Kondo effect of cobalt adatoms on nanostructured Cu-O surfaces: Scanning tunneling spectroscopy experiments and first-principles calculations A. Gumbsch, 1 G. Barcaro, 2 M. G. Ramsey, 1 S. Surnev, 1 A. Fortunelli, 2, * and F. P. Netzer 1,† 1 Institute of Physics, Surface and Interface Physics, Karl-Franzens University Graz, A-8010 Graz, Austria 2 Molecular Modeling Laboratory, Istituto per i Processi Chimico-Fisici (IPCF), CNR, v. G. Moruzzi 1, 56124 Pisa, Italy Received 11 November 2009; revised manuscript received 7 March 2010; published 12 April 2010 The Kondo response of single Co adatoms on a nanostructured Cu110-O stripe phase is studied using scanning tunneling spectroscopy STS and first-principles calculations. The nanostructured Cu-O substrate consists of a regular array of clean Cu110 and oxygen-reconstructed Cu110 2  1-O stripes and allows us to measure STS of Co adatoms in different chemical environments under identical experimental conditions. The characteristic Kondo parameters are obtained from the Fano line-shape analysis of the STS data, finding a qualitatively different behavior on clean Cu110 and Cu110 2  1-O adsorption sites, with a Fano-type peak around the Fermi energy in STS on Cu110 and a Fano dip on Cu2  1-O, and mean Kondo temperatures of 125 K and 93 K, respectively. Density-functional calculations are performed to reveal the detailed geom- etry and the electronic structure of the Co adsorption complexes, and are used in conjunction with simple models of the Kondo effect to rationalize the present experimental observations and the trends with respect to literature data on other Cu surfaces. DOI: 10.1103/PhysRevB.81.165420 PACS numbers: 72.10.Fk, 68.37.Ef, 72.15.Qm, 68.43.h I. INTRODUCTION The interaction of the conduction-band electrons of a non- magnetic host metal with the localized spins of an embedded magnetic impurity atom gives rise to spin-flip scattering pro- cesses, which at sufficiently low temperatures create a corre- lated many-body singlet ground state forming a narrow Abrikosov-Suhl resonance in the local density of states of the impurity around the Fermi energy. This is the spectroscopic fingerprint of the Kondo effect, 1 formulated some 45 years ago to explain the previously observed anomalous transport properties at low temperatures in simple metals containing magnetic impurities. 2,3 Although the Kondo phenomenon as a prototypical example of correlation effects in condensed- matter physics has remained of interest in the following years, quantitative theoretical predictions of the properties of the Kondo many-body ground state from first principles are still under debate. Empirical Hamiltonians able to capture the basic physics of the Kondo phenomena have been proposed 4–6 but the quantitative connection between the pa- rameters of these Hamiltonians and the complicated elec- tronic picture obtained from sophisticated first-principles cal- culations still remains elusive, due to the intrinsically many- body character of the underlying physics and the complexity of the systems in which the Kondo effect is realized. The Kondo effect has received new actuality by the recent ad- vances in experimental nanoscale techniques, in particular, in scanning tunneling microscopy STM and scanning tunnel- ing spectroscopy STS, which enabled the observation of the effect down to the single atom level. The Abrikosov-Suhl or Kondo resonances, as they are generally called, of single magnetic adatoms on surfaces of nonmagnetic metals have been first observed as sharp asymmetric features around the Fermi energy in STS spectra on Ag111 and Au111 surfaces. 7,8 The spectral features obtained in the STS spectra taken on top of the magnetic adatoms are often described by Fano line shapes, 9 which are interpreted by the interference between two electron-tunneling channels, one direct tunnel- ing channel through the resonance localized at the magnetic impurity and an indirect channel into the conduction band of the metal substrate. The form and asymmetry parameter q of the Fano line shape depends on the relative strengths of di- rect and indirect channels whereas the half width at half maximum  is related to the so-called Kondo temperature T K , which is characterized by the binding energy of the screened, many-body singlet ground state. 10 The Kondo states of single Co adatoms on Cu100 and Cu111 surfaces have been studied by STS in the group of Kern et al. 11–13 The spectroscopic Kondo fingerprints of the Co adatoms on Cu100 and 111 surfaces consist of char- acteristic Fano dips around the Fermi level, as they have been observed on most other noble-metal surfaces. 10 Schneider et al. 13 have suggested a simple scaling behavior of the Kondo temperature with the number of nearest- neighbor substrate atoms: increasing the coordination num- ber n in the particular adsorption site increases T K . In accord with the latter, for the case of Co on Cu111, n =3 and T K  54 K whereas on Cu100, n =4 and T K  88 K. The ra- tionale behind this scaling relation is that the Kondo tem- perature depends on the hybridization between the adatom and the substrate electronic states which increases with the number of nearest neighbors on the supporting surface. How- ever, a fully satisfactory atomistic understanding of the Kondo phenomenon has not been yet achieved, even though several approaches ranging from mapping onto Anderson im- purity models 14 to quantum Monte Carlo methods 15 have been proposed in the literature. A route for connecting first- principles calculations to geometry-dependent empirical models, then solved via numerical renormalization-group techniques, and thus leading to explicit conductance calcula- tions has been very recently proposed 16 but this approach has been so far demonstrated on a very simple model system, and its application to those considered in the present work, exhibiting a complex band structure and several open inter- PHYSICAL REVIEW B 81, 165420 2010 1098-0121/2010/8116/1654207 ©2010 The American Physical Society 165420-1