1. a I.B.M. Research Center , P.O. Box 218, Yorktown Heights , New York , 10598 , U.S.A.
2. b UA CNRS 783, Département de Physique des Solides, Faculté des Sciences de Luminy , 13288 , Marseille , France
3. c Department of Chemistry , The University of North Carolina , Chapel Hill , North Carolina , 27514 , U.S.A.