Controlling the Kondo Effect inCoCunClusters Atom by Atom

Abstract

Clusters containing a single magnetic impurity were investigated by scanning tunneling microscopy, spectroscopy, and ab initio electronic structure calculations. The Kondo temperature of a Co atom embedded in Cu clusters on Cu(111) exhibits a non-monotonic variation with the cluster size. Calculations model the experimental observations and demonstrate the importance of the local and anisotropic electronic structure for correlation effects in small clusters.PACS numbers: 68.37. Ef,72.15.Qm,73.20.Fz Nanometer scaled electronic devices require the understanding and the control of electron behavior, in particular of correlation effects, at the atomic scale. Experimental techniques such as scanning tunneling microscopy (STM) and spectroscopy and angle-resolved photoemission demonstrate the relevance of many-body phenomena beyond standard band theory [1]. In many cases, especially for compounds of d and f elements, strong electron correlations should be taken into account even for a qualitative description of electron energy spectra and physical properties [2]. The Kondo effect, which is related to the resonant scattering of conduction electrons by quantum local centers, is one of the key correlation effects in condensed matter physics [3]. It arises from the interaction of a single localized magnetic moment with a continuum of conduction electrons and results in a sharp Abrikosov-Suhl resonance of the local impurity spectral function at the Fermi level and below a characteristic Kondo temperature, T K [3]. Originally detected as a resistance increase below T K of materials with dilute magnetic impurities, this many-body phenomenon has been observed for semiconductor quantum dots [4,5] and for atoms [6,7,8,9] and molecules [10] at surfaces. Whereas the understanding of correlation phenomena of bulk materials and single impurities has made significant progress much less is known about these effects in structures such as clusters where typical dimensions are on an atomic scale. STM offers unique opportunities to fabricate such structures by manipulation of single atoms and a few attempts have been made to achieve a degree of control over the electronic [11,12,13] and magnetic structure [14,15] of atoms and clusters on surfaces.Here we show that the Kondo effect of a magnetic impurity does not simply scale with the number of nearest neighbors. Rather, the local and anisotropic electronic structure at the impurity site is important for correlations in atomic scale structures, where "each atom counts" [16]. We investigate the evolution of the Abrikosov-Suhl resonance induced by a Co atom adsorbed on Cu (111) as we change the number of nearest neighbor Cu atoms. A non-monotonic behavior of the Kondo temperature with the cluster size is at variance with bulk models which are typically invoked to interpret the Kondo effect of a single adsorbed atom (adatom). Scanning tunneling spectroscopy measurements and ab initio calculations clearly reveal that the Kondo effect in atomic scaled clusters depends crucially on their deta...

Dynamical mean-field theory for molecular electronics: Electronic structure and transport properties

“…Recently, however, nanocontacts made from Fe, Co, or Ni have been reported to display Kondo effect 10,11 . This has been infered from the observation of Fanolineshapes 12 in the low-voltage conductance characteristics similar to those observed in recent STM experiments with magnetic adatoms on metal surfaces [13][14][15][16] . The observation of the Kondo effect in Fe, Co, and Ni nanocontacts is rather surprising since these materials are strong ferromagnets as bulk materials.…”

Ammonia adsorption on iron phthalocyanine on Au(111): Influence on adsorbate–substrate coupling and molecular spin

“…These results fit in well with previous studies, which have shown the possibility of tailoring the electronic structure of metal atoms and complexes by ligand attachment [31][32][33][34] and atom-by-atom manipulation. 35 In addition, we find evidence for adorption of NH 3 in other sites of the FePc monolayer/Au(111) system. We have used temperature-programmed XPS (TP-XPS) to characterize these adsorbate states and find that NH 3 in these sites is more weakly bonded than when adsorbed on the FePc iron center.…”

Theory of Fano resonances in graphene: The influence of orbital and structural symmetries on STM spectra

“…To this end, we first consider the single impurity Anderson model with graphene providing the host electronic structure and compare to a simple model of a usual metal surface. With this background, we turn to a more realistic ab initio based description of magnetic impurities on graphene and discuss the case of Co adatoms as in the recent experiment by Manoharan et al 19 By comparison to Co on Cu ͑111͒, an extensively studied system, [5][6][7] possessing also hexagonal symmetry of the surface, we will demonstrate the importance of impurityinduced resonances in graphene 20 and the particular sensitivity of the q factor to orbital symmetries. Furthermore, we analyze model s-wave impurities being bound to different sites of the graphene lattice and show that a strong adsorption site dependence of Fano factors in STM experiments can be expected there.…”

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