1. Department of Physics and Research, Institute of Natural Science, College of Natural Science, Gyeongsang National University, Jinju 660-701, Korea
2. Faculté des Sciences, Département de Physique, Université Ibnou Zohr, Agadir, Morocco
3. Abdus Salam International Centre for Theoretical Physics, Strada Costiera, 11 I-34151 Trieste, Italy