[1] IPN, CINVESTAV, Dept Fis, Mexico City 07000, DF, Mexico
[2] Univ Austral Chile, Inst Ciencias Fis & Matemat, Valdivia, Chile
[3] Univ Libre Bruxelles, B-1050 Brussels, Belgium
[4] Int Solvay Inst, B-1050 Brussels, Belgium
[5] Univ Talca, Inst Matemat & Fis, Talca, Chile