Acknowledgements This work was partially supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 60736005 and 60425101-1), the Foundation for Innovative Research Groups of the NSFC (No. 60721001), the Provincial Program (No. 9140A02060609DZ0208), Doctoral Fund of Ministry of China (No. 20090185110020), the National Hi- Tech Research and Development Program of China (No. 2007AA03Z424), the Project Sponsored by SRF for ROCS, SEM (No. GGRYJJ08-05), and Young Excellence Project of Sichuan (No. 09ZQ026-074).
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