SiC(0001): A surface Mott-Hubbard insulator

We present ab initio electronic structure calculations for the Si-terminated SiC(0001)root 3 X root 3 surface. While local-density approximation (LDA) calculations predict a metallic ground state with a half-filed narrow band, Coulomb effects, included by the spin-polarized LDA+U method, result in a magnetic (Mott-Hubbard) insulator with a gap of 1.5 eV, comparable with the experimental value of 2.0 eV. The calculated value of the intersite exchange parameter, J=30 K, leads to the prediction of a paramagnetic Mott state, except at very low temperatures. The observed Si 2p surface core-level doublet can naturally be explained as an on-site exchange splitting.