1. Department of Physics, Indian Institute of Technology Guwahati, Guwahati, Assam 781039, India
2. Department of Materials Science and Engineering, Uppsala University, Box 534, SE-75121 Uppsala, Sweden
3. School of Materials Science and Engineering, Hubei University, Wuhan, Hubei 430062, China
4. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA