Formation of Intermediate Coupling Optical Polarons and Bipolarons in Two-Dimensional Systems

TL;DR

This paper investigates the formation and stability of optical polarons and bipolarons in two-dimensional systems within the intermediate electron-phonon coupling regime, using various theoretical methods to analyze their energies and critical coupling constants.

Contribution

It provides a detailed analysis of 2D polaron and bipolaron formation, including critical coupling constants, using the Buimistrov-Pekar method and compares results with other established approaches.

Findings

Electron-phonon correlation lowers 2D polaron energy.

Critical coupling constant for localization in 2D is ~2.94.

Bipolaron stability depends on Coulomb repulsion parameter.

Abstract

he formation of the optical polaron and bipolaron in two-dimensional (2D) systems are studied in the intermediate electron-phonon coupling regime. The total energies of 2D polaron and bipolaron are calculated by using the Buimistrov-Pekar method of canonical transformations and analyzed in the weak, intermediate and strong coupling regimes. It is shown that the electron-phonon correlation significantly reduces the total energy of 2D polaron in comparison with the energy of the strong-coupling (adiabatic) polaron. A charge carrier in polar crystals remains localized in a 2D potential well when the electron-phonon coupling constant $\alpha$ is greater than the critical value $\alpha_{c}\simeq2.94$, which is much lower than a critical value of the electron-phonon coupling constant $\alpha$ for a 3D system. The critical values of the electron-phonon coupling constant $\alpha$ and the parameter of Coulomb repulsion between two carriers $\beta=1/(1-\epsilon_{\infty}/\epsilon_{0})$ (where $\epsilon_{\infty}$ and $\epsilon_{0}$ are the high frequency and static dielectric constants, respectively), which determine the bipolaron stability region, are numerically calculated. The obtained results are compared with the ones obtained by using the Feynman path integral method and the modified Lee-Low-Pines unitary transformation method.