[1] Univ Michigan, Ann Arbor, MI 48109 USA
[2] Hong Kong Univ Sci & Technol, Kowloon, Hong Kong, Peoples R China
[3] Tsing Hua Univ, Beijing 100084, Peoples R China
[4] Graz Univ Technol, A-8043 Graz, Austria
[5] Micron Technol Inc, NO-0349 Oslo, Norway