We review recent optical experiments on dilute two-dimensional electron systems (2DES) at very low temperatures ( K) and high magnetic fields. In photoluminescence experiments on a 2DES subjected to a quantizing magnetic field around , we have observed an anomalous dispersion of the charged excitons. We have found that the anomaly exists only at a very low temperature K) and an intermediate electron density cm . It is explained to occur due to the perturbation of the incompressible liquid at . The perturbation is induced by the close proximity of a localized charged exciton which creates a fractionally-charged quasihole in the liquid. The intriguing experimentally observed puzzle that the anomaly (2 meV) can be destroyed by applying a small thermal energy of 0.2,meV is thereby resolved, as this energy is enough to close the quasihole energy gap. This work presents a first ever probe of the quasihole gap in a quantum Hall system.
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