1. Center
for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States
2. Center
for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
3. Department
of Electrical and Computer Engineering, University of California San Diego, La Jolla, California 92093, United States
4. Materials
Physics Department, Sandia National Laboratories, Livermore, California 94551, United States