Quantum modulation of the Kondo resonance of Co adatoms on Cu/Co/Cu(100): Low-temperature scanning tunneling spectroscopy study

Low-temperature scanning tunneling spectroscopy reveals that the Kondo temperature $T_K$ of Co atoms adsorbed on Cu/Co/Cu(100) multilayers varies between 60 and 134 K as the Cu film thickness decreases from 20 to 5 atomic layers. The observed change of $T_K$ is attributed to a variation of the density of states at the Fermi level ${\rho }_F$ induced by quantum well states confined to the Cu film. A model calculation based on the quantum oscillations of ${\rho }_F$ at the belly and the neck of the Cu Fermi surface reproduces most of the features in the measured variation of $T_K$.

Figure 1

(Color online) (a) Sketch of Cu/Co/Cu(100) multilayer with single adsorbed Co atoms. STM images of (b) a 10-ML-thick Co overlayer grown on a Cu(100) substrate ($I=100\text{pA}$, $V=200\text{mV}$), (c) a 15-ML-thick Cu overlayer grown on a Co/Cu(100) multilayer ($I=100\text{pA}$, $V=200\text{mV}$), and (d) of Co adatoms deposited on a Cu/Co/Cu(100) multilayer ($I=100\text{pA}$, $V=300\text{mV}$).

Figure 2

(Color online) (a) Series of $dI/dV$ spectra measured on different surfaces of Cu/Co/Cu(100) multilayers. Prior to spectroscopy, the tunneling gap was set at 1 nA and 2.5 V. Spectra are offset for clarity. The labels beside the graph show the thicknesses of the Cu overlayers determined by referring to theoretical curves in (c). The relative thicknesses within each group (6–8 ML, 9–13 ML, 14–20 ML) were determined by a simultaneously observed surface topography. (b) Schematic dispersion curve of the Cu $sp$ band along the [100] direction $\left(\overline{\Gamma X}\right)$. (c) Peak energies of $dI/dV$ spectra in Fig. 2a as a function of Cu overlayer thickness $d_{\text{Cu}}$ (dots). Calculated energies of QW states (solid lines) and IPES data (squares) (Ref. 20) are included.

Figure 3

(Color online) (a) $dI/dV$ spectra measured on Co adatoms on Cu/Co/Cu(100) multilayers. Top: 14 ML Cu. Middle: 11 ML Cu. Bottom: pure Cu(100). Spectra are offset for clarity. Dashed lines indicate fits to experimental data according to Eq. (3). (b) Averaged Kondo temperature $T_K$ of Co adatoms on Cu/Co/Cu(100) multilayers as a function of Cu overlayer thickness $d_{\text{Cu}}$ (circles connected by a dashed line). Half the error bar represents the standard error of $T_K$ calculated for each $d_{\text{Cu}}$. Calculated $T_K$ is plotted as a solid line. The fit parameters used are $A_b=0.046{\text{eV}}^{-1}\text{ML}$, ${\Phi }_b=0.26\pi$, $A_n=0.011{\text{eV}}^{-1}\text{ML}$, ${\Phi }_n=-0.03\pi$. Averaged $T_K=94\text{K}$ for Co adatoms on pure Cu(100) surfaces appears as a dotted line.