1. Faculty of Physics, Vilnius University, Sauletekio Avenue 9, LT-10222 Vilnius, Lithuania
2. Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, PL-50-950 Wroclaw 2, Poland
3. Felix Bloch Institute for Solid State Physics, Leipzig University, Linnéstrasse 5, D-04103 Leipzig, Germany