The ultrafast spin dynamics of the bright exciton in CdSe/ZnS nanocrystal quantum dots has been investigated using a circularly polarized pump-probe experiment. A remarkably fast spin flip (∼500 fs) of the bright exciton was observed at 4 K, which is attributed to the anisotropic
electron–hole exchange interaction and the random positioning of nanocrystal quantum dots. In the presence of an applied magnetic field (5 T), the exciton spin parallel to the external magnetic field was favored due to Zeeman splitting. We found that this imbalance can possibly be suppressed
by the state-blocking and the mixing of the 1L and 1U states in asymmetric quantum dots.
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