1. Kernforschungszentrum Karlsruhe, Institut für Nukleare Festkörperphysik, P.O.Box 3640, W-7500 Karlsruhe, Germany
2. University of Patras, School of Engineering, 26110 Patras, Greece
3. Department of Electrical and Computer Engineering, University of California, San Diego, Mail Code R-007, La Jolla, CA92093–0407, USA
4. Department of Physical Electronics, Electrical Engineering Institute, Prof. Popov Str. 5, St. Petersburg, 197376, Russia