1. Department of Low Temperature Physics, Roland Eötvös University, Puskin u. 5-7, 1088 Budapest, Hungary
2. CNRS Centre de Recherches sur les très Basses Températures, 38042 Grenoble, France
3. Research Institute for Technical Physics of the Hungarian Academy of Sciences, P.O. Box 76, 1325 Budapest, Hungary
4. A.F. Ioffe Physico-Technical Institute of the Russian Academy of Sciences, 194021 St. Petersburg, Russia
5. Institute of Nuclear Research of the Hungarian Academy of Sciences, Ρ.O.Βox 51, 4001 Debrecen, Hungary