TABLE OF CONTENTS So far in this chapter we considered the behaviour of magnetic impurities in insulating systems, in which only spin degrees of freedom possessed dynamics. It is interesting to investigate how impurities behave in correlated electron chains using exact methods. From the previous chapter the reader already knows one possible model of an impurity: it pertains to a local field or potential. For the Bethe ansatz integrable models this kind of impurity can be considered only when it is situated at the edge of an open chain. Now we shall study the behaviour of an integrable impurity of another kind: the one, studied in the previous section, which is introduced into the Bethe ansatz scheme via a special L-operator, which, though, satisfies Yang Baxter (intertwining) relations with R-matrices of the host model.
Let us start our consideration with the supersymmetric antiferromagnetic t-J model for the most popular case V = ? J/2 and J =2. We shall first work in the framework of the graded scheme of the algebraic Bethe ansatz, introduced in Chapter 5. We begin with the gl(12) invariant R-matrix, which satisfies the Yang Baxter equation. We already proved in Chapter 5 that one can construct monodromy operators, which traces, transfer matrices, with different spectral parameters mutually commute. This constitutes the exact integrability. The only condition we demanded from a monodromy matrix was the following. The action of diagonal matrix elements of the monodromy T ??
