We discuss the transition from a metal to charge- or spin-insulating phases characterized by the opening of a gap in the charge- or spin-excitation spectra, respectively. These transitions are addressed within the context of two exactly solvable Hubbard and t-J chains with long-range, 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-insulator transition is accompanied by an infinite discontinuity in the Drude weight of the electrical conductivity. While the magnetic properties of these models reflect the genuine features of strongly correlated electron systems, the charge-transport properties, especially near the Mott-Hubbard transition, display a nongeneric behavior.
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Gebhard et al. (1994) studied this question.