1. State Key Laboratory of Electronic Thin Film and Integrated DevicesSchool of Electronic Science and EngineeringUniversity of Electronic Science and Technology of China Chengdu 610054 P. R. China
2. College of Resources and EnvironmentHuazhong Agricultural University Wuhan 430070 P. R. China
3. School of PhysicsUniversity of Electronic Science and Technology of China Chengdu Sichuan 610054 China
4. School of Materials Science and EngineeringZhengzhou University Zhengzhou 450002 P. R. China