Affiliation: | 1Department of Physics, Faculty of Science, University of Yaounde I, PO Box 812, Cameroon ; 2Ecole Nationale Supérieure Polytechnique, University of Yaounde I, PO Box 8390, Cameroon ; 3The Abdus Salam International Centre for Theoretical Physics, PO Box 586, Strada Costiera, Ⅱ-34014, Trieste, Italy ) |