  A new decoupling scheme is developed for the Hubbard model which provides a
unified description of the spin-symmetric (paramagnetic metallic and
insulating) phases as well as the broken-symmetry AFI phase. Independent of
magnetic ordering, the scheme yields, in the lowest order, the correct
strong-coupling bandwidth of order J (for the NN hopping model) and band gap of
order U, a non-zero critical interaction strength (above which the band gap
opens) only if same sublattice hopping (e.g., NNN hopping) is also present, and
the correct integrated spectral weights (1/2 per spin) in each band for the
half-filled model. The effects of charge and spin fluctuations, including spin
twisting due to finite spin correlation length, are investigated with a static,
random approximation. A self-consistent evaluation of the disorder-averaged
self energy within the CPA is carried out numerically. Fluctuations activate
the hopping term at first order resulting in band broadening, and the
consequent decrease in the band gap with fluctuation strength is obtained for
several U values in two and three dimensions. We find that the band gap shrinks
to zero continuously, and subsequently the density of states N(0) between the
bands grows continuously, leading to a continuous metal-insulator transition.
For finite doping there is transfer of spectral weight between the Hubbard
bands, and a qualitative change in the nature of the quasiparticle band
dispersion, with an effective doping-induced hopping strength and bandwidth of
order xt.
