[1] St Petersburg State Tech Univ, St Petersburg 195251, Russia
[2] RAS, AF Ioffe Physicotech Inst, St Petersburg 194021, Russia
[3] Univ Leipzig, Inst Expt Phys 2, D-04103 Leipzig, Germany
[4] Univ Paris 11, Inst Elect Fondamentale, F-91405 Orsay, France