1. Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa
2. Energy Centre, Council for Scientific and Industrial Research (CSIR), Pretoria 0001, South Africa
3. Ames Laboratory, U.S. Department of Energy, Iowa State University, Ames, Iowa 50011-2416, United States
4. Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011-1096, United States