1. Department of Electrical and Computer Engineering , University of Wisconsin-Madison , Madison , Wisconsin 53706 , USA
2. Institute of Solid State Physics, Friedrich Schiller University Jena , Jena , 07743 , Germany ,
3. Department of Materials Science and Engineering , University of Wisconsin-Madison , Madison , Wisconsin 53706 , USA
4. School of Materials Engineering , Purdue University , West Lafayette , IN 47907 , USA
5. Department of Physics , University of Wisconsin-Madison , Madison , Wisconsin 53706 , USA