IMPURITY EFFECTS IN CORRELATED ELECTRON-SYSTEMS

We consider spin vacancies as well as spin-one impurities in RVB state s. In the flux phase a spin vacancy site induces a bound state with a free spin. The spin distribution in the vicinity of the vacancy is cal culated as well as the interaction potential between two vacancies. Wh en a spin-one impurity is considered, its low energy properties can be described in terms of an underscreened Kondo problem. These findings may be of relevance for Zn and Ni impurities in underdoped HTSC materi als.