Abstract
We report the spectroscopic observation of bound excitons in natural ${\mathrm{Cu}}_{2}\mathrm{O}$ samples, which are generated by impurity capture of free excitons. Due to the broken symmetry of a bound exciton, its radiative recombination rate increases, causing a greatly enhanced luminescence intensity compared with that for a free exciton. Moreover, the bound exciton luminescence serves as an internal monitor of the free ortho- and free para-exciton densities, whereas the free para-exciton luminescence is allowed only via weak ${\ensuremath{\Gamma}}_{25}^{\ensuremath{-}}$ phonon emission. At high excitation levels, the bound exciton luminescence shows a density-dependent loss of excitons that is well explained by a recently proposed biexciton formation model. The temperature dependence of the bound ortho- and bound para-exciton luminescence indicates a ${T}^{\ensuremath{-}1}$-dependence for the exciton-impurity capture rate.
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