Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, Shanghai 200241, People’s Republic of China, College of Materials and Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China, and Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China (At 2010-06-21)
