[1] Univ S Florida, Dept Phys, Tampa, FL 33620 USA
[2] Georgia Inst Technol, Sch Mat Sci & Engn, Atlanta, GA USA
[3] Georgia Inst Technol, GW Woodruff Sch Mech Engn, Atlanta, GA USA
[4] Queens Univ Belfast, Ctr Quantum Mat & Technol, Sch Math & Phys, Belfast, North Ireland
[5] Trinity Coll Dublin, Sch Phys, Dublin, Ireland
[6] Ctr Res Adapt Nanostruct & Nanodevices CRANN, Adv Microscopy Lab, Dublin, Ireland