PHYSICAL REVIEW B 103, 085423 (2021)

Interplay between point symmetry, oxidation state, and the Kondo effect in 3d transition metal
acetylacetonate molecules on Cu(111)
Hongyan Chen ,1,*,† Timo Frauhammer,1,† Satoru Sasaki,2 Toyo Kazu Yamada ,2,3 and Wulf Wulfhekel1
1

Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang Gaede Strasse 1, 76131 Karlsruhe, Germany
2
Department of Materials Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan3
Molecular Chirality Research Centre, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan(Received 18 November 2020; revised 26 January 2021; accepted 27 January 2021; published 15 February 2021)
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(acac)3 , with M = Cr(III) or Co(III)] adsorbed on a Cu(111) single crystal were investigatedto demonstrate the interplay. After deposition, Cr(acac)3 molecules formed threefold symmetric Cr(acac)3 andtwofold symmetric Cr(acac)2 by releasing a ligand, while Co(acac)3 molecules only formed twofold symmetricCo(acac)2 . Threefold symmetric Cr(acac)3 molecules with a total electron spin S = 23 exhibited no Kondo effect,
while a clear Kondo resonance was observed in twofold Cr(acac)2 molecules. Co(acac)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 ofmagnetic molecule. The present approach provides a feasible design strategy for single molecule magnets on
metallic surfaces.
DOI: 10.1103/PhysRevB.103.085423

I. INTRODUCTION

Single molecule magnets (SMMs) have attracted muchattention due to their potential application as building blocks
for bits in quantum computers [1–7]. Among the different
approaches toward SMMs, 3d transition metal-based systems
have the advantage that the 3d orbitals are not strongly localized and even contribute to the coordinative bonds between
the ligands and the central metal ion, making them readily
accessible to scanning tunneling microscopy (STM) [8–11].
Further, due to the relatively strong coupling of the spins to
the environment, the magnetic properties of 3d magnetic ionscan be tuned with the help of external environment [10,12–15]. That is, both detection and manipulation of 3d transition
metal SMMs are easily possible. For example, by modifying
the point symmetry of the attached ligands, the ligand field canbe varied, including the magnetic anisotropy or the zero-field
splitting (ZFS) of the SMMs [16–18]. In general, the ZFSof the magnetic states can be expressed by the ligand field
Hamiltonian, which consists of the sum of Stevens operators[19]. Depending on the symmetry, some terms are allowed,
and others are forbidden, which gives an excellent opportunity to modify the magnetic behavior of SMMs adsorbed on
conductive surface or even to develop design rules for systems[3,18,20].
In this paper, we present an experimental STM studyof two different transition metal acetylacetonate compounds
M(acac)3 (with M = Cr or Co) on Cu(111) at low temper-*†

hongyan.chen@kit.edu
These authors contributed equally to this work.

2469-9950/2021/103(8)/085423(8)ature. We find that Cr(acac)3 molecules form two different
species with twofold symmetry [Cr(acac)2 ] or threefold symmetry [Cr(acac)3 ] when deposited on Cu(111). Especially, thetwofold symmetric Cr(acac)2 molecule with a total electron
spin S = 23 of the Cr 3+ exhibits a pronounced Kondo resonance at the Fermi level and one inelastic excitation, while forthe threefold symmetric Cr(acac)3 , no spectral features wereobserved. For Co-acetylacetonate compounds, only twofold
symmetric molecules [Co(acac)2 ] were observed, showing
featureless scanning tunneling spectroscopy (STS) as well.
We demonstrate that, by combining the consideration of thetotal electron spin of magnetic ions and the point symmetryof local environment of the magnetic molecules, the observed
results can be well understood by a model based on ZFS,
which will be discussed later. This is essential for starting todesign a SMM system.
II. EXPERIMENTAL DETAILSThe trivalent Cr and Co acetylacetonates are chelate complexes composed of three organic acac ligands such thatthe central metal ion is surrounded by six oxygen atoms inoctahedral symmetry [21]. Figures 1(a) and 1(b) show themolecule structure and the spin configurations of the twocompounds. The 3d orbitals split into eg and t2g orbitals inoctahedral symmetry. In Cr(III)(acac)3 , the three remaining
3d electrons each occupy one of the t2g orbitals and form a
total spin S = 23 [22]. In contrast, Co(III)(acac)3 was reported
to be diamagnetic with S = 0 [22], i.e., the six remaining
electrons doubly occupy the t2g orbitals, and the Co ion isin a low spin state. The molecules were deposited on a cleanCu(111) single crystal substrate in ultrahigh vacuum (UHV) at

085423-1©2021 American Physical Society

HONGYAN CHEN et al.

