Controlling the Kondo effect in CoCu(n) clusters atom by atom

Kondo Effect of Co-Porphyrin: Remarkable Sensitivity to Adsorption Sites and Orientations

We investigated the Kondo effect of cobalt(II)-5-15-bis(4'-bromophenyl)-10,20-bis(4'-iodophenyl)porphyrin (CoTPPBr2I2) molecules on Au(111) with low-temperature scanning tunneling microscopy under ultrahigh vacuum conditions. The molecules exhibit four adsorption configurations at the top and bridge sites of the surface with different molecular orientations. The Kondo resonance shows extraordinary sensitivity to the adsorption configuration. By switching the molecule between different configurations, the Kondo temperature is varied over a wide range from ≈8 up to ≈250 K. Density functional theory calculations reveal that changes of the adsorption configuration lead to distinct variations of the hybridization between the molecule and the surface. Furthermore, we show that surface reconstruction plays a significant role for the molecular Kondo effect.

Manipulating the Spin Orientation of Co Atoms Using Monatomic Cu Chains

Harnessing the spin of single atoms is at the heart of quantum information nanotechnology based on magnetic concepts. By attaching single Co atoms to monatomic Cu chains, we demonstrate the ability to control the spin orientation by the atomic environment. Due to spin-orbit coupling (SOC), the spin is tilted by ≈58° from the surface normal toward the chain as evidenced by inelastic tunneling spectroscopy. These findings are reproduced by density functional theory calculations and have implications for Co atoms on pristine Cu(111), which are believed to be Kondo systems. Our quantum Monte Carlo calculations suggest that SOC suppresses the Kondo effect of Co atoms at chains and on the flat surface. Our work impacts the fundamental understanding of low-energy excitations in nanostructures on surfaces and demonstrates the ability to manipulate atomic-scale magnetic moments, which can have tremendous implications for quantum devices.

Interfacial Roughness Facilitated by Dislocation and a Metal-Fuse Resistor Fabricated Using a Nanomanipulator

Granular magnetic systems consisting of magnetic nanoparticles embedded in a nonmagnetic metallic matrix have emerged as an attractive building block for nanodevices. A key challenge for building interface-based nanodevice applications, such as magnetic memory devices, is to clearly know about the influences of interfacial roughness on the scattering of conduction electrons. Here, we demonstrate a granular magnetic system composed of Co and Cu nanoparticles and further link the atomic structure of the Co/Cu interface to the scattering mechanism of conduction electrons. The multiple scattering is caused by the dislocations at the rough interface, which lead to a reduction of conduction efficiency and an increase of energy consumption. These dislocations mostly originate from the lattice defects on the surface of nanoparticles, the lattice mismatch of two crystal structures, and the different surface energies. Based on the negative effects of a rough interface on electronic transport, we first develop a nanometal-fuse resistor, which could hopefully be used in the protection circuits of nanodevices. Our results may open up the possibility of implementing the low-dimensional granular magnetic materials in nanodevice applications.

Quantum-Chemical Design of Molecular Structures of Tetra-, Penta- and Hexanuclear Metal Clusters Containing Aluminum and 3d-Element Atoms

Various data on the structural and thermodynamic characteristics of polynuclear metal clusters containing atoms of aluminum and various d-elements with the general formula Al_nM_m where (n + m) is 4, 5, or 6, and which can be precursors for the formation of nanoparticles of elemental metals or intermetallic compounds, have been systematized and discussed. It has been noted that each of these metal clusters in principle is able to exist in very diverse structural isomers, differing significantly among themselves in terms of the total energy and spin multiplicity of the ground state, the number of which is determined by both the specific values of n and m, and the nature of d-elements in their compositions. The presence of very complex dynamics with respect to the changes of the individual thermodynamic characteristics of the metal clusters under consideration as well as the thermodynamic parameters of the reactions of their formation, depending on the nature of the d-element, were also ascertained. In the main, the given review is devoted to the authors’ works published over the last 10 years. Bibliography – 96 references.

Manipulation of the two-site Kondo effect in linear CoCu n CoCu m clusters

Artificially assembled linear atomic clusters, CoCu _n CoCu _m , are used to explore variations of the Kondo effect at the two Co sites. For all investigated Cu _n chain lengths ([Formula: see text]) the addition of a single Cu atom to one edge Co atom of the chain ([Formula: see text]) strongly reduces the amplitude of the Abrikosov-Suhl-Kondo resonance of that Co atom. Concomitantly, the resonance line width is more than halved. On the contrary, the Kondo effect of the opposite edge Co atom remains unaffected. Hybridization together with the linear geometry of the cluster are likely to drive the effect.

