Affiliation: | 1. Department of Applied Physics,Chongqing University,Chongqing 400044,China; Department of Physics,Chongqing Normal College,Chongqing 400047,China 2. Department of Applied Physics,Chongqing University,Chongqing 400044,China 3. Department of Physics,Chongqing Normal College,Chongqing 400047,China |