1. *) Present address: Royal Institute of Technology, Material Science, 10044 Stockholm, Sweden. #) Permanent address: Istituto di Struttura della Materia, CNR, Via E. Fermi 38, 00044 Frascati, Italy. **) Permanent address: Faculte des Sciences du Luminy, Departement de Physique, Case 901, 13288 Marseille Cedex 9, France.
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