  When a scalar wave perturbation is properly scattering off a charged or
rotating black hole, the energy of the reflected scalar wave may be amplified.
This is a superradiant process. If this amplification process can occur back
and forth through certain confining mechanism, it will lead to strong
instability of the black hole. In this paper, the superradiant stability is
investigated for a special kind of dyonic black holes in string theory.
Although the dynoic black hole has a similar spacetime metric with a
electrically charged RN black hole, it is found that the dyonic black hole is
more unstable than a RN black hole due to the coupling between magnetic charge
of the dyonic black hole and the imping electrically charged scalar wave. We
find two superradiantly stable regions in the parameter space for the dyonic
black holes and charged massive scalar perturbation.
