1. Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University, Hlavova 8, Prague 2 128 43, Czech Republic
2. Institute of Physics, Academy of Sciences of the Czech Republic, v.v.i., Na Slovance 2, Prague 8 182 21, Czech Republic
3. Department of Low-Temperature Physics, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, Prague 8 180 00, Czech Republic