[1] Int Lab High Magnet Fields & Low Temp, PL-53421 Wroclaw, Poland
[2] Bulgarian Acad Sci, Inst Solid State Phys, BU-1784 Sofia, Bulgaria
[3] Zent Inst Festkorperphys & Werkstofforsch, D-01171 Dresden, Germany
[4] Bulgarian Acad Sci, Inst Elect, BU-1784 Sofia, Bulgaria
[5] Univ Birmingham, Sch Phys & Astron, Birmingham B15 2TT, W Midlands, England
[6] Polish Acad Sci, Inst Phys, PL-02668 Warsaw, Poland