Affiliation: | (1) Department of Physics, Nankai University, 300071 Tianjin, P.R. China;(2) Departamento de Física, Universidade. Federal do Paraná, Curitiba, Caixa Postal 19044, 81531-990, Brazil;(3) Department of Physics, University of Arizona, Tucson, AZ, US, 85721 |