1. Departamento de Física, Facultad de Ciencias Naturales, Universidad de Atacama, Copayapu 485, Copiapó, Chile
2. Department of Applied Mathematics, Hong Kong Polytechnic University, Hong Kong, Hong Kong S.A.R., P. R. China
3. Astronomical Observatory, 19 Ciresilor Street, 400487 Cluj-Napoca, Romania
4. Department of Physics, Babes-Bolyai University, Kogalniceanu Street, Cluj-Napoca 400084, Romania
5. School of Physics, Sun Yat-Sen University, Guangzhou 510275, P. R. China