PHYSICAL REVIEW B 103, 085423 (2021)

Co) molecules could not be attributed to the different adsorption site of M(acac)3 on the Cu(111) surface. A clear height
difference between the triangular-shaped and the dumbbellshaped molecules is estimated to be about 80 pm and can beextracted from Figs. 1(d) and 1(e).
III. RESULTS

FIG. 1. (a) Molecule structure of the trivalent acetylacetonates:
the molecule is a chelate complex, where the central transition metalion (Cr or Co) is surrounded by six oxygen atoms in octahedral
symmetry. (b) Simplified electronic configurations of the molecules.
In the gas phase, Cr(acac)3 is paramagnetic with a spin S = 23 , while
the Co(acac)3 is diamagnetic with S = 0. (c) Large-scale scanningtunneling microscopy (STM) image of Cr-acetylacetonates adsorbed on Cu(111) surface (V = 1.0 V, I = 100 pA, T = 800 mK).
Both triangular-shaped Cr(acac)3 and dumbbell-shaped Cr(acac)2
molecules can be observed, which were marked by yellow andblue dashed lines, respectively. (d) and (e) are zoom-in images ofCr(acac)3 and Cr(acac)2 molecules, respectively. The height scales
show clear height difference (about 80 pm) between the two different
species.

room temperature using a homemade Knudsen cell. The UHV
system is further equipped with a low temperature STM usinga Joule-Thomson cooling stage with 3 He and an operation
temperature of about 700 mK [23]. The samples were transferred to STM immediately after growth.
It has been reported that Cr(acac)3 can be sublimed without
molecule decomposition [24]. In this paper, however, we observed two different species of Cr-acetylacetonate molecules.
More precisely, we obtained threefold symmetric molecules
consisting of three lobes in triangular shape [see Fig. 1(d) and
Fig. 2(b)] and twofold symmetric dumbbell-shaped molecules[see Fig. 1(e) and Fig. 2(a)] at the same time [see Fig. 1(c)]after deposition of Cr(acac)3 at 62 °C and an evaporatingpressure around 10−8 mbar. If every lobe corresponds to
one individual acac ligand, then the triangles and dumbbells
represent Cr(acac)3 and Cr(acac)2 , respectively. We always
observed a fraction of partially decomposed molecules.
For Co-acetylacetonate molecules, we only observed
twofold symmetric molecules [Co(acac)2 ] after deposition,even at a lower deposition temperature of only 44 °C [seeFig. 3(a)]. Indeed, we can confirm a strong tendency for partial
disintegration of Co(acac)3 in agreement with the literature
which shows that Co(acac)3 may decompose at temperatures
around 220 °C, leading to the generation of divalent Co(acac)2[25,26]. Note that the dumbbell-shaped M(acac)2 (M = Cr or

To study the magnetic behavior of the 3d magnetic ions,
we measured dI/dV spectra on different species of Cracetylacetonates at the center of the molecules. Interestingly,
the two different objects show very different behavior. As
shown in Fig. 2(a), the dumbbell-shaped Cr(acac)2 molecules
show a clear peak at the Fermi level in combination with
two faint shoulders marked in blue arrows. Such sharp features on a scale of a few millielectronvolts cannot be causedby molecular orbitals but indicates a Kondo resonance. The
faint shoulders can be interpreted as inelastic excitations, as
will be discussed in more detail below. The triangular-shapedCr(acac)3 molecules, however, show an almost featurelessspectrum, as displayed in Fig. 2(b). Note that, due to thesurface state of Cu(111) and scattering of the two-dimensional
electron gas by the adsorbed molecules [27], the densityof states is not entirely flat, as evidenced by the spectrum recorded directly on Cu(111) [dI/dV spectra in red inFig. 2(a) and 2(b)].
To investigate the nature of the sharp peak in the Cr(acac)2
molecules, we recorded position-dependent spectra.
Figure 2(c) shows the topography of the molecule withhigher lateral resolution. For every pixel, the feedback loop
was opened, and dI/dV spectra were recorded. Figure 2(d)displays the corresponding laterally resolved dI/dV signal
at zero bias. Clearly, the resonance is in the molecule center,
where the Cr ion is expected. This indicates that the resonanceresults from the unpaired spins of the magnetic ion rather thanthe ligands [9]. To better understand the resonance feature andthe symmetric shoulders, we fitted the experimental spectrumwith the following model function like that proposed inRef. [28]:dI() = c + m + AK f (, q, F )dV+ AS [(ex + , T ) + (ex − , T )].

