1. Department of Materials Science and Engineering and Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Unit 3136, Storrs, Connecticut 06269-3136, USA
2. Institute of Theoretical Physics, Goethe University, Frankfurt am Main, D-60438 Frankfurt am Main, Germany
3. Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA