Strong correlation effects in theoretical STM studies of magnetic adatoms

Abstract: We present a theoretical study for the scanning tunneling microscopy (STM) spectra of surface-supported magnetic nanostructures, incorporating strong correlation effects. As concrete examples, we study Co and Mn adatoms on the Cu(111) surface, which are expected to represent the opposite limits of the Kondo physics and local moment behavior, using a combination of density functional theory and both quantum Monte Carlo and exact diagonalization impurity solvers. We examine in detail the effects of temperature $T$, correlation strength $U$, and the impurity $d$ electron occupancy $N_d$ on the local density of states. We also study the effective coherence energy scale, i.e., the Kondo temperature $T_K$, which can be extracted from the STM spectra. Theoretical STM spectra are computed as a function of the STM tip position relative to each adatom. Because of the multi-orbital nature of the adatoms, the STM spectra are shown to consist of a complicated superposition of orbital contributions, with different orbital symmetries, self-energies and Kondo temperatures. For the Mn adatom, which is close to half-filling, the STM spectra are featureless near the Fermi level. On the other hand, the quasiparticle peak for Co adatom gives rise to strongly position-dependent Fano line-shapes.