Unified theory for photoemission and light absorption spectra of a one-dimensional interacting electron system

Abstract

Very recently, the angle-resolved photoemission spectrum (ARPES) of $[\mathrm{Ni}(\mathrm{chxn}{)}_{2}\mathrm{Br}]{\mathrm{Br}}_{2}$ $(\mathrm{chxn}=1R,$ $2R$-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 $[\mathrm{Ni}(\mathrm{chxn}{)}_{2}\mathrm{Br}]{\mathrm{Br}}_{2}$ 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.