1. College of Science Xi'an University of Architecture and Technology Xi'an Shaanxi 710055 P. R. China
2. College of Science Chang'an University Xi'an Shaanxi 710064 P. R. China
3. Functional Materials Laboratory (FML) School of Materials Science & Engineering Xi'an University of Architecture and Technology Xi'an Shaanxi 710055 P. R. China
4. Department of Physics Georgia Southern University Statesboro GA 30460 USA