Monte-Carlo phase diagram of a Hubbard-Peierls model in the search for spin crossover transition in $\pi$-conjugated polymers

TL;DR

This study uses Monte Carlo simulations to explore the finite temperature phase diagram of an extended Hubbard-Peierls model in one-dimensional $\pi$-conjugated polymers, revealing a temperature-induced spin crossover driven by electron interactions.

Contribution

It introduces a Monte Carlo approach to analyze the interplay of electron-electron and electron-phonon interactions in a Hubbard-Peierls model, uncovering a novel entropy-driven spin crossover phenomenon.

Findings

Identification of a diamagnetic to antiferromagnetic transition at low temperature.

Observation of a transition from homogeneous to Peierls dimerized geometry with increasing electron-phonon coupling.

Discovery of a temperature-induced spin crossover from low-spin to high-spin states.

Abstract

We present a Monte Carlo study of the finite temperature properties of an extended Hubbard-Peierls model describing one dimensional $\pi$-conjugated polymers. The model incorporates electron-phonon and hyperfine interaction and it is solved at the mean field level for half filling. In particular we explore the model as a function of the strength of electron-electron and electron-phonon interactions. At low temperature the system presents a diamagnetic to antiferromagnetic transition as the electron-electron interaction strength increases. At the same time by increasing the electron-phonon coupling there is a transition from a homogeneous to a Peierls dimerized geometry. As expected such a Peierls dimerized phase disappears at finite temperature as a result of thermal vibrations. More intriguing is the interplay between the electron-phonon and the electron-electron interactions at finite temperature. In particular we demonstrate that for a certain region of the parameter space there is a spin-crossover, where the system transits from a low-spin to a high-spin state as the temperature increases. In close analogy to standard spin-crossover in divalent magnetic molecules such a transition is entropy driven. Finally we discuss the role played by the hyperfine interaction over the phase diagram.