1. Department of Physics, Stockholm University, S-106 91 Stockholm, Sweden
2. Department of Mathematics, Faculty of Physics, M. V. Lomonosov Moscow State University, Leninskie Gory, 119991 Moscow, Russia
3. CIMAP, UMR 6252, CEA/CNRS/ENSICAEN/Université de Caen Basse-Normandie, bd Henri Becquerel, BP 5133, F-14070 Caen Cedex 05, France
4. Université de Caen Basse-Normandie, Esplanade de la Paix, F-14032 Caen, France
5. Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands