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The biexciton puzzle

The Stochastic Variational Method (SVM) is used to show that the effective mass model correctly estimates the binding energies of excitons and trions, but fails to predict the experimental binding energy of the biexciton. Using high-accuracy variational calculations, it is demonstrated that the biexciton binding energy in transition metal dichalcogenides is smaller than the trion binding energy, contradicting experimental findings. It is also shown that an excited state of the biexciton is in very good agreement with experimental data. This excited state corresponds to an hole attached to a negative trion and may be a possible resolution of the discrepancy between theory and experiment.