1. Branch of Molecular Engineering, Instituto Mexicano del Petróleo, Apartado Postal 14-805,07730, México, D.F, México, Department of Physics, Universidad Autónoma Metropolitana-Iztapalapa, Apartado Postal 55-534,09340, México, D.F., México, and Institute of Physics, Universidad Autónoma de Guanajuato, Apartado Postal E-143, 37150, León, Guanajuato, México