1. Applied Science and Technology Graduate Group, University of California, Berkeley California 94720, Department of Physics, University of California, Berkeley California 94720, Faculty of Science and Technology, University of Twente, P. O Box 217, 7500 AE, Enschede (The Netherlands), Advanced Research Projects Agency − Energy, U.S. Department of Energy, Washington, DC 20585, and Department of Materials Science and Engineering, University of California, Berkeley California 94720