Very recently, the angle-resolved photoemission spectrum (ARPES) of
[Ni(chxn)2Br]Br2 (chxn=1R, 2R-cyclohexanediamine), which is a one-dimensional insulator driven by the strong Coulomb repulsion, has been experimentally observed. The experiment has clearly shown that a one-body gap appearing in the ARPES is much smaller than an optical gap observed in the light-absorption spectrum (LAS). The apparent difference between these two gaps indicates the breakdown of a mean-field description of this strongly correlated electron system. In this paper, we show that the ARPES and LAS of
[Ni(chxn)2Br]Br2 are consistently explained within the framework of the one-dimensional extended Hubbard model. Lehmann spectra of the one-body and light-absorption-type two-body Green functions are calculated by a quantum Monte Carlo method. Our numerical results reproduce the experimentally observed ARPES and LAS, confirming the small one-body gap and large optical gap. The apparent difference in the gap structures can be explained by a dynamical Zeeman field induced by quantum fluctuations.
Using an efficient variational exact diagonalization method, we computed the electron removal spectral function within the framework of the Holstein-Hubbard model containing two electrons with opposite spins coupled to dispersive quantum optical phonons. We focus on how the interplay between the electron-phonon coupling and on-site Coulomb repulsion shapes the spectral function, a quantity directly relevant to angle-resolved photoemission spectroscopy (ARPES). Tuning the strengths of the electron-phonon coupling and the Hubbard interaction allows us to examine the evolution of the spectral properties of the system as it crosses over from a bound bipolaron to separate polarons. We find that phonon dispersion plays an important role: It substantially redistributes the spectral weight across the low- and the high-energy features and controls the emergence and the structure of distinct multiphonon continua. In regimes of strong electron-phonon coupling and low Coulomb repulsion, the spectral function exhibits a broad distribution reflecting bound bipolaronic states. Increasing the Coulomb repulsion weakens bipolaron binding, shifts the primary quasiparticle peak closer to polaronic behavior and confines the spectral weight near the Brillouin-zone center. Our results provide a framework for interpreting ARPES measurements in systems where phonon–mediated attraction competes with direct Coulomb repulsion.