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Oscillator strengths of dark charged excitons at low electron filling factors
- By direct absorption spectroscopy and comparison to photoluminescence (PL), we investigate negatively charged excitons in dilute two-dimensional electron systems at temperatures down to (formula presented) in the regime of the fractional quantum Hall effect. At very low temperatures, for filling factor (formula presented) an additional excitation appears in the PL spectrum, between the well-known singlet and triplet excitons. The observation of a similar excitation by PL was reported very recently [G. Yusa et al., Phys. Rev. Lett. 87, 216402 (2001)], and the excitation was assigned, in spite of the PL intensity similar to that of the neutral exciton, to be due to a dark triplet exciton. By comparing PL and direct absorption spectra in optically thin samples at (formula presented) we can identify the new excitation indeed as a “dark” mode, since we find that the oscillator strength is much smaller than those of the “bright” modes. © 2002 The American Physical Society.
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