1. Department of Physics, University of California, Berkeley, Berkeley, CA 94720, USA.
2. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
3. Max Planck Institute for Chemical Physics of Solids, Dresden D-01187, Germany.
4. Department of Physics, Temple University, Philadelphia, PA 19122, USA.
5. Department of Applied Physics, The University of Tokyo, Hongo, Tokyo 113-8656, Japan.