1. Institute of Technical Physics and Materials Science, Centre for Energy Research, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 49, Hungary
2. Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 49, Hungary
3. Department of Physics, Shiga University of Medical Science, Shiga 520-2192, Japan