Project supported by National Natural Science Foundation of China(10674113,10847103);Program for New Century Excellent Talents in University(NCET-06-0707);Hunan Provincial Natural Science Foundation of China (06JJ50006);Scientific Research Fund of Hunan Provincial Education Department(06A071,07B073);Partially supported by the Open Project Program of Low Dimensional Materials & Application Technology (Xiangtan University) of Ministry of Education of China (KF0713)~~