1. Department
of Physics, Oregon State University, Corvallis, Oregon 97331-6507, United States
2. Science
Faculty, Ningbo University, Ningbo 315211, People’s Republic of China
3. Laboratory
of Atomic and Solid-State Physics, Cornell University, Ithaca, New York 14883, United States
4. Institute
of Theoretical Physics and Astrophysics, University of Kiel, Kiel D-24098, Germany