Quantum phase transitions in a quarter-filled Hubbard ladder

We study the phase diagram of a quarter-filled two-leg extended Hubbard ladder by bosonization and renormalization group (RG) approach. We obtain various charge ordered and bond ordered insulating states. We discuss the phase transitions between these various phases with antiferromagnetic and ferromagnetic interactions. We find second order transitions in the Ising or in the SU(2)(2) universality class, as well as first order transitions. In the presence of a magnetic field the spin gap can be closed by a commensurate-incommensurate transition, leading to insulating states with gapless spin excitations. We discuss the spin density wave correlations obtained in these gapless states. (C) 2004 Elsevier B.V. All rights reserved.