1. Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okólna 2, 50-422 Wrocław, Poland
2. Faculdade de Fisica, Universidade Federal do Ceara, P.O. Box 6030, 60455-970 Fortaleza, Brazil
3. Faculty of Physics, Vilnius University, Sauletekio av. 9, LT-10222 Vilnius, Lithuania
4. Felix Bloch Institute for Solid State Physics, Leipzig University, Linnéstrasse 5, D-04103 Leipzig, Germany