1. Department
of Physics, University of California at Berkeley, Berkeley, California 94720, United States
2. National Institute of Standards and Technology, Boulder, Colorado 80305, United States
3. Department
of Physics, Department of Chemistry, and JILA, University of Colorado, Boulder, Colorado 80309, United States
4. Kavli Energy NanoSciences Institute at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States