[1] Inst Politecn Nacl, Ctr Invest & Estudios Avanzados, Dept Fis, Mexico City 07000, DF, Mexico
[2] Univ Konstanz, Fak Phys, D-78457 Constance, Germany
[3] Univ Autonoma Metropolitana Iztapalapa, Dept Fis, Mexico City 09340, DF, Mexico
[4] Univ Guanajuato, Inst Fis, Guanajuato 37150, Mexico