The Holstein Polaron Problem Revisited

TL;DR

This paper presents an exact solution to the two-site Holstein model, revealing its structure, symmetries, and energy levels, and introduces a method for accurate energy calculations across all interaction strengths.

Contribution

It provides the first exact solution to the two-site Holstein model using orthogonal polynomials and recurrence relations, advancing understanding of polaronic systems.

Findings

Exact eigenfunctions and energy levels for the two-site Holstein model.

Identification of symmetry-breaking points and level crossings.

Development of a basis for efficient numerical analysis of multi-site systems.

Abstract

The Holstein Hamiltonian was proposed half a century ago; since then, decades of research have come up empty handed in the pursuit of a closed-form solution. An exact solution to the two-site Holstein model is presented in this paper. The obtained results provide a clear image of the Hamiltonian structure and allow for the investigation of the symmetry, energy level crossings and polaronic characteristics of the system. The main mathematical tool is a three-term recurrence relation between the wave function amplitudes that was obtained using the properties of a family of orthogonal functions, namely the Poisson-Charlier polynomials. It is shown that, with the appropriate choice of basis, the eigenfunctions of the problem naturally fall into two families (parities) associated with the discrete $\mathbb{Z}_{2}$ symmetry of the Hamiltonian. The asymptotic solution to the recurrence relation is found by using the Birkhoff expansion. The asymptotic sets the truncation criterion for the wave function, which ensures the accurate calculation of the energy levels for any strength of electron-phonon interaction. Level crossing of states with different parities is discussed and the exact points of broken symmetry are found analytically. The results are used as the building blocks for studying a four-site system. The inherited symmetries lead to the formation of a sparse matrix that is convenient for numerical calculations.