1. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China
2. Department of Physics and Astronomy, Vanderbilt University, Nashville Tennessee 37235
3. Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506
4. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831