1. Department of Physics, Universiteit Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium, Departamento de Física, Universidade Federal do Ceará, Fortaleza, Ceará 60455-760, Brazil, and Department of Physics, Concordia University, 7141 Sherbrooke Ouest, Montreal, Quebec, Canada H4B 1R6