Magneto-optical spectroscopy of highly aligned carbon nanotubes: Identifying the role of threading magnetic flux

We have investigated excitons in highly aligned single-walled carbon nanotubes (SWCNTs) through optical spectroscopy at low temperatures down to 1.5 K and high magnetic fields (B) up to 55 T. SWCNT/polyacrylic acid films were stretched, giving SWCNTs that are highly aligned along the direction of stretch (n). Utilizing two well-defined measurement geometries, n B and n B, we provide unambiguous evidence that the photoluminescence energy and intensity are only sensitive to the B -component parallel to the tube axis. A theoretical model of one-dimensional magnetoexcitons, based on exchange-split "bright" and "dark" exciton bands with Aharonov-Bohm-phase-dependent energies, masses, and oscillator strengths, successfully reproduces our observations and allows determination of the splitting between the two bands as ∼4.8 meV for (6,5) SWCNTs. © 2008 The American Physical Society.