TABLE OF CONTENTS In this chapter we shall present exact results for thermodynamic characteristics of impurities in quantum spin and correlated electron chains and compare their behaviours with those of free edges of homogeneous quantum chains, and with magnetic and hybridization impurities in three- dimensional metals (the Kondo and Anderson impurities).
In previous chapters we considered mostly homogeneous quantum spin and correlated electron chains. The only inhomogeneity, considered so far, was the possibility of cutting a periodic chain. In that case the behaviour of edges of open quantum chains was different from the behaviour of bulk sites of that chain. This situation (with open chains) can be realized if one introduces a nonmagnetic impurity into a periodic spin chain. However, more generic situation with an impurity in a quantum correlated chain is the following:
An impurity can have different local characteristics ( e.g.,aneffective magnetic moment or local potential energy) from those of other sites of a chain;
an impurity can be coupled to other sites of a chain with an interaction, different in its strength (and, generally speaking, in the way of coupling) from interactions between other sites of a chain.
Actually, an open boundary, studied in the previous chapter, can be considered as a special impurity. The strength of an interaction of the link between the first and the last sites of a periodic chain is zero for an open chain, and, hence, it is different from couplings in the bulk. Also, an effective magnetic...
