1. Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, Faculty of Engineering and Graduate School of Science & Technology, Chiba University, Chiba, Japan, Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot, Israel, Experimental Physics II, University of Würzburg, Am Hubland, Würzburg, Germany, and School of Physics, University of New South Wales, Sydney, Australia