  Both nonmagnetic and magnetic impurity effects in spin singlet and triplet
excitonic insulators were investigated. The bound state energies caused by
single impurity were given. The different compositions of the bound states can
be used to detect the symmetry of the excitonic insulators. In finite
concentration problems, nonmagnetic impurities showed same pair-breaking effect
in singlet and triplet excitonic insulators while magnetic impurities showed
weaker pair-breaking effect in triplet excitonic insulators than in singlet
ones. The pair-breaking effects suppressed the ferromagnetic range via doping
and gave a natural explaination for experimental results.
