1. I. Physikalisches Institut, Georg-August-Universität Göttingen, 37077 Göttingen, Germany.
2. Department of Physics, Kyoto University, Kyoto 606-8502, Japan.
3. Experimental Physics VI, Center for Electronic Correlations and Magnetism, University of Augsburg, 86159 Augsburg, Germany.
4. Ames Laboratory, U.S. Department of Energy, and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA.