1. Department of Physics, University of California, Berkeley, California 94720, USA;
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3. Department of Applied Physics, The University of Tokyo, Hongo, Tokyo 113-8656, Japan
4. Department of Physics, Temple University, Philadelphia, Pennsylvania 19122, USA
5. Temple Materials Institute, Temple University, Philadelphia, Pennsylvania 19122, USA
6. Materials Department, University of California, Santa Barbara, California 93106, USA
7. Department of Physics, California Institute of Technology, Pasadena, California 91125, USA;
8. Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, CA 91125, USA