1. a A.F. Ioffe Physical-Technical Institute , St. Petersburg , 194021 , Russia
2. c INFM Dip. di Fisica A. Volta , Università di Pavia , Pavia , 27100 , Italy
3. b Institute of Physics AS CR , Praha 8 , 18221 , Czech Republic
4. d FB Physik , University of Osnabrück , Osnabrück , D-49069 , Germany
5. e Solid State Division , Oak Ridge Nat. Lab. , Oak Ridge , Tennessee , 37831