Evidence of Singlet-Triplet Crossing in Photoluminescence of Positively Charged Excitons in GaAs Quantum Wells

Positively charged excitons in a two-dimensional hole gas in symmetric and asymmetric GaAs/$Ga_{1-x}Al_{x}As$ quantum wells are studied in polarization-resolved photoluminescence experiments in high magnetic fields B (up to 23 T) and low temperatures (down to 300 mK). The experiments are accompanied by numerical calculations of a real structure. The whole family of trions (the singlet and a pair of triplets) are observed. The Coulomb energies crossing of singlet and triplet is found: hidden in symmetric and visible in asymmetric structures.