1. Faculty of Physics, "Babes-Bolyai" university, 3400 Cluj-Napoca, Romania
2. Université Libre de Bruxelles, Physique des Solides, CP 233, Bvd. du Triomphe, 1050 Bruxelles, Belgique
3. Faculty of Physics, Moldova State University, 277014 Chişinău, R. Moldova
4. Institute of Applied Physics, 277028 Chişinău, R. Moldova