1. Department of Materials Physics, ELTE Eötvös Loránd University, P.O. Box 32, H-1518 Budapest, Hungary
2. HUN-REN Wigner Research Centre for Physics, P.O. Box 49, H-1525 Budapest, Hungary
3. Institute for Technical Physics and Materials Science, HUN-REN Centre for Energy Research, Konkoly-Thege Miklós. út 29-33, H-1121 Budapest, Hungary