1. Department of Materials Science and Engineering and Department of PhysicsUniversity of California, Berkeley Berkeley CA 94720 USA
2. State Key Laboratory for Low‐Dimensional Quantum PhysicsDepartment of PhysicsTsinghua University Beijing 100084 P. R. China
3. Department of PhysicsNational Cheng Kung University Tainan 701 Taiwan
4. Department of PhysicsDurham University Durham DH1 3LE UK
5. Department of Materials Science and EngineeringNational Chiao Tung University Hsinchu 30010 Taiwan
6. Materials Sciences DivisionLawrence Berkeley National Laboratory Berkeley CA 94720 USA