Optical properties of small polarons from dynamical mean-field theory

TL;DR

This paper uses dynamical mean-field theory to analyze the optical properties of small polarons in the Holstein model, revealing new insights into their spectra across different regimes and conditions, with implications for experimental interpretation.

Contribution

It provides a comprehensive DMFT-based analysis of small polaron optical spectra, including new analytical formulas and a detailed study of temperature and bandwidth effects.

Findings

Optical absorption peaks depend on temperature and frequency, contradicting previous models.

In the adiabatic regime, polaron absorption is significantly affected by electron bandwidth.

Reentrant behavior with sharp features emerges at intermediate coupling and increasing temperature.

Abstract

The optical properties of polarons are studied in the framework of the Holstein model by applying the dynamical mean-field theory. This approach allows to enlighten important quantitative and qualitative deviations from the limiting treatments of small polaron theory, that should be considered when interpreting experimental data. In the antiadiabatic regime, accounting on the same footing for a finite phonon frequency and a finite electron bandwidth allows to address the evolution of the optical absorption away from the well-understood molecular limit. It is shown that the width of the multiphonon peaks in the optical spectra depends on the temperature and on the frequency in a way that contradicts the commonly accepted results, most notably in the strong coupling case. In the adiabatic regime, on the other hand, the present method allows to identify a wide range of parameters of experimental interest, where the electron bandwidth is comparable or larger than the broadening of the Franck-Condon line, leading to a strong modification of both the position and the shape of the polaronic absorption. An analytical expression is derived in the limit of vanishing broadening, which improves over the existing formulas and whose validity extends to any finite-dimensional lattice. In the same adiabatic regime, at intermediate values of the interaction strength, the optical absorption exhibits a characteristic reentrant behavior, with the emergence of sharp features upon increasing the temperature -- polaron interband transitions -- which are peculiar of the polaron crossover, and for which analytical expressions are provided.