formation of polaron quasiparticles. The optical conductivity and optical absorption spectrum of a material are affected by this electron-phonon coupling, most notably leading to an absorption peak in the mid-infrared region. Recently, a model Hamiltonian for anharmonic electron-phonon coupling was derived [M. Houtput and J. Tempere, Phys. Rev. B \textbf{103}, 184306 (2021)], that includes both the conventional Fr\"ohlich interaction as well an interaction where an electron interacts with two phonons simultaneously. In this article, we calculate and investigate the optical conductivity of the anharmonic large polaron gas, and show that an additional characteristic absorption peak appears due to this 1-electron-2-phonon interaction. We calculate a semi-analytical expression for the optical conductivity $\sigma(\omega)$ at finite temperatures and weak coupling using the Kubo formula. The electronic and phononic contributions can be split and treated separately, such that the many-body effects of the electron gas may be taken into account through the well-known dynamical structure factor $S(\mathbf{k},\omega)$. From the resulting optical conductivity, we calculate the polaron effective mass, an estimate for the electron-phonon scattering times, and the optical absorption spectrum of the anharmonic polaron gas. We show that alongside the well-known polaron absorption peak at the phonon energy $\hbar \omega_{\text{LO}}$, the 1-electron-2-phonon interaction leads to an additional absorption peak at $2 \hbar \omega_{\text{LO}}$. We propose this absorption peak as an experimentally measurable indicator for nonnegligible 1-electron-2-phonon interaction in a material, since the height of this peak is proportional to the strength of this anharmonic interaction.