1. Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin-Dahlem, Germany
2. Department of Physics and Astronomy, Uppsala University, Box 516, SE-751, 20 Uppsala, Sweden
3. Laboratory of High-Tech Materials, School of Engineering, University of Patras, 26504 Patras, Greece
4. Materials Science Department, University of Patras, 26504 Patras, Greece
5. Chemical Process & Energy Resources Institute (CPERI), Centre for Research & Technology Hellas (CERTH), 57001 Thermi, Thessaloniki, Greece