Anomalous dispersion of charged excitons in dilute two-dimensional electron systems at low temperatures

We report the observation of a strong anomaly in the magnetic-field dispersion of negatively charged excitons in dilute two-dimensional electron systems. The anomaly exists in an intermediate density range around $1\times {10}^{11}\hspace{0.5em}{\mathrm{cm}}^{-2},$ and at temperatures $T<\mathrm{}1\mathrm{}\hspace{0.5em}\mathrm{K},$ only. The strength of the anomaly at $T=0.1\mathrm{}\hspace{0.5em}\mathrm{K}$ is about 2.5 meV, and the most striking feature is that it can be completely destroyed by a small thermal energy of about 0.2 meV (2 K). Furthermore, the anomaly is observed both in photoluminescence and direct absorption measurements, which gives a strong indication that it is of intrinsic origin.