1. Department of Physics, Chemistry and Biology, Linköping University, 581 83 Linköping, Sweden
2. Graduate Program of Materials Science and
Engineering, University of California—San Diego, La Jolla, California 92093, United States
3. Department of Physics, University of California—San Diego, La Jolla, California 92093, United States
4. Department
of Electrical and Computer Engineering, University of California—San Diego, La Jolla, California 92093, United States