Delocalization of Wannier-Stark ladders by phonons: tunneling and stretched polarons

TL;DR

This paper investigates how phonons can delocalize Wannier-Stark polarons in strong electric fields, revealing phenomena like tunneling and stretched polarons with complex spectral structures.

Contribution

It introduces a novel analytical and numerical approach to study phonon-induced delocalization and complex spectral features in driven polaron systems.

Findings

Polaron states can delocalize via resonance with phonon modes.

Discovery of stretched polarons with trailing phonon clouds.

Polaron spectrum exhibits a nearly-fractal structure.

Abstract

We study the coherent dynamics of a Holstein polaron in strong electric fields. A detailed analytical and numerical analysis shows that even for small hopping constant and weak electron-phonon interaction, polaron states can become delocalized if a resonance condition develops between the original Wannier-Stark states and the phonon modes, yielding both tunneling and `stretched' polarons. The unusual stretched polarons are characterized by a phonon cloud that {\em trails} the electron, instead of accompanying it. In general, our novel approach allows us to show that the polaron spectrum has a complex nearly-fractal structure, due to the coherent coupling between states in the Cayley tree which describes the relevant Hilbert space. The eigenstates of a finite ladder are analyzed in terms of the observable tunneling and optical properties of the system.