[1] Univ Zaragoza, Dept Fis Teor, E-50009 Zaragoza, Spain
[2] Univ Carlos III Madrid, ICMAT, Madrid 28911, Spain
[3] Univ Carlos III Madrid, Dept Matemat, Madrid 28911, Spain
[4] Univ Fed II, Dipartimento Fis, I-80126 Naples, Italy
[5] Sez INFN Napoli, I-80126 Naples, Italy