[1] Tohoku Univ, Dept Phys, Sendai, Miyagi 9808578, Japan
[2] CREST, Sendai, Miyagi 9808578, Japan
[3] MIT, Dept Mat Sci & Engn, Cambridge, MA 02139 USA
[4] MIT, Dept Elect Engn & Comp Sci, Cambridge, MA 02139 USA
[5] MIT, Francis Bitter Magnet Lab, Cambridge, MA 02139 USA
[6] MIT, Dept Phys, Cambridge, MA 02139 USA