1. Division of Solid
State Physics, Lund University, Box 118,
S-221 00 Lund, Sweden
2. I.E.M.N., UMR CNRS 8520, Avenue Poincaré, BP 60069, F-59652 Villeneuve
d’Ascq, France
3. Department
of Electronics and
Key Laboratory for the Physics and Chemistry of Nanodevices, Peking University, Beijing 100871, China