1. Department of Physics, Politehnica University of Bucharest, 313 Splaiul Independentei, Bucharest RO-060042, Romania
2. Department of Solid State Physics, Yerevan State University, Alex Manoogian 1, 0025 Yerevan, Armenia
3. Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile