[1] Institut für Festkörperforschung, Kernforschungsanlage Jülich GmbH, P.O. Box 1931, D-5170 Jülich, Germany
[2] Synchrotron Radiation Center, Stoughton, WI 53589, United States
[3] University of Pennsylvania, Department of Physics, Philadelphia, PA 19104, United States
[4] Laboratorium für Festkörperphysik, ETH Zürich, CH-8093 Zürich, Switzerland