1. Institute
of Low Temperature and Structure Research, Polish Academy of Sciences, Box 1410, 50-950 Wrocław 2, Poland
2. Faculdade
de Física, Universidade Federal do Pará, 66075-110 Belém, Pará, Brazil
3. Department
of Experimental Physics, Wrocław University of Technology, Wybrzeże
Wyspiańskiego 27, 50-370 Wrocław, Poland