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We find a new instability in the four-dimensional Reissner-Nordstr\"om--de Sitter black holes against charged scalar perturbations with vanishing angular momentum, l=0. We show that such an instability is caused by superradiance. The instability does not occur for a larger angular index, as explicitly proved for l=1. Our results are obtained from a numerical investigation of the time-domain profiles of the perturbations.
Zhu et al. (Fri,) studied this question.