1. Department of Materials Science and Engineering and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, USA
2. Department of Materials Science and Engineering/Materials Research Laboratory, University of Illinois, Urbana−Champaign, Urbana, Illinois 61801, USA
3. Department of Physics, University of Connecticut, Storrs, Connecticut 06269, USA
4. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA