1. Key Laboratory for Renewable EnergyBeijing National Laboratory for Condensed Matter PhysicsInstitute of PhysicsChinese Academy of SciencesP. O. Box 603Beijing 100190P.R. China
2. Department of Electrical and Computer EngineeringRutgers University94 Brett RoadPiscatawayNJ 08854USA
3. Key Laboratory of Advanced Materials and Electron MicroscopyBeijing National Laboratory for Condensed Matter PhysicsInstitute of PhysicsChinese Academy of SciencesP. O. Box 603Beijing 100190P.R. China