1. Department of Materials Science and Engineering, State University of New York, Stony Brook, New York 11794-2275; XFD/APS Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439; Institut für Experimentelle und Angewandte Physik der Universität Kiel 24098 Kiel, Germany; and ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801