1. Physics Department of Basic Department, The Army Engineering University of PLA, Nanjing 211101, P. R. China
2. Department of Physics, Nanjing Xiaozhuang College, Nanjing 210017, P. R. China
3. Research Center Jülich, Institute for Neuronscience and Medicine INM-2, D-52425 Jülich, Germany
4. Department of Physics, Southeast University, Nanjing 210096, P. R. China