  We calculate the differential conductance (dI/dV) corresponding to scanning
tunneling spectroscopy (STS) measurements for two magnetic atoms adsorbed on a
metal surface with the aid of the numerical renormalization group (NRG)
technique. We find that the peak structure of the dI/dV spectra near the Fermi
level changes gradually as a function of the adatom separation and the coupling
between the adatoms and the metal surface conduction band. When the coupling
becomes small, the peak disappears and, instead, a dip structure appears near
the Fermi level. This dip structure is the manifestation of the strong
antiferromagnetic correlation between the localized spins. The gradual change
of the dI/dV structure from a peak structure to a dip structure originates from
the crossover transition in the two impurity Kondo problem.
