Magnetic impurities in half-integer-spin Heisenberg antiferromagnetic chains.

We study the efFect of an isolated impurity on the low-energy properties of a half-odd-integerspin Heisenberg antiferromagnetic chain using both numerical and conformal field theory techniques. The impurity corresponds to the substitution of a magnetic ion by a difFerent ion with the same or different spin, or else to the coupling of a magnetic impurity to a single spin on the chain. Depending on which kind of impurity is present, the low-energy behavior corresponds either to a "healing" of the defect or else to a severing of the chain at the impurity location. In both cases there is a couplingdependent length scale over which magnetic screening takes place. Analogies with the Kondo effect are elucidated.