  A system of conduction electrons can mediate an indirect magnetic exchange
between magnetic impurites. The nonlocal exchange typically competes with the
local Kondo screening of the impurity magnetic moments. In case of magnetic
adatoms on non-magnetic surfaces this competition is expected to be affected by
different confinement effects. Here we study this situation in the regime of
intermediate hybridization strengths by means of real-space dynamical
mean-field theory for a one-dimensional two-impurity Anderson model. Depending
on the presence of a boundary and depending on the system size, a crossover
between nonlocal and local singlet formation is observed in corresponding
magnetic susceptibilities. The crossover is driven by the strong sensitivity of
the Kondo temperature on the local density of states.
