1. Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
2. Jiangsu Provincial Key Laboratory of Nanotechnology, School of Electronic Science and Engineering, Nanjing University, 210093 Nanjing, China
3. Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden
4. Materials and NanoPhysics, School of Engineering Sciences, KTH-Royal Institute of Technology, Electrum 229, 164 40 Kista, Sweden