1. Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University, Qinhuangdao 066004, China
2. Department of Physics, Xinzhou Teachers University, Xinzhou 034000, China
3. Department of Physics, Tangshan Normal University, Tangshan 063000, China
4. Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, U.S.A.