[1] Univ Paris 11, CNRS, UMR 8626, LPTMS, F-91405 Orsay, France
[2] Univ Duisburg Essen, Fachbereich Phys, D-47048 Duisburg, Germany
[3] St Petersburg Univ, Inst Phys, St Petersburg 198504, Russia
[4] Univ Toulouse, CNRS, Phys Theor Lab, F-31062 Toulouse, France