Heavy-Electron Formation and Bipolaronic Transition in the Anharmonic Holstein Model

TL;DR

This study explores how anharmonicity influences bipolaron formation and heavy-electron states in the Holstein model, revealing a transition from first-order to crossover behavior as anharmonicity increases.

Contribution

It demonstrates the impact of anharmonicity on polaron-bipolaron transitions and heavy-electron formation using dynamical mean-field theory with exact diagonalization.

Findings

Polaron-bipolaron transition is first-order at low anharmonicity.

Transition temperature and critical coupling decrease with increased anharmonicity.

Transition becomes a crossover at strong anharmonicity.

Abstract

The emergence of the bipolaronic phase and the formation of the heavy-electron state in the anharmonic Holstein model are investigated using the dynamical mean-field theory in combination with the exact diagonalization method. For a weak anharmonicity, it is confirmed that the first-order polaron-bipolaron transition occurs from the observation of a discontinuity in the behavior of several physical quantities. When the anharmonicity is gradually increased, the polaron-bipolaron transition temperature is reduced as well as the critical values of the electron-phonon coupling constant for polaron-bipolaron transition. For a strong anharmonicity, the polaron-bipolaron transition eventually changes to a crossover behavior. The effect of anharmonicity on the formation of the heavy-electron state near the polaron-bipolaron transition and the crossover region is discussed in detail.