[1] Carnegie Mellon Univ, Dept Phys, Pittsburgh, PA 15213 USA
[2] Univ Sci & Technol China, Hefei Natl Lab Phys Sci Microscale, Int Ctr Quantum Design Funct Mat ICQD, Hefei, Anhui, Peoples R China
[3] Oak Ridge Natl Lab, Mat Sci & Technol Div, Oak Ridge, TN USA
[4] Univ Tennessee, Dept Mat Sci & Engn, Knoxville, TN 37996 USA
[5] Univ Tennessee, Dept Phys & Astron, Knoxville, TN 37996 USA