Zero-bias conductance anomaly of a FeO-bound Au atom triggered by CO adsorption

Single gold-carbonyl species adsorbed on a FeO thin film on Pt(111) exhibit a pronounced zero-bias conductance anomaly in scanning tunneling spectroscopy, while bare Au atoms reveal a smooth conductance behavior. The phenomenon is attributed to the Kondo effect, which seems to be triggered by AuCO attachment to the oxide surface but originates in fact from the interplay between the Fe magnetic moments and the Pt conduction electrons. This interpretation is supported by the observation of a similar conductance anomaly even for the pristine oxide film under certain circumstances. Apparently, the FeO/Pt(111) system is intrinsically susceptible to a Kondo-type behavior, which needs however to be initiated by a small, e.g., adsorbate-induced modification of its magnetic structure. The role of the AuCO in triggering the effect is explored by analyzing the geometric and electronic properties of the ad-complex on the FeO thin film via tunneling spectroscopy and density-functional theory.

Figure 1

(Color online) (a) Model of the Moiré cell formed between the FeO bilayer and Pt(111). (b) Sketch of the local binding configuration of Fe atoms in top and hcp domains and of an hcp-bound AuCO complex.

Figure 2

(Color online) (a) STM topographic image of Au and AuCO species on a thin FeO film on Pt(111) ($13\times 9{\text{nm}}^2$, 500 mV, 0.1 nA). (b) Differential conductance spectra of Au and AuCO bound to FeO hcp regions. The set point for spectroscopy was set with 750 mV and 0.1 nA.

Figure 3

(Color online) (a) $dI/dV$ spectra taken around the Fermi level of hcp-bound Au and AuCO species on FeO/Pt(111). The AuCO spectrum was fitted with a Fano line centered at 1 mV and broadened by 112 meV (dotted line). Spectra taken in the hcp and top regions of the pristine oxide are shown for comparison. (b) $d^{2}I/d{V}^2$ spectra of the same Au and AuCO species as in (a) (set point in both cases: 100 mV and 0.1 nA).

Figure 4

(Color online) (a) Topography ($16.5\times 16.5{\text{nm}}^2$, 0.1 nA) and $d^{2}I/d{V}^2$ maps of CO/Au/FeO/Pt(111) taken at (b) $-30\text{mV}$ and (c) $+30\text{mV}$ sample bias. The AuCO species (marked by circles) are clearly identified by a contrast reversal induced by the opposite slope of the $dI/dV$ signal above and below the Fermi level.

Figure 5

(Color online) (a) Calculated state density of Fe and O atoms located in hcp (solid line) and top regions (dashed line) of the FeO Moiré cell. No minimum in the single-electron LDOS is observed at the Fermi level. (b) Spin-polarized LDOS of Au and AuCO species on FeO/Pt(111). The resonance of the bare Au at 0.3 eV marks the $\text{Au}6s$ orbital hybridized with the FeO states, while the AuCO peak at 1.0 eV has mixed $\text{Au}5d\text{-CO}2{\pi }^{\ast }$ character. Similar intensities for spin-up and spin-down states (especially below $E_F$) indicate the nonmagnetic character of Au and AuCO.