(1)

The model contains a constant c, a linear background m, aFano-Frota function f (, q, F ) [29,30], and two thermallybroadened step functions (ex ± , T ) with
(x) =

1 + (x − 1)exp(x)[exp (x) − 1]2

and

x=


,
kB T

(2)

describing indirect tunneling through the Kondo cloud andinterference with direct tunneling to the substrate, as wellas inelastic excitations. Figure 2(e) shows the fitting results;
the experimental data were plotted in black and the result of
the fit in red. The fitting function reproduces very well all
details of the experimental spectrum. The fit gives an estimated half-width at half-maximum (HWHM) of the Kondo
peak of F = (1.69 ± 0.02) meV and an excitation energyof ex = (0.76 ± 0.03) meV. This can be more easily recognized from the inset of Fig. 2(e), which shows the fitting
resonance in blue and the inelastic steps in orange, sepa-

085423-2

INTERPLAY BETWEEN POINT SYMMETRY, OXIDATION …

PHYSICAL REVIEW B 103, 085423 (2021)

FIG. 2. (a) and (b) dI/dV spectra on dumbbell-shaped Cr(acac)2 and triangular-shaped Cr(acac)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 onCr(acac)2 molecules, while the spectra of triangular-shaped Cr(acac)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 (Vmod = 0.1 mV) andfrequency of 5.51 kHz ( fmod = 5.51 kHz) with set point V = 40 mV, I = 13 nA. The spectra in (b) were measured at Vmod = 1 mV and fmod =2.12 kHz with set point V = 100 mV, I = 300 pA. (c) Scanning tunneling microscopy (STM) topography of a single Cr(acac)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 Cr(acac)2 molecule. The black curve presents the experimental data, and the red curve denotes thefit with a Fano-Frota function and two thermally broadened step function. The fitting results are shown as follows: F = (1.69 ± 0.02) meV,q = 60.6 ± 4.7, ∈ex = (0.76 ± 0.03) meV, and Tfit = (13.0 ± 0.2) 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 dependenceof the maximum intensity of the zero-bias Kondo resonance. The black dashed line indicates the fit according to numerical renormalizationgroup (NRG) theory. (h) Magnetic field dependence measurement, V = 40 mV, I = 1 nA, Vmod = 0.1 mV, fmod = 5.51 kHz. Some of thespectra are vertically offset for clarity.

rately. To obtain the Kondo temperature, we measured thetemperature dependence of the Kondo resonance by tunnelingspectroscopy. Figure 2(f) shows the HWHM as a functionof temperature in the range from the base temperature of
our instrument to 7 K. The resonance width was fitted using
the Fermi liquid model [31]: 2 F = (αkB T)2 +2(2kB Tk )2 ,where α is a free fit parameter, and kB is the Boltzmann
constant. The fit results in a Kondo temperature of Tk =(19.80 ± 1.20) K and α = (24.64 ± 0.47). The Kondo temperature is much lower than that of a single Co adatom on an
Cu(111) surface [32]. We attribute the relatively low Kondo
temperature to the ligands which weaken the coupling between the central magnetic ion and the metal substrate. The
value of α deviates strongly from that expected (α = 2π )based on strong coupling theory. This behavior has beenreported by others in similar magnetic molecules on sur-

085423-3

HONGYAN CHEN et al.

PHYSICAL REVIEW B 103, 085423 (2021)

