1. NSF Nanoscale Science and Engineering Center (NSEC), University of California, Berkeley 3112 Etcheverry Hall, UC Berkeley, California 94720, United States
2. Department of Physics, University of Paderborn, Warburger Strasse 100, 33098 Paderborn, Germany
3. Department of Mechanical Engineering, Vanderbilt University, VU Station B 351592, Nashville, Tennessee 37235, United States
4. Materials Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States