Strongly anharmonic collective modes in a coupled electron-phonon-spin problem

TL;DR

This paper investigates the complex collective mode dynamics in a strongly anharmonic electron-phonon-spin system near a polaronic instability, revealing novel low-energy spectral features and phase transitions relevant to manganites.

Contribution

It introduces a coupled Langevin equation approach to study finite temperature collective modes in a strongly anharmonic regime, near a polaronic instability, with detailed spectral analysis.

Findings

Transition from ferromagnetic metal to paramagnetic insulator with charge order

Large amplitude phonon dynamics emerge near the crossover temperature

Low energy spectral weight appears due to thermal domain tunneling

Abstract

We solve for the finite temperature collective mode dynamics in the Holstein-double exchange problem, using coupled Langevin equations for the phonon and spin variables. We present results in a strongly anharmonic regime, close to a polaronic instability. For our parameter choice the system transits from an `undistorted' ferromagnetic metal at low temperature to a structurally distorted paramagnetic insulator at high temperature, through a short range charge ordered (CO) phase near the ferromagnetic crossover at $T_{FM}$. The small amplitude harmonic phonons at low temperature cross over to large amplitude dynamics around $0.5 T_{FM}$ due to thermally generated short range correlated polarons. The rare thermal ``tunneling'' of CO domains generates a hitherto unknown momentum selective spectral weight at very low energy. We compare our results to inelastic neutron data in the manganites and suggest how the singular low energy features can be probed.