Heavy-Electron Formation and Polaron-Bipolaron Transition in the Anharmonic Holstein Model

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