1. Department of Physics, University of California, Berkeley, Berkeley, California 94720, United States
2. Departmento de Ciencias Físicas, Universidad Andres Bello, Santiago 837-0136, Chile
3. Kavli Energy NanoSciences Institute at Berkeley, Berkeley, California 94720, United States
4. Walter Schottky Institut and Physik Department, Technische Universitat München, 95748 Garching, Germany