ACKNOWLEDGMENT
The authors thank the Brazilian agencies CNPq and
F APERJ for financial support. A.A.M.M. gratefully
acknowledges the agencies CAPES and FAPESPA. R. C.B.
acknowledges the partial financial support from the Abdus
Salam International Centre for Theoretical Physics (ICTP,
Trieste, Italy) through the Associate Scheme. The authors
thank Dr. Giuliana Tromba for the support during the
experiment.