[1] Univ Concepcion, Dept Fis, Casilla 160-C, Concepcion 4070386, Chile
[2] Univ Estadual Paulista, Inst Fis Teor, R Dr Bento T Ferraz 271,Bl 2, BR-01140070 Sao Paulo, SP, Brazil
[3] Pontificia Univ Catolica Valparaiso, Inst Fis, Ave Brasil 2950, Valparaiso, Chile
[4] Politecn Torino, Dipartimento Sci Applicata & Tecnol, Corso Duca Abruzzi 24, I-10129 Turin, Italy