Lower and upper Hubbard bands: A slave-boson treatment

We analyze the single-particle spectral density and the optical conductivity of the single-band Hubbard model in the insulating Mott phase within the slave-boson technique. We show that the boson fluctuations around the saddle-point solution build up the lower and upper Hubbard bands and give rise to the optical conductivity which describes transitions between these incoherent bands. We find that the Brinkman-Rice metal-insulator transition is characterized by the simultaneous vanishing of the jump in the chemical potential, the single-particle spectral density gap, and the optical gap.