1. Department of Physics, University of California San Diego, La Jolla, California 92093, United States
2. Leibniz Institute for Solid State and Materials Research, IFW, 01069 Dresden, Germany
3. Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States
4. Department of Physics, Columbia University, New York, New York 10027-6902, United States