1. Department of Physics, University of California, Berkeley, California 94720, USA
2. Department of Electrical Engineering and Computer Science, Nagoya University, Nagoya 464-8603, Japan
3. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
4. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
5. Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya 464-8603, Japan
6. Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, USA