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.
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