Local susceptibilities in semi-infinite antiferromagnet chains: elementary perspectives

Using conformal field theory methods Eggert and Affleck have shown that the semi-infinite S=1/2 Heisenberg antiferromagnetic chain exhibits a remarkable alternation in its local response to a uniform field at low temperatures. Such alternation is not an essentially quantum effect: similar, and sometimes stronger, susceptibility alternation is a feature of classical Heisenberg-Ising chains at T=0. In S=1/2 chains, susceptibility alternation is not unique to the Heisenberg model, but can be seen in expansions about the Ising model and also in the XY model.