Giornale di Fisica, 48 4 2007 [Figure 1 in this article is already published in Phys. Rev. Lett. 47 460 (1981). Copyright (1981) - American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the Americal Physical Society.]
