  The impurity effects on the transition temperature Tc and the isotope effect
are examined in multiband superconductors with magnetic and nonmagnetic
impurities, where the effect of Coulomb repulsion is considered. It is shown
that an internal transition of the superconducting state is induced by impurity
doping, and that the transition is accompanied by a jump of the isotope-effect
coefficient alpha. In particular, the transition is illustrated in a system
with two electron bands. In some special cases, extended Abrikosov and Gor'kov
(AG) equations for Tc and the expressions of the isotope-effect coefficient
alpha are obtained. Possible relevance of the present mechanism to the
experimental results of Sr2RuO4 is discussed.
