Affiliation: | a Center for Theoretical Physics, Institute of Physics, 10 Dao Tan, Ba Dinh, Hanoi, Viet Nam b Physics Department, University of Education No. 1, 136 Xuan Thuy, Cau Giay, Hanoi, Viet Nam c Physics Department, Hanoi National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam |