1. Key
Laboratory of Photonics Technology for Information, Key Laboratory
for Physical Electronics and Devices of the Ministry of Education,
School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China
2. Department
of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, U.K.
3. State
Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, P. R. China