  We apply the recently developed slave fermion approach to study the doped
Mott insulator in the one-band Hubbard and Hubbard-Heisenberg models. Our
results produce several subtle features in the electron spectra and confirm the
key role of antiferromagnetic (AFM) correlations in the appearance of the
pseudogap. Upon hole doping, the electron spectra exhibit a single peak near
the Fermi energy in the local approximation of the Hubbard model where AFM
correlations are not included. When AFM correlations are included through an
explicit mean-field Heisenberg interaction, a second peak emerges at slightly
lower energy and pushes the other peak to higher energy, so that a pseudogap
emerges between the two peaks at small doping. Both peaks grow rapidly with
increasing doping and eventually merge together, where the pseudogap no longer
exists. Detailed analyses of the spectral evolution with doping and the
strength of the Heisenberg interaction confirm that the lower-energy peak comes
from a polaronic mechanism due to the holon-spinon interaction in the
AFM-correlated background and the higher-energy peak arises from the holon
hybridization to form the electron quasiparticles. Thus, the pseudogap arises
from the interplay of the polaronic and hybridization mechanisms. Our results
are in good agreement with previous numerical calculations using the dynamical
mean-field theory and its cluster extensions, but give a clearer picture of the
underlying physics. Our work provides a promising perspective for clarifying
the nature of doped Mott insulators and may serve as a starting point for more
elaborate investigations in the future.
