1. Department of Physics and Center for Advanced Nanoscience, University of California, San Diego, La Jolla, California 92093, USA
2. Department of Physics, Universidad de Chile, Santiago 7800024, Chile
3. Center for the Development of Nanoscience and Nanotechnology, CEDENNA, Santiago 9170124, Chile
4. Materials Science Division, Lawrence Livermore National Laboratory, Livermore, California 94550, USA
5. Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas 78249, USA
6. General Atomics, PO Box 85608, San Diego, California 92186, USA