1. Department of Physics and Center for Advanced Nanoscience, University of California San Diego, La Jolla, California 92093, USA
2. Materials Science and Engineering Program, University of California San Diego, La Jolla, California 92093, USA
3. Department of Physics, Universidad de Chile, Santiago 7800024, Chile
4. Center for the Development of Nanoscience and Nanotechnology, CEDENNA, Santiago 9170124, Chile