1. Institute of Materials Physics, University of Goettingen, Friedrich-Hund-Platz 1, Goettingen D-37077, Germany
2. Department of Low-Temperature Physics, Charles University, Prague V Holešovičkách 2, CZ-18000 Praha 8, Czech Republic
3. Institute for Applied Materials (IAM-WK), Karlsruhe Institute of Technology (KIT), Engelbert-Arnold-Strasse 4, Karlsruhe D-76131, Germany