The ground state of negatively charged excitons (trions) in high magnetic fields is shown to be a dark triplet state, confirming long-standing theoretical predictions. Photoluminescence (PL), reflection, and PL excitation spectroscopy of CdTe quantum wells reveal that the dark triplet trion has lower energy than the singlet trion above 24 Tesla. The singlet-triplet crossover is "hidden" (i.e., the spectral lines themselves do not cross due to different Zeeman energies), but is confirmed by temperature-dependent PL above and below 24 T. The data also show two bright triplet states. PACS numbers: 71.35.Pq, 71.35.Ji, 78.66.Hf A central problem found in atomic, solid state, and nuclear physics is the case of a three-particle system of fermions, bound together by long-range Coulomb interactions. In atomic physics, this situation is most simply realized by the two-electron hydrogen ion, H − , in which the two identical electrons can exist in either a singlet or triplet state with total electron spin S e = 0 or 1, depending on external parameters. The semiconductor analog of the H − ion is the negatively charged exciton (trion), consisting of two conduction electrons bound to a single valence hole. Optical signatures from trions have been observed in GaAs, CdTe, and ZnSe quantum wells (QWs) [1,2,3,4]. Unlike the H − ion, the hole and two electrons comprising the trion have comparable masses and typically experience strong QW confinement in one dimension, making trions a genuine quantum three-particle system with Coulomb interactions for which no general analytical solutions exist.Much attention has focused on the evolution of trion optical signatures with applied magnetic field [5,6,7,8,9]. In the limit of zero magnetic field, theory predicts just one bound trion state: the S e = 0 singlet trion (T s ) [10,11]. This is consistent with Hill's theorem [12], which states that the H − ion (with an infinitely massive proton) supports exactly one bound singlet state. In the opposite limit of extremely high magnetic fields, it can be rigorously shown that a S e = 1 triplet is the only bound trion state in a strictly 2D system [11,13]. Modelindependent symmetry considerations [14] demonstrate that this lowest triplet state is "dark" (T td ) (i.e., optically inactive), due to the exact selection rules imposed by spatial axial and translational symmetries that exist in a disorder-free QW. Thus, at finite magnetic fields one expects both singlet and triplet bound trions [11,13]. More importantly, at some critical magnetic field B c the spin configuration of the trion ground state must cross over from the singlet to the triplet. Theoretical estimates suggest this crossover field is very large (B c > 20 T) and depends sensitively on the strength of the Coulomb interaction (dielectric constant) and the details of the QW confinement [11,15,16,17,18]. Numerical calculations also point to the existence of weakly-bound, optically active "bright" triplet states (T tb ), although there is large disparity amongst the predicted regions of...

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Thermally stimulated luminescence (TSL) and thermally stimulated exoelectron emission (TSEE) methods were used in combination with cathodoluminescence to probe electronically induced defects in solid Ne. The defects were generated by a low energy electron beam. For spectroscopic study we used Ar * centers in Ne matrix as a model system. At a temperature of 10.5 K a sharp decrease in the intensity of «defect» components in the luminescence spectrum was observed. From the analysis of the corresponding peak in the TSL and TSEE yields the trap depth energy was estimated and compared with available theoretical calculations. The obtained data support the model suggested by Song, that stable electronically induced defects have the configuration of second-neighbour Frenkel pairs.