1. a Department of Materials Science and Mineral Engineering , University of California , Berkeley , CA , 94720 , USA
2. b Department of Physics , University of California , Berkeley , CA , 94720 , USA
3. c Materials Sciences Division Lawrence Berkeley Laboratory , Berkeley , CA , 94720 , USA
4. d ISOBE Semiconductor Laboratory, Shin-Etsu Handoutai Co. , Gunnma, ISOBE AN-NAKA City , Japan