Orbital-selective Mott transitions in two-band Hubbard models

The anisotropic two-orbital Hubbard model is investigated at low temperatures using high-precision quantum Monte Carlo (QMC) simulations within dynamical mean-field theory (DMFT). We demonstrate that two distinct orbital-selective Mott transitions (OSMTs) occur for a bandwidth ratio of 2 even without spin-flip contributions to the Hund exchange, and we quantify numerical errors in earlier QMC data which had obscured the second transition. The limit of small inter-orbital coupling is introduced via a new generalized Hamiltonian and studied using QMC and Potthoff's self-energy functional method, yielding insight into the nature of the OSMTs and the non-Fermi-liquid OSM phase and opening the possibility for a new quantum-critical point.Comment: 2 pages, 4 figures, presented at ICM2006 and accepted for JMM

Minimal charge gap in the ionic Hubbard model

We study the ionic Hubbard model at temperature T=0 within the mean-field approximation, and show that the charge gap does not close completely at the ionic-band insulator to antiferromagnetic insulator transition, contrary to previous expectations. Furthermore, we find an intermediate phase for on-site repulsions $U>U_c$ for different lattices, and calculate the phase diagram for the ionic Hubbard model with alternating U, corresponding to a Cu-O lattice.Comment: 5 pages with 7 figures; minor correction