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Sander Otte, University of Leiden The Kondo Effect of a Single Atom with Large Spin A localized spin interacting with a sea of conduction electrons (such as a magnetic atomic impurity embedded in a metal) can lead to the formation of a multi-body spin singlet state at sufficiently low temperatures. This intriguing phenomenon, known as the Kondo effect, dramatically changes the electronic properties of the host material. In experimental studies of individual Kondo impurities the magnitude of the localized spin is for simplicity often assumed to be 1/2, the smallest possible value. However, in the case of atomic spins adsorbed onto metal surfaces it could in reality be larger. In contrast to spin-1/2, larger spins are subject to the influence of magnetic anisotropy. Using an STM operating at 0.5 K, we study the Kondo effect of a single Co atom adsorbed onto Cu_2 N. By performing spin excitations we demonstrate that it is a spin-3/2 that is quenched by the anisotropic crystal field into an effective S=1/2 doublet. When applying magnetic fields along various crystalline axes we find that the Kondo resonance splits at a rate that varies for different field directions, precisely reproducing the known anisotropy of the Cu_2 N surface.
