1. Department of Physics, Center for Nanophysics and Advanced Materials, University of Maryland, College Park, Maryland 20742-4111, USA
2. B. Verkin Institute for Low Temperature Physics and Engineering, National Academy of Sciences of Ukraine, UA-61103 Kharkov, Ukraine
3. Department of Physics, National University of Singapore, Singapore 117551
4. NUSNNI-Nanocore, National University of Singapore, Singapore 117411
5. Ames Laboratory, Ames, Iowa 50011, USA