1. Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, U.S.A.
2. Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843, U.S.A.
3. Department of Chemistry, Texas A&M University, College Station, TX 77843, U.S.A.
4. WPI—Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan