1. School of Materials Science and Engineering; Shanghai Jiao Tong University; Shanghai 200240 China
2. Department of Material Science and Engineering; Materials Research Laboratory; University of Illinois at Urbana-Champaign; Urbana 61801 Illinois
3. Frederick Seitz Materials Research Laboratory; University of Illinois at Urbana-Champaign; Urbana 61801 Illinois
4. Department of Physics; University of Illinois at Urbana-Champaign; Urbana 61801 Illinois