1. Department of Physics
2. University of Calabria
3. 87036 Rende (CS)
4. Italy
5. College of Science
6. Institute of Materials Physics and Chemistry
7. Nanjing Forestry University
8. Nanjing 210037
9. P. R. China
10. Department of Physical and Chemical Sciences
11. University of L’Aquila
12. 67100 L’Aquila (AQ)