1. Philips Technologie GmbH Research Laboratories, Weisshausstrasse 2, D-52066 Aachen, Germany, Department of Condensed Matter, Debye Institute, Utrecht University, P.O. Box 80 000, 3508 TA Utrecht, The Netherlands, Centre for Optical and Electromagnetic Research, Zhejiang University, Hangzhou 310058, China, and Department Chemie and Biochemie, Lehrstuhl für Anorganische Festkörperchemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13 (D), 81377 München, Germany