Origin of low-energy photoluminescence peaks in single carbon nanotubes: K -momentum dark excitons and triplet dark excitons

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We performed photoluminescence (PL) spectroscopy on single carbon nanotubes to investigate the satellite PL peaks, which are much lower in energy than the lowest (E11) bright exciton peak. From the temperature and tube-diameter dependences of the PL spectra, we clarified two origins of the low-energy PL peaks. The weak peak, lying about 130 meV below the E11 bright exciton state, is well explained by the phonon sideband of the K-momentum dark exciton states above the lowest-bright exciton state. In addition, a strong PL peak appears after pulsed-laser irradiation, and its peak energy depends strongly on the tube diameter. This PL peak comes from the triplet dark exciton states.