`Unscreening' Effect on Fe-Pnictide Superconductor

We study a microscopic mechanism of Fe-pnictide superconductor, considering the screening effects of Coulomb interaction in addition to the conventional spin-fluctuation mechanism. It is shown that, by electron doping, the transition temperature of superconductivity increases due to the `unscreening' effect even though the density of states decrease, while that of spin-density wave rapidly decreases due to breaking of nesting conditions. Our results give a clear interpretation to the mystery of interrelation between $T$c and the density of states observed in the Fe-pnictide superconductors.