1. Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany
2. Faculty of Physics Babes Bolyai University, Cluj-Napoca, Romania
3. H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, UK
4. Institute of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland