1. Physics Department, Florida State University, Tallahassee, Florida 32310, USA
2. National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA
3. Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA
4. Institute of physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China
5. Texas Materials Institute, University of Texas at Austin, Austin, Texas 78712, USA