faces [33]. Figure 2(g) shows the temperature dependenceof the peak height of the Kondo resonance together witha fit according to numerical renormalization group (NRG)atheory: G(T) = G(0)[1+( TTk ) (21/b −1)]−b . The fitting parameters shown in Ref. [34] for a spin of S = 23 , i.e., a =0.483 and b = 0.67, are adopted. Thus, the only fitting parameter here is the scale of the conductance. As it shows,
G(T) fits the theoretical prediction well. Additionally, thebroadening of the inelastic step functions of the fit shownin Fig. 2(e) can be expressed as 2 ∼= 3.4kB Tfit , i.e.,  =(1.90 ± 0.03) meV, which is very similar to that of the Kondo
peak F = (1.69 ± 0.02) meV. This indicates that it is nota thermal broadening but rather a lifetime broadening. This isalso expected, as both are caused by the coupling of the local
spin to the conduction electrons [35]. Figure 2(h) shows theeffect of a magnetic field on the shape of the Kondo resonance.
As the HWHM with ∼1.69 meV is much larger than the
expected Zeeman splitting of both the Kondo resonance andthe inelastic excitations of δE = gμB B at 2 T (2 Tesla is the
maximal field of our magnet, g is the Landé factor and μB
is the Bohr magneton), which is of the order of 0.2 meV, wecannot observe the splitting of the Kondo peak or the inelastic
excitations for this high lifetime broadening. However, thesharpness of the Kondo peak is indeed reduced upon applying
a magnetic field.
Returning to the triangular-shaped Cr(acac)3 molecules, nosuch features as a Kondo resonance or an inelastic excitation was observed even when investigating several molecules
exemplarily shown in Fig. 2(b). To gain signal-to-noise in
the dI/dV signal, a 1 mV lock-in modulation (Vmod = 1 mV)
was used for these spectra trading signal-to-noise for energy
resolution. Note that, even at low lock-in modulations (Vmod 
0.25 mV), no Kondo feature or molecular excitation could
be observed, and even the second derivative of the tunnelingcurrent was featureless (not shown).
To better understand the relation between the point symmetry and the presence of a Kondo resonance, a second
3d metal acetylacetonate [Co(acac)3 ] was studied. As mentioned above, only one type of object could be observed
when deposited on Cu(111). These objects consist of two
lobes and exhibit twofold symmetry like the dumbbellshaped molecules observed on the chromium samples.
Figure 3(a) shows the topography of these molecules. Threefold symmetric triangles, like those observed on Cr(acac)3
samples, could not be found for Co-acetylacetonates. We infer that, again, these dumbbell-shaped objects represent thedivalent Co(acac)2 . This is compatible with previous reports,
where the Co(acac)3 decomposed upon sublimation [25,26].
To investigate the magnetic behavior of the Co ions, werecorded dI/dV spectra on different Co(acac)2 at the centerof the molecules, as displayed in Fig. 3(b). No spectroscopic
features, such as a Kondo resonance nor an inelastic excitation
could be observed.
IV. DISCUSSION

After presenting the experimental findings, we turn toanalyzing the results based on the point symmetry of the individual molecules, the oxidation states of the central ions, andthe magnetic properties. To model the effect of geometry, i.e.,

FIG. 3. (a) Topography of the dumbbell-shaped Co(acac)2
molecules (V = 0.1 V, I = 1nA). Left: Large-scale image. Right:
Zoom-in high-resolution image. (b) dI/dV spectra on the dumbbellshaped Co(acac)2 and Cu(111) surface for comparison. The spectraof 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 Vmod = 0.5 mV and fmod = 5.51 kHz with set point
V = 50 mV, I = 1 nA.

number of ligands, on the magnetic properties, we represent
the spin state of the magnetic ion in the molecules by theeffective spin Hamiltonian describing the ZFS. In general, thecrystal-field Hamiltonian can be written as
 n

∞
n



m m
m m
B̃n Õn ,H=
Bn On +
(3)
n=0

m=0

m=1

where Onm and Õnm are Stevens operators, and Bnm and B̃nm arecorresponding coefficients related to the quantum mechanical
ZFS [3,19]. Here, B00 is a constant and can be omitted. The
operators Onm and Õnm are n th degree polynomials composedof Jz , J+ , and J− operators (z component, raising and lowering operator), and m indicates the order of ladder operators.
Owing to time reversal symmetry, the states with opposite
magnetic momentum should be degenerate when no external
magnetic field is applied. Thus, only terms with even n are
allowed. All terms of n > 2J vanish, where J is the quantum
number of total angular momentum, because the operators
only act on 2J +1 states. Similarly, all terms of n > 2 vanish,085423-4

