[1] Humboldt Univ, Dept Phys, Newtonstr 15, D-12489 Berlin, Germany
[2] Natl Res Univ Higher Sch Econ, Moscow Inst Elect & Math, Moscow 123458, Russia
[3] Moscow MV Lomonosov State Univ, Fac Phys, Moscow 119991, Russia
[4] Univ Potsdam, Inst Phys & Astron, D-14476 Potsdam, Germany
[5] Kharkov Inst Phys & Technol, Akhiezer Inst Theoret Phys, UA-61108 Kharkov, Ukraine