Marginal quantum criticality of metal-insulator transitions

Critical exponents of metal-insulator transitions are studied for an extended Hubbard model within the Hartree-Fock approximation. At zero temperature, we find that the first-order transition line between a symmetry broken (antiferromagnetic or charge-ordered) metal and an insulator terminates at the "marginally quantum critical point'', where an unconventional universality class emerges. This universality is in agreement with the scaling theory and explains the exponents obtained by recent experiments on (V, Cr)(2)O-3 and k-(ET)(2)Cu(Cn)(2)Cl in a unified way as a crossover from the marginally quantum universality and the Ising universality. (c) 2006 Elsevier B.V. All rights reserved.