1. Department
of Physics, University of California, Berkeley, California 94720, United States
2. Department
of Physics, Umeå University, SE-901 87 Umeå, Sweden
3. Materials
Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Kavli Energy NanoSciences Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States