Affiliation: | a Helsinki Institute of Physics, PO Box 9 (Siltavuorenpenger 20 C), FIN-00014 University of Helsinki, Helsink, Finland b Department of Physics, Box 1843, Brown University, Providence, RI 02912, USA c Department of Physics, University of Jyväskylä, PO Box 35, FIN-40351, Jyväskylä, Finland d Laboratory of Physics, Tampere University of Technology, PO Box 692, FIN-33101, Tampere, Finland |