  The interface between LaAlO3 and SrTiO3 hosts a two-dimensional electron
system of itinerant carriers, although both oxides are band insulators.
Interface ferromagnetism coexisting with superconductivity has been found and
attributed to local moments. Experimentally, it has been established that Ti 3d
electrons are confined to the interface. Using soft x-ray angle-resolved
resonant photoelectron spectroscopy we have directly mapped the interface
states in k-space. Our data demonstrate a charge dichotomy. A mobile fraction
contributes to Fermi surface sheets, whereas a localized portion at higher
binding energies is tentatively attributed to electrons trapped by O-vacancies
in the SrTiO3. While photovoltage effects in the polar LaAlO3 layers cannot be
excluded, the apparent absence of surface-related Fermi surface sheets could
also be fully reconciled in a recently proposed electronic reconstruction
picture where the built-in potential in the LaAlO3 is compensated by surface
O-vacancies serving also as charge reservoir.