INTERPLAY BETWEEN POINT SYMMETRY, OXIDATION …

PHYSICAL REVIEW B 103, 085423 (2021)

where is the orbital quantum number of the open shell of themagnetic ion, i.e., for = 2 of the 3d states, n is limited to
4. More importantly, all terms vanish that are incompatible
with the point symmetry of system. Thus, only terms withm = 0, 2, and 4 for twofold symmetry and m = 0 and 3 forthreefold symmetry are allowed in 3d metal system. For thispaper, the orbital angular momentum L is quenched for the 3d
metal such that their magnetic properties arise merely fromthe total electron spin S, i.e., J = S. Finally, in case a verticalmirror plane is present, all conjugate Stevens operators Õnm
vanish. This is the case for the twofold symmetric molecules
where the acetylacetonate groups are in one plane.
As a last ingredient, the spin S must be determined. While
S is known for the bulk phase of the starting M(III)(acac)3(M = Cr or Co) complexes, taking away one of the acac ligands potentially changes the oxidation state of the ion M(III)to M(II). Additionally, upon adsorption of the molecules on
the Cu(111) surface, charge transfer may arise [9,13]. Unfortunately, STM does not have the capability to directly
determine the oxidation state. We, however, will use the results from tunneling spectroscopy to indirectly evidence theoxidation state and thus the spin S.
First, we discuss the case of Cr(acac)2 . We expect that,upon removal of one acac ligand, the oxidation state of Cr(III)changes to Cr(II), but adsorption onto the surface leads tochemisorption and a transfer of one electron from Cr to the Cu
substrate [Cr(II) to Cr(III)]. This would lead to the electron
configuration of [Ar]3d 3 and a total spin S = 23 . Then the
Hamiltonian for this twofold symmetric Cr(III)(acac)2 with
one mirror plane can be written as follows:states with Sz = ±1 and ±2 leading to nonmagnetic symmetric and antisymmetric combinations of the Sz components[18]. As a result, only singlet states are left. To be more
precise, for S = 0, only one nonmagnetic state would arise;for S = 1, three nonmagnetic states result; and for S = 2, fiveexist. This situation is sketched in Fig. 4(b). This analysis fully
agrees with the absence of a Kondo peak in the dI/dV spectra[see Fig. 3(b)]. As no inelastic excitations were observed [seeFig. 3(b)], our measurements indicate a singlet state, whichmeans the system is in the S = 0 diamagnetic state like thebulk counterpart [22]. Nevertheless, it is also possible thatthe excitations which would arise for S = 1 or 2, which areof too low energy for our experimental energy resolution. To
conclude this section, the molecules with nonmagnetic singlet
ground state are not suited for the application as SMM orquantum bits.
Lastly, for threefold symmetric Cr(acac)3 with S = 23 , thecrystalline molecules have already been studied using electron
paramagnetic resonance [21]. The crystal field parameters ofthe model HamiltonianH = gμB B · S + D Sz2 − (1/3)S(S + 1) + E Sx2 − Sy2 ,(6)−1
were determined to be |D| = (0.592 ± 0.002) cm
and|E | = (0.052 ± 0.002) cm−1 . Assuming a positive value ofD [36], the ground state doublet would consist mainly of thestates with Sz = ± 21 . Hence, in STS, one would expect a
Kondo resonance with one inelastic excitation at 2D ≈ 150
μeV. While in the bulk phase, a D3 point-group symmetry
is present, and the adsorbed molecules on Cu(111) show athreefold symmetry due to planarization of the molecule uponadsorption. Thus, O22 , i.e., the E term in the model above,
would vanish in our case, but O40 and O43 would be allowed
for S = 2 or larger. In the case of no charge transfer, Cr would
be in the (III) oxidation state and S = 23 , while for the case ofa transfer of one electron, Cr would be in the (IV) oxidation
state with S = 1. In both cases, the Hamiltonian simplifies to

H = B20 O20 + B22 O22 .

(4)

Since theoperator mixes the states with Sz = 2,
the four eigenstates can be expressed as linear combinationsof Sz = − 23 , 21 , and Sz = − 21 , 23 , respectively. As plotted inFig. 4(a), the system consists of two subsets of states, marked
in magenta and blue. Regardless of the precise values of
the crystal field parameters B20 and B22 (=0), we obtain aground state Kramers doublet with two states of opposite
color. As discussed above, the composition of these two states
contains neighboring values of Sz . Thus, single electron scattering by substrate electrons can flip the spin of the molecule
between the two degenerate ground states without energy
cost, and a Kondo resonance should arise [18]. On top ofthis, inelastic excitations by the tunneling current may excite
the molecule from the ground state doublet to the excitedstate doublet, which would manifest in the dI/dV spectra as
symmetric steps. The experimental results of twofold symmetric Cr(acac)2 molecules show both expected features [seeFig. 2(e)]. A sketch of the two possibilities of this twofold
symmetry case is shown in Fig. 4(a).
Taking a similar line of thought for Co(acac)2 molecules
yields an integer total spin, i.e., Co3+ ([Ar]3d 6 ) This would
allow, depending on the size of the crystal field splitting,
Co3+ with S = 0, 1, and 2. Thus, we expect the following
Hamiltonian:O22

H = B20 O20 + B22 O22 + B40 O40 + B42 O42 + B44 O44 .

(5)

For all three values of S, no Kondo effect is predicted as O22(and eventually also O44 ) would mix the otherwise degenerate

H = B20 O20 .

(7)

Our experimental results showed no Kondo feature or
inelastic excitation [see Fig. 2(b)] in this symmetry. Since
Kondo resonances are typically easily seen in STS experiments, we may conclude an absence of a Kondo resonance.
Inelastic excitations can cause small signals and might beoverlooked in experiments. For all cases theoretically expected, one inelastic excitation is predicted, as will be
discussed below. Thus, we believe that the amplitude or energy of the inelastic scattering is too small to be observed on
the background on the bare Cu(111) spectrum [see Fig. 2(b)].
Therefore, we focus on the Kondo resonance and discuss thedifferent oxidation states.
(1) Charge transfer does not occur. Two different cases canarise for the resulting S = 23 system. As shown in Fig. 4(c),B20 > 0 will result in a Kondo effect, while B20 < 0 will givean unscreened ground state doublet | ± 23 . Thus, in this case,due to the absence of a Kondo peak in the experiments, weconclude that the crystal field parameter B20 is expected to be
negative.
(2) Charge transfer occurs. Depending on the ligand fieldparameters, there are two possibilities for the resulting spin

085423-5

HONGYAN CHEN et al.

PHYSICAL REVIEW B 103, 085423 (2021)

FIG. 4. Overview of the results for Cr- and Co-acetylacetonates adsorbed on Cu(111) surface. For every symmetry and spin state, theeffective spin Hamiltonian, the corresponding spin spectrum, and the expected spectroscopic features are denoted. (a) and (b) Different possible
cases of twofold symmetric Cr(acac)2 and Co(acac)2 molecules, respectively. (c) With and without charge transfer process, the possible casesof threefold symmetric Cr(acac)3 molecules.

085423-6

INTERPLAY BETWEEN POINT SYMMETRY, OXIDATION …

PHYSICAL REVIEW B 103, 085423 (2021)

S = 1 system: B20 > 0 yields a singlet ground state |0, while
B20 < 0 results in an unscreened ground state doublet | ± 1.
In both cases, no Kondo resonance can be expected, which isconsistent with our experimental results. One inelastic excitation is expected but was not observed in the experiment. Thiscan be due to a low excitation energy below the energy resolution of our instrument or due to a small excitation probability
leading to too low signal intensity.

symmetry and electron spin of the molecule. Our results indicate that the twofold symmetric dumbbell configurations areeither a Kondo system [Cr(acac)2 ] or display a nonmagnetic
singlet [Co(acac)2 ] and are thus not suited for application
as SMM. Whether threefold symmetric systems are suitable
candidates for SMMs crucially depends on the sign of thecrystal field parameter B20 . These experimental measurements
indicate that symmetry plays an essential role in determining
the magnetic properties of molecules when adsorbed onto
conductive substrates.

V. SUMMARYACKNOWLEDGMENTS

Figure 4 summarizes the theoretical expectation for thedifferent systems (symmetry of the crystal field and chargetransfer). We conclude that—except for in part missing inelastic excitations—the observations can be broken down
and understood in the rather simple framework analyzing theT.F. acknowledges funding by the Landesstiftung
Baden-Württemberg. W.W. acknowledges funding by theDeutsche Forschungsgemeinschaft (DFG) under the Grant
No. WU349/13-1.

[1] L. Bogani and W. Wernsdorfer, Molecular spintronics using
single-molecule magnets, Nat. Mater. 7, 179 (2008).
[2] S. Thiele, F. Balestro, R. Ballou, S. Klyatskaya, M. Ruben, and
W. Wernsdorfer, Electrically driven nuclear spin resonance insingle-molecule magnets, Science 344, 1135 (2014).
[3] T. Miyamachi, T. Schuh, T. Markl, C. Bresch, T. Balashov, A.
Stohr, C. Karlewski, S. Andre, M. Marthaler, M. Hoffmann, M.
Geilhufe, S. Ostanin, W. Hergert, I. Mertig, G. Schon, A. Ernst,
and W. Wulfhekel, Stabilizing the magnetic moment of single
holmium atoms by symmetry, Nature 503, 242 (2013).
[4] L. Gu and R. Q. Wu, Origins of Slow Magnetic Relaxationin Single-Molecule Magnets, Phys. Rev. Lett. 125, 117203(2020).
[5] J. M. Frost, K. L. M. Harriman, and M. Murugesu, The rise of
3d single-ion magnets in molecular magnetism: towards materials from molecules? Chem. Sci. 7, 2470 (2016).
[6] F. S. Guo, B. M. Day, Y. C. Chen, M. L. Tong, A.
Mansikkamaki, and R. A. Layfield, Magnetic hysteresis up to
80 kelvin in a dysprosium metallocene single-molecule magnet,
Science 362, 1400 (2018).
[7] C. Karlewski, M. Marthaler, T. Märkl, T. Balashov, W.
Wulfhekel, and G. Schön, Magnetic adatoms as memory bits:
a quantum master equation analysis, Phys. Rev. B 91, 245430(2015).
[8] G. Czap, P. J. Wagner, J. Li, F. Xue, J. Yao, R. Wu, andW. Ho, Detection of Spin-Vibration States in Single Magnetic
Molecules, Phys. Rev. Lett. 123, 106803 (2019).
[9] U. G. E. Perera, H. J. Kulik, V. Iancu, L. G. G. V. Dias da Silva,
S. E. Ulloa, N. Marzari, and S. W. Hla, Spatially Extended
Kondo State in Magnetic Molecules Induced by Interfacial
Charge Transfer, Phys. Rev. Lett. 105, 106601 (2010).
[10] P. Wahl, L. Diekhoner, G. Wittich, L. Vitali, M. A. Schneider,and K. Kern, Kondo Effect of Molecular Complexes at Surfaces: Ligand Control of the Local Spin Coupling, Phys. Rev.
Lett. 95, 166601 (2005).
[11] L. Zhang, A. Bagrets, D. Xenioti, R. Korytár, M. Schackert,
T. Miyamachi, F. Schramm, O. Fuhr, R. Chandrasekar,
M. Alouani, M. Ruben, W. Wulfhekel, and F. Evers, Kondo
effect in binuclear metal-organic complexes with weakly interacting spins, Phys. Rev. B 91, 195424 (2015).

[12] K. Yang, H. Chen, T. Pope, Y. B. Hu, L. W. Liu, D. F. Wang, L.
Tao, W. D. Xiao, X. M. Fei, Y. Y. Zhang, H. G. Luo, S. X. Du,
T. Xiang, W. A. Hofer, and H. J. Gao, Tunable giant magnetoresistance in a single-molecule junction, Nat. Commun. 10, 3599
(2019).
[13] J. Hu and R. Wu, Control of the Magnetism and Magnetic
Anisotropy of a Single-Molecule Magnet with an Electric Field,
Phys. Rev. Lett. 110, 097202 (2013).
[14] N. Tsukahara, K. I. Noto, M. Ohara, S. Shiraki, N. Takagi,
Y. Takata, J. Miyawaki, M. Taguchi, A. Chainani, S. Shin,and M. Kawai, Adsorption-Induced Switching of Magnetic
Anisotropy in a Single Iron(II) Phthalocyanine Molecule on
an Oxidized Cu(110) Surface, Phys. Rev. Lett. 102, 167203(2009).
[15] E. del Barco, A. D. Kent, E. M. Rumberger, D. N. Hendrickson,
and G. Christou, Symmetry of Magnetic Quantum Tunneling inSingle Molecule Magnet Mn-12-Acetate, Phys. Rev. Lett. 91,
047203 (2003).
[16] R. Boca, Zero-field splitting in metal complexes, Coordin.
Chem. Rev. 248, 757 (2004).
[17] J. Chen, H. Isshiki, C. Baretzky, T. Balashov, and W. Wulfhekel,
Abrupt switching of crystal fields during formation of molecular contacts, ACS Nano 12, 3280 (2018).
[18] T. Balashov, C. Karlewski, T. Märkl, G. Schön, and W.
Wulfhekel, Electron-assisted magnetization tunneling in single
spin systems, Phys. Rev. B 97, 024412 (2018).
[19] K. W. H. Stevens, Matrix elements and operator equivalentsconnected with the magnetic properties of rare earth ions,P. Phys. Soc. Lond. A 65, 209 (1952).
[20] M. Marciani, C. Hübner, and B. Baxevanis, General scheme forstable single and multiatom nanomagnets according to symmetry selection rules, Phys. Rev. B 95, 125433 (2017).
[21] L. S. Singer, Paramagnetic resonance absorption in some Cr+3
complexes, J. Chem. Phys. 23, 379 (1955).
[22] P.-L. Wang, J.-H. Lee, S.-M. Huang, and L.-P. Hwang,
Proton spin relaxation in solutions of the complex
tris(acetylacetonato)chromium(III), J. Magn. Reson. 73,
277 (1987).
[23] L. Zhang, T. Miyamachi, T. Tomanić, R. Dehm, andW. Wulfhekel, A compact sub-Kelvin ultrahigh vacuum

085423-7

HONGYAN CHEN et al.

PHYSICAL REVIEW B 103, 085423 (2021)

scanning tunneling microscope with high energy resolution and high stability, Rev. Sci. Instrum. 82, 103702(2011).
[24] M. A. Siddiqi, R. A. Siddiqui, and B. Atakan, Thermal
stability, sublimation pressures and diffusion coefficients ofsome metal acetylacetonates, Surf. Coat. Technol. 201, 9055
(2007).
[25] G. Beech and R. M. Lintonbon, Thermal and kinetic studies ofsome complexes of 2,4-pentanedione, Thermochim. Acta 3, 97(1971).
[26] A. Rautiainen, M. Lindblad, L. B. Backman, and R. L.
Puurunen, Preparation of silica-supported cobalt catalysts
through chemisorption of cobalt(II) and cobalt(III) acetylacetonate, Phys. Chem. Chem. Phys. 4, 2466 (2002).
[27] M. F. Crommie, C. P. Lutz, and D. M. Eigler, Imaging standing waves in a 2-dimensional electron-gas, Nature 363, 524(1993).
[28] K. von Bergmann, M. Ternes, S. Loth, C. P. Lutz, and A. J.
Heinrich, Spin Polarization of the Split Kondo State, Phys. Rev.
Lett. 114, 076601 (2015).
[29] U. Fano, Effects of configuration interaction on intensities andphase shifts, Phys. Rev. 124, 1866 (1961).
[30] H. O. Frota, Shape of the Kondo resonance, Phys. Rev. B 45,
1096 (1992).

[31] K. Nagaoka, T. Jamneala, M. Grobis, and M. F. Crommie,
Temperature Dependence of a Single Kondo Impurity, Phys.
Rev. Lett. 88, 077205 (2002).
[32] N. Knorr, M. A. Schneider, L. Diekhöner, P. Wahl, and K. Kern,
Kondo Effect of Single Co Adatoms on Cu Surfaces, Phys. Rev.
Lett. 88, 096804 (2002).
[33] S. Mishra, D. Beyer, K. Eimre, S. Kezilebieke, R. Berger, O.
Gröning, C. A. Pignedoli, K. Müllen, P. Liljeroth, P. Ruffieux,
X. Feng, and R. Fasel, Topological frustration induces unconventional magnetism in a nanographene, Nat. Nanotechnol. 15,
22 (2020).
[34] J. J. Parks, A. R. Champagne, T. A. Costi, W. W. Shum, A. N.
Pasupathy, E. Neuscamman, S. Flores-Torres, P. S. Cornaglia,
A. A. Aligia, C. A. Balseiro, G. K. L. Chan, H. D. Abruña,
and D. C. Ralph, Mechanical control of spin states in spin-1
molecules and the underscreened Kondo effect, Science 328,
1370 (2010).
[35] I. Riess and A. Ron, Kondo-resistivity suppression due toinelastic scattering of conduction electrons by magnetic impurities in alloys, Phys. Rev. B 4, 4099 (1971).
[36] J. Miller, N. Schaefle, and R. Sharp, Calculating NMR paramagnetic relaxation enhancements without adjustable parameters:
the spin-3/2 complex Cr(III)(acac)3 , Magn. Reson. Chem. 41,806 (2003).

085423-8