1Materials Research Laboratory, Samsung Advanced Institute of Technology, Yongin 446-712, Korea
2Department of Physics, Sung Kyun Kwan University, Suwon 440-746, Korea
3Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Korea
4Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854-8019, USA
Correspondence and requests for materials should be addressed to S.M.L. ([email protected]).
Received 22 January; accepted 20 April 2009.
Acknowledgements We thank D. Johnson, K. Koumoto and B. I. Min for discussions. We also thank H. R. Choi for TEM measurements. J.H.S. was supported by the WCU programme (KOSEF: R32-2008-000-10180-0).
