1. Department of Physics and Astronomy, Rice University, Houston, TX 77005, U.S.A.
2. National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, U.S.A.
3. Department of Physics, Renmin University of China, Beijing 100872, China
4. Max Planck Institute for the Physics of Complex Systems, 01187 Dresden, Germany
5. Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany