Interplay between point symmetry, oxidation state, and the Kondo effect in $3d$ transition metal acetylacetonate molecules on Cu(111)

We report that the occurrence of a Kondo effect in magnetic molecules crucially depends on the point symmetry and oxidation state of the adsorbed species. Two different transition metal acetylacetonate (acac) compounds [$M{\left(\mathrm{acac}\right)}_3$, with $M=\mathrm{Cr}\left(\mathrm{III}\right)$ or Co(III)] adsorbed on a Cu(111) single crystal were investigated to demonstrate the interplay. After deposition, $\mathrm{Cr}{\left(\mathrm{acac}\right)}_3$ molecules formed threefold symmetric $\mathrm{Cr}{\left(\mathrm{acac}\right)}_3$ and twofold symmetric $\mathrm{Cr}{\left(\mathrm{acac}\right)}_2$ by releasing a ligand, while $\mathrm{Co}{\left(\mathrm{acac}\right)}_3$ molecules only formed twofold symmetric $\mathrm{Co}{\left(\mathrm{acac}\right)}_2$. Threefold symmetric $\mathrm{Cr}{\left(\mathrm{acac}\right)}_3$ molecules with a total electron spin $S=\frac{3}{2}$ exhibited no Kondo effect, while a clear Kondo resonance was observed in twofold $\mathrm{Cr}{\left(\mathrm{acac}\right)}_2$ molecules. $\mathrm{Co}{\left(\mathrm{acac}\right)}_2$ molecules surprisingly showed no Kondo resonance, even in the case of twofold symmetry, which is explained by a low-spin state of $S$ = 0. To analyze the results, a simple model is proposed based on the total electron spin and the symmetry of magnetic molecule. The present approach provides a feasible design strategy for single molecule magnets on metallic surfaces.

Figure 1

(a) Molecule structure of the trivalent acetylacetonates: the molecule is a chelate complex, where the central transition metal ion (Cr or Co) is surrounded by six oxygen atoms in octahedral symmetry. (b) Simplified electronic configurations of the molecules. In the gas phase, $\mathrm{Cr}{\left(\mathrm{acac}\right)}_3$ is paramagnetic with a spin $S$ = $\frac{3}{2}$, while the $\mathrm{Co}{\left(\mathrm{acac}\right)}_3$ is diamagnetic with $S$ = 0. (c) Large-scale scanning tunneling microscopy (STM) image of Cr-acetylacetonates adsorbed on Cu(111) surface ($V=1.0\mathrm{V}, I=100\mathrm{pA}, T=800\mathrm{mK}$). Both triangular-shaped $\mathrm{Cr}{\left(\mathrm{acac}\right)}_3$ and dumbbell-shaped $\mathrm{Cr}{\left(\mathrm{acac}\right)}_2$ molecules can be observed, which were marked by yellow and blue dashed lines, respectively. (d) and (e) are zoom-in images of $\mathrm{Cr}{\left(\mathrm{acac}\right)}_3$ and $\mathrm{Cr}{\left(\mathrm{acac}\right)}_2$ molecules, respectively. The height scales show clear height difference (about 80 pm) between the two different species.

Figure 2

(a) and (b) $dI/dV$ spectra on dumbbell-shaped $\mathrm{Cr}{\left(\mathrm{acac}\right)}_2$ and triangular-shaped $\mathrm{Cr}{\left(\mathrm{acac}\right)}_3$ molecules, respectively. The spectra on Cu(111) surface are also presented for comparison. No background is subtracted. A clear resonance including two shoulders occur on $\mathrm{Cr}{\left(\mathrm{acac}\right)}_2$ molecules, while the spectra of triangular-shaped $\mathrm{Cr}{\left(\mathrm{acac}\right)}_3$ molecules are featureless. The blue arrows in (a) mark the position of the two shoulders. The inserted topography images show the tip locations during taking these spectra and are marked in different colored dots. The colors correspond to the curves in each figure. The spectra in (a) were measured at a lock-in modulation of 0.1 mV $\left(V_{\mathrm{mod}}=0.1\mathrm{mV}\right)$ and frequency of 5.51 kHz $\left(f_{\mathrm{mod}}=5.51\mathrm{kHz}\right)$ with set point $V=40\mathrm{mV}, I=13\mathrm{nA}$. The spectra in (b) were measured at $V_{\mathrm{mod}}=1\mathrm{mV}$ and $f_{\mathrm{mod}}=2.12\mathrm{kHz}$ with set point $V=100\mathrm{mV}, I=300\mathrm{pA}$. (c) Scanning tunneling microscopy (STM) topography of a single $\mathrm{Cr}{\left(\mathrm{acac}\right)}_2$ molecule and (d) corresponding differential conductance map obtained at zero bias, respectively. For every pixel, the feedback loop was opened and $dI/dV$ spectra were recorded. The mapping shows that the position of the Kondo resonance is at the center of the molecule (images size: 2.9 × 2.4 nm). (e) The fit of the Kondo resonance on the $\mathrm{Cr}{\left(\mathrm{acac}\right)}_2$ molecule. The black curve presents the experimental data, and the red curve denotes the fit with a Fano-Frota function and two thermally broadened step function. The fitting results are shown as follows: ${\mathrm{\Gamma }}_F=\left(1.69\pm 0.02\right)\mathrm{meV}, q=60.6\pm 4.7, {\in }_{\mathrm{ex}}=\left(0.76\pm 0.03\right)\mathrm{meV}$, and ${\mathrm{T}}_{\mathrm{fit}}=(13.0\pm 0.2)\mathrm{K}$. The inset shows the fitting resonance with linear offset (blue curve) and the inelastic steps (orange curve), separately. (f) Temperature dependence of the Kondo resonance. The half widths at half maximum (HWHMs) were extracted from a Frota fit. The black dashed line indicates the fit using the Fermi-liquid model. (g) Temperature dependence of the maximum intensity of the zero-bias Kondo resonance. The black dashed line indicates the fit according to numerical renormalization group (NRG) theory. (h) Magnetic field dependence measurement, $V=40\mathrm{mV}, I=1\mathrm{nA}, V_{\mathrm{mod}}=0.1\mathrm{mV}, f_{\mathrm{mod}}=5.51\mathrm{kHz}$. Some of the spectra are vertically offset for clarity.

Figure 3

(a) Topography of the dumbbell-shaped $\mathrm{Co}{\left(\mathrm{acac}\right)}_2$ molecules ($V=0.1\mathrm{V}, I=1\mathrm{nA}$). Left: Large-scale image. Right: Zoom-in high-resolution image. (b) $dI/dV$ spectra on the dumbbell-shaped $\mathrm{Co}{\left(\mathrm{acac}\right)}_2$ and Cu(111) surface for comparison. The spectra of molecules do not show a Kondo resonance nor an inelastic excitation. The inset shows the tip locations during the measurements. The marked colored dots correspond to the colored curves. The spectra were measured at $V_{\mathrm{mod}}=0.5\mathrm{mV}$ and $f_{\mathrm{mod}}=5.51\mathrm{kHz}$ with set point $V=50\mathrm{mV}, I=1\mathrm{nA}$.

Figure 4

Overview of the results for Cr- and Co-acetylacetonates adsorbed on Cu(111) surface. For every symmetry and spin state, the effective spin Hamiltonian, the corresponding spin spectrum, and the expected spectroscopic features are denoted. (a) and (b) Different possible cases of twofold symmetric $\mathrm{Cr}{\left(\mathrm{acac}\right)}_2$ and $\mathrm{Co}{\left(\mathrm{acac}\right)}_2$ molecules, respectively. (c) With and without charge transfer process, the possible cases of threefold symmetric $\mathrm{Cr}{\left(\mathrm{acac}\right)}_3$ molecules.