Ground state features and spectral properties of large polaron liquids from low to high charge densities

TL;DR

This paper introduces a variational method to analyze large polaron liquids across different charge densities, revealing how electron-phonon interactions influence spectral features and phase transitions from insulating to metallic states.

Contribution

It develops a new variational approach that simultaneously accounts for polaron formation and polaron-polaron correlations at finite charge densities.

Findings

Spectral weight shifts from incoherent to coherent peaks with decreasing coupling or increasing density.

Identification of three density regimes: single polaron, crossover, and metallic phase.

Results help interpret tunneling and photoemission experiments in SrTiO3 systems.

Abstract

A new variational approach is proposed at zero temperature for a finite density of charge carriers in order to study ground state features of the Frohlich model including electron-electron and electron-phonon interactions. Within the intermediate electron-phonon coupling regime characteristic of large polarons, the approach takes into account on the same footing polaron formation and polaron-polaron correlations which play a relevant role going from low to high charge densities. Including fluctuations on top of the variational approach, the electronic spectral function is calculated from the weak to the intermediate electron-phonon coupling regime finding a peak-dip-hump line shape. The spectra are characterized by a transfer of spectral weight from the incoherent hump to the coherent peak with decreasing the electron-phonon coupling constant or with increasing the particle density. Three different density regimes stem out: the first, at low densities, where the features of a single large polaron with a substantial incoherent spectral weight are not modified by charge carrier interactions; a second one, at intermediate densities, where the polaronic liquid shows a rapid crossover from incoherent to coherent dynamics; the third one, at high densities, where screening effects are so prominent that the system presents a conventional metallic phase. The results obtained in the low to intermediate density regime turn out to be relevant for the interpretation of recent tunneling and photoemission experiments in SrTiO3-based systems.