1. a Department of Physics , University of Florence, and INFM , via G. Sansone 1, 50019 , Sesto Fiorentino (Florence) , Italy
2. b Huntsman Tioxide , Stabilimento di Scarlino , 58020 , Scarlino (Gr) , Italy
3. c Department of Mathematics and Informatic Science , University of Yantai , 264005 , Yantai , P. R. China