  We study the charge dynamics of the half-filled Hubbard model on the square
lattice at zero temperature. We employ a slave-fermion formulation in which the
charge degrees of freedom are represented by fermionic holons and doublons and
the bosonic spin degrees of freedom are assumed to be described by the
antiferromagnetic Heisenberg model. The ground state has long-ranged magnetic
(N\'eel) order and its Mott-insulating characteristics are the consequence of
holon-doublon bound-state formation. Within this framework, we calculate the
average double occupancy, the electron density of states, and the spectral
function in the self-consistent Born approximation. The lower and upper Hubbard
bands are reproduced, there is spectral-weight transfer into a coherent
quasiparticle band at their lower edges, and the Mott gap, associated with
holon-doublon binding, is renormalized due to the interactions of the holons
and doublons with the magnons. We compute the zeros of the Green function at
the chemical potential to discuss the Luttinger volume and show that the poles
of the self-energy reflect the underlying quasiparticle dispersion with a
spin-renormalized hopping parameter. Optical conductivity results relate the
optical gap to the Mott gap of the insulating system. We conclude that a
self-consistent treatment of the spin fluctuation effects on the charge degrees
of freedom captures all the essential physics of the antiferromagnetic
Mott-Hubbard insulator.
