Kondo effect of cobalt adatoms on nanostructured Cu-O surfaces: Scanning tunneling spectroscopy experiments and first-principles calculations

A. Gumbsch, G. Barcaro, M. G. Ramsey, S. Surnev, A. Fortunelli, and F. P. Netzer

Phys. Rev. B 81, 165420 – Published 12 April 2010

Abstract

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) $2 \times 1 -O$ 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) $2 \times 1 -O$ adsorption sites, with a Fano-type peak around the Fermi energy in STS on Cu(110) and a Fano dip on $Cu (2 \times 1) -O$ , and mean Kondo temperatures of $\sim 125 K$ and $\sim 93 K$ , respectively. Density-functional calculations are performed to reveal the detailed geometry 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.

Received 11 November 2009

DOI:https://doi.org/10.1103/PhysRevB.81.165420

Authors & Affiliations

A. Gumbsch¹, G. Barcaro², M. G. Ramsey¹, S. Surnev¹, A. Fortunelli^2,*, and F. P. Netzer^1,†

¹Institute of Physics, Surface and Interface Physics, Karl-Franzens University Graz, A-8010 Graz, Austria
²Molecular Modeling Laboratory, Istituto per i Processi Chimico-Fisici (IPCF), CNR, v. G. Moruzzi 1, 56124 Pisa, Italy

^*fortunelli@ipcf.cnr.it
^†falko.netzer@uni-graz.at

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Vol. 81, Iss. 16 — 15 April 2010

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Images

Figure 1
(Color online) (a) Constant current topographic STM image of the $Cu (110) (2 \times 1) -O$ stripe phase, displaying bare Cu(110) and $Cu- (2 \times 1) -O$ stripes (with darker contrast), covered with $\sim 10^{- 3}$ monolayer of cobalt adatoms (bright contrast) ( $400 \times 400 Å^{2}$ ; tunneling conditions: 100 mV sample bias; 0.4 nA tunneling current). (b) High-resolution STM image of a single Co adatom on clean Cu(110) ( $48 \times 48 Å^{2}$ ; 1 V, 1 nA). The inset shows a scan along the dark line across the Co adatom. (c) High-resolution STM image of a single Co adatom on the $Cu (110) (2 \times 1) -O$ reconstructed surface ( $48 \times 48 Å^{2}$ ; 1 V, 0.6 nA). The inset shows a DFT-simulated STM image of Co adatoms on Cu-O. (d) Schematic model of Cu(110) (left) and $Cu (110) (2 \times 1) -O$ (right) with Co adatoms, specifying the Co adsorption sites. Co atoms are in dark grey (blue in color) and Cu-O rows in grey (dark grey in color).Reuse & Permissions
Figure 2
(Color online) Differential conductance STS spectra ( $d I / d V$ versus $V$ ) from the $Cu (110) (2 \times 1) -O$ stripe phase. Top curve: STS on top of a Co adatom on clean Cu(110); middle curves: STS from clean Cu(110) and bare $Cu (110) 2 \times 1 -O$ , respectively; bottom curve: STS on top of a Co adatom on $Cu (110) (2 \times 1) -O$ . STS set point: 10 mV, 1 nA.Reuse & Permissions
Figure 3
(Color online) Fano line-shape fits to the STS spectra of Co on clean (a) Cu(110) and of Co on (b) $Cu (110) (2 \times 1) -O$ . The spectra have been processed by subtracting the background of the bare substrates before fitting. The corresponding fit parameters are given in Table .Reuse & Permissions
Figure 4
(Color online) LDOS (arbitrary units) projected onto Co orbitals for Co adatoms adsorbed on (a) Cu(110), (b) $Cu (110) (2 \times 1) -O$ , (c) Cu(100), and (d) Cu(111) substrates in an energy window of $[- 4.0 - 2.0] eV$ . The Fermi energy (full line) is at 0 eV. The values pointed by arrows correspond to the number of electrons in the $Co 3 d$ bands: majority (red) and minority (dark green) spin, respectively.Reuse & Permissions
Figure 5
(Color online) Kondo temperature $T_{K}$ (experimental values in kelvin from the present work and Ref. 12) vs $Δ$ , derived from the present DF calculations for Co adatoms on different Cu substrates. $Δ$ is related to the coordination number of the Co impurity atom.Reuse & Permissions
Figure 6
Shift, $δ$ , of the Kondo resonance in millielectron volt with respect to the Fermi energy (experimental values from the present work and Ref. 12) as a function of the occupation number of the $3 d$ states, $n_{d}$ , of a Co adatom on the four substrates considered in the present work. $n_{d}$ is calculated according to Eq. (6) (see text).Reuse & Permissions

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