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

We study the boundary impurity-induced critical behavior in an integrable SU(2)-invariant model consisting of an open Heisenberg chain of arbitrary spin-S (Takhatajian-Babujian model) interacting with an impurity of spin ~ S′ located at one of the boundaries. For S = 1/2 or S′ = 1/2, the impurity interaction has a very simple form J ~ S1 · ~ S′ which describes the deformed boundary bond between the impurity ~ S′ and the first bulk spin ~ S1 with an arbitrary strength J . With a weak coupling 0 < J < J0/[(S + S ′)2 − 1/4], the impurity is completely compensated, undercompensated, and overcompensated for S = S′, S > S′ and S < S′ as in the usual Kondo problem. While for strong coupling J ≥ J0/[(S + S′)2 − 1/4], the impurity spin is split into two ghost spins. Their cooperative effect leads to a variety of new critical behaviors with different values of |S′ − S|. 75.10.Jm, 75.20.Hr, 72.15.Qm, 05.70.Jk Typeset using REVTEX 1