Dirac quasinormal modes of the Reissner-Nordstr\"om de Sitter black hole

The quasinormal modes of the Reissner-Nordstr"om de Sitter black hole for the massless Dirac fields are studied using the P"oshl-Teller potential approximation. We find that the magnitude of the imaginary part of the quasinormal frequencies decreases as the cosmological constant or the orbital angular momentum increases, but it increases as the charge or the overtone number increases. An interesting feature is that the imaginary part is almost linearly related to the real part as the cosmological constant changes for fixed charge, and the linearity becomes better as the orbital angular momentum increases. We also prove exactly that the Dirac quasinormal frequencies are the same for opposite chirality.