1. a Department of Physics , Carnegie-Mellon University , Pittsburgh , Pennsylvania , 15213 , USA
2. b Division of Engineering, Brown University , Providence , Rhode Island , 02912 , USA
3. c Metals and Ceramics Division, Oak Ridge National Laboratory , Oak Ridge , Tennessee , 37831-6115 , USA
4. d Department of Physics , Washington University , St Louis , Missouri , 63130-4899 , USA