1. Department of Materials Science and Engineering; Pohang University of Science and Technology (POSTECH); Pohang Gyungbuk 790-784 South Korea
2. School of Electrical and Electronic Engineering; Yonsei University; Seoul 120-749 South Korea
3. Theoretical Division; Los Alamos National Laboratory; Los Alamos NM 87545 USA
4. Center for Integrated Nanotechnologies; Los Alamos National Laboratory; Los Alamos NM 87545 USA