Spectral properties of the single-band Holstein model away from half-filling

Evidence for electron-phonon (e-ph) coupling has accumulated for many different families of correlated materials and highlighted the need to improve our understanding of these interactions, even within the context of simple model Hamiltonians. The most widely studied model in this context is the Holstein Hamiltonian, which incorporates an e-ph interaction by coupling the local electron density to dispersionless phonon modes. Nevertheless, this model's spectral properties have not been systematically studied, particularly in higher dimensions and far from dilute carrier concentrations. Here we use determinant Monte Carlo (DQMC) in conjunction with novel analytic continuation methods to derive the Holstein model's single particle spectral function and optical conductivity for a range of coupling strengths, phonon frequencies, temperatures, and electron densities. We find evidence for the formation of a gap in the single-particle excitation spectrum that is related to polaron formation.