STM differential conductance of a pair of magnetic adatoms*

Competition between screening of local moments through the Kondo effect and magnetic ordering of those moments through direct or induced (RKKY) exchange interactions is a key feature of heavy-fermion systems and of metals containing dilute magnetic impurities. It should be possible to probe this competition in its purest form through scanning tunneling microscopy (STM) studies of pairs of magnetic adatoms on metallic surfaces. However, in order to interpret the STM differential conductance through a pair of nearby adatoms, it is necessary to understand the effects of Fano-like interference between different tunneling paths from the STM tip into the substrate. We report preliminary results of numerical renormalization-group calculations of the impurity spectral function and the differential tunneling conductance for the two-impurity Anderson model, and compare our results with those for the well-studied one-impurity case.