Ghost spins and quantum critical behavior in a spin chain with local bond deformation

We study the impurity-induced critical behavior in an integrable SU(2)-inva riant model consisting of an open spin chain of arbitrary spin S (Takhataji an-Babujian model) interacting with an impurity of spin S' located at one o f the boundaries. For S=1/2 or S'=1/2, the impurity interaction takes a ver y simple form J (S) over right arrow(1).(S) over right arrow' that describe s the deformed boundary bond between the impurity (S) over right arrow' and the first bulk spin (S) over right arrow(1) with an arbitrary coupling str ength J. For a weak coupling 0<J<J(0)/[(S+S')(2)-1/4], the impurity is comp letely compensated, undercompensated, and overcompensated for S'=S, S'>S, a nd S'<S as in the usual Kondo problem. While for strong coupling J greater than or equal to J(0)/[(S+S')(2)-1/4], the impurity spin is split into two ghost spins. Their cooperative effect leads to a variety of new critical be haviors with different values of IS'-SI. [S0163-1829(99)03530-4].