[1] Oak Ridge Natl Lab, Condensed Matter Sci Div, Oak Ridge, TN 37831 USA
[2] Peking Univ, State Key Lab Mesoscop Phys, Dept Phys, Beijing 100871, Peoples R China
[3] Oak Ridge Natl Lab, Div Met & Ceram, Oak Ridge, TN 37831 USA
[4] Univ Tennessee, Dept Phys & Astron, Knoxville, TN 37996 USA