1. Physik Department E20, TU München, Germany, Donostia International Physics Center (DIPC) and Departamento de Fisica de Materiales and Unidad de Fisica de Materiales, E-20018 San Sebastian, Spain, Instituto de Óptica—CSIC, Serrano 121, 28006 Madrid, Spain, and Karlsruhe Institute of Technology (KIT), Forschungszentrum Karlsruhe, Karlsruhe, Germany