[1] Univ Texas, Dept Phys & Astron, San Antonio, TX 78249 USA
[2] Univ Houston, Texas Ctr Superconduct, Houston, TX 77204 USA
[3] Univ Sci & Technol China, Hefei Natl Lab Phys Sci Microscale, Hefei, Peoples R China
[4] Univ Sci & Technol China, Dept Phys, Hefei 230026, Peoples R China
[5] Univ Houston, Dept Chem, Houston, TX 77204 USA
[6] Univ Houston, Dept Phys, Houston, TX 77204 USA