1. Condensed Matter Physics Department, Jožef Stefan Institute, P.O. Box 3000, SI-1001 Ljubljana, Slovenia
2. Electronic Ceramics Department, Jožef Stefan Institute, P.O. Box 3000, SI-1001 Ljubljana, Slovenia
3. Institute of Molecular Physics, Polish Academy of Sciences, PL-60179 Poznan, Poland
4. Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802