Excitons in Carbon Nanotubes with Broken Time-Reversal Symmetry
  Near-infrared magneto-optical spectroscopy of single-walled carbon nanotubes reveals two absorption peaks with an equal strength at high magnetic fields ($>$ 55 T). We show that the peak separation is determined by the Aharonov-Bohm phase due to the tube-threading magnetic flux, which breaks the time-reversal symmetry and lifts the valley degeneracy. This field-induced symmetry breaking thus overcomes the Coulomb-induced intervalley mixing which is predicted to make the lowest exciton state optically inactive (or ``dark''). 
S. Zaric, G. N. Ostojic, J. Shaver, J. Kono, O. Portugall, P. H.   Frings, G. L. J. A. Rikken, M. Furis, S. A. Crooker, X. W...