1. Department of Physics, Columbia University, New York, NY 10027, USA.
2. Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA.
3. Department of Physics, University of California–San Diego, La Jolla, CA 92093, USA.
4. Department of Physics, Harvard University, Cambridge, MA 02138, USA.
5. Departamento de Teoría y Simulación de Materiales, Instituto de Ciencia de Materiales de Madrid, CSIC, E-28049 Madrid, Spain.
6. National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan.