1. Department of Solid State Physics, Debrecen University, P.O. Box. 2, 4010 Debrecen, Hungary
2. Institute of Nuclear Research, Hungarian Academy of Sciences (ATOMKI), P.O. Box 51, 4001 Debrecen, Hungary
3. Department of Experimental Physics, Debrecen University, P.O. Box 105, 4010 Debrecen, Hungary