1. Department of Mechanical Engineering
and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States
2. Beijing National Laboratory
for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s
Republic of China
3. Woodruff
School of Mechanical
Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
4. Center for Integrated Nanotechnologies
(CINT), Sandia National Laboratories, Albuquerque,
New Mexico 87185, United States