1. Department of Materials Science and EngineeringCollege of EngineeringPeking University Beijing 100871 China
2. Condensed Matter Science and Technology InstituteSchool of Instrumentation Science and EngineeringHarbin Institute of Technology Harbin 150080 China
3. Key Laboratory for Photonic and Electronic Bandgap MaterialsMinistry of EducationSchool of Physics and Electronic EngineeringHarbin Normal University Harbin 150025 China
4. Department of Mathematics and Materials Research InstituteThe Pennsylvania State University University Park PA 16802 USA