CHARGE-GAP AND SPIN-GAP FORMATION IN EXACTLY SOLVABLE HUBBARD CHAINS WITH LONG-RANGE HOPPING

We discuss the transition from a metal to charge- or spin-insulating p hases characterized by the opening of a gap in the charge- or spin-exc itation spectra, respectively. These transitions are addressed within the context of two exactly solvable Hubbard and t-J chains with long-r ange, 1/r hopping. We discuss the specific heat, compressibility, and magnetic susceptibility of these models as a function of temperature, the band filling, and the interaction strength. We then use conformal- field-theory techniques to extract ground-state correlation functions. Finally, by employing the g-ology analysis we show that the charge-in sulator transition is accompanied by an infinite discontinuity in the Drude weight of the electrical conductivity. While the magnetic proper ties of these models reflect the genuine features of strongly correlat ed electron systems, the charge-transport properties, especially near the Mott-Hubbard transition, display a nongeneric behavior.