1. Graduate School of Biomedical
Engineering, Tohoku University, 6-6 Aoba,
Aramaki, Aoba-ku, Sendai 980-8579, Japan
2. PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi,
Saitama 332-0012, Japan
3. Laboratory
for Nanoelectronics
and Spintronics, Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai,
Miyagi 980-8577, Japan