1. Department of Electrical and
Computing Systems, College of Engineering and Applied Science, University of Cincinnati, Cincinnati, Ohio 45221-0030,
United StatesUSA
2. Laboratoire
de Physique Théorique
de la Matière Condensée, Université Pierre et Marie Curie, Boite 121, 4, Place Jussieu,
75252 Paris Cedex 05, France