Facilitating charge transfer via a giant magnetoresistance effect for high-efficiency photocatalytic hydrogen production

CoCu alloy magnetic unit was implanted in photocatalytic system to improve photoinduced charge separation efficiency by regulating electron transfer pathway via giant magnetoresistance (GMR) effect, achieving significantly H₂ production activity.

Spin-fluctuation and spin-relaxation effects of single adatoms from first principles

Single adatoms offer an exceptional playground for studying magnetism and its associated dynamics at the atomic scale. Here we review recent results on single adatoms deposited on metallic substrates, based on time-dependent density functional theory. First we analyze quantum zero-point spin-fluctuations (ZPSF) as calculated from the fluctuation-dissipation theorem, and show how they affect the magnetic stability by modifying the magnetic anisotropy energy. We also assess the impact of ZPSF in the limit of small hybridization to the substrate characteristic of semi-insulating substrates, connecting to recent experimental investigations where magnetic stability of a single adatom was achieved for the first time. Secondly, we inspect further the dynamics of single adatoms by considering the longitudinal and transverse spin-relaxation processes, whose time-scales are analyzed and related to the underlying electronic structure of both the adatom and the substrate. Thirdly, we analyze spin-fluctuation modes of paramagnetic adatoms, i.e. adatoms where the Stoner criterion for magnetism is almost fulfilled. Interestingly, such modes can develop well-defined peaks in the meV range, their main characteristics being determined by two fundamental electronic properties, namely the Stoner parameter and the density of states at the Fermi level. Furthermore, simulated inelastic scanning tunneling spectroscopy curves reveal that these spin-fluctuation modes can be triggered by tunneling electrons, opening up potential applications also for paramagnetic adatoms. Lastly, an overview of the outstanding issues and future directions is given.

Asymmetry parameter of peaked Fano line shapes

The spectroscopic line shape of electronic and vibrational excitations is ubiquitously described by a Fano profile. In the case of nearly symmetric and peaked Fano line shapes, the fit of the conventional Fano function to experimental data leads to difficulties in unambiguously extracting the asymmetry parameter, which may vary over orders of magnitude without degrading the quality of the fit. Moreover, the extracted asymmetry parameter depends on initially guessed values. Using the spectroscopic signature of the single-Co Kondo effect on Au(110) the ambiguity of the extracted asymmetry parameter is traced to the highly symmetric resonance profile combined with the inevitable scattering of experimental data. An improved parameterization of the conventional Fano function is suggested that enables the nonlinear optimization in a reduced parameter space. In addition, the presence of a global minimum in the sum of squared residuals and thus the independence of start parameters may conveniently be identified in a two-dimensional plot. An angular representation of the asymmetry parameter is suggested in order to reliably determine uncertainty margins via linear error propagation.

Insights into the structural, electronic and magnetic properties of V-doped copper clusters: comparison with pure copper clusters

The structural, electronic and magnetic properties of Cu_n+1 and Cu_nV (n = 1–12) clusters have been investigated by using density functional theory. The growth behaviors reveal that V atom in low-energy Cu_nV isomer favors the most highly coordinated position and changes the geometry of the three-dimensional host clusters. The vibrational spectra are predicted and can be used to identify the ground state. The relative stability and chemical activity of the ground states are analyzed through the binding energy per atom, energy second-order difference and energy gap. It is found that that the stability of Cu_nV (n ≥ 8) is higher than that of Cu_n+1. The substitution of a V atom for a Cu atom in copper clusters alters the odd-even oscillations of stability and activity of the host clusters. The vertical ionization potential, electron affinity and photoelectron spectrum are calculated and simulated for all of the most stable clusters. Compare with the experimental data, we determine the ground states of pure copper clusters. The magnetism analyses show that the magnetic moments of Cu_nV clusters are mainly localized on the V atom and decease with the increase of cluster size. The magnetic change is closely related to the charge transfer between V and Cu atoms.

Towards a full ab initio theory of strong electronic correlations in nanoscale devices

In this paper I give a detailed account of an ab initio methodology for describing strong electronic correlations in nanoscale devices hosting transition metal atoms with open d- or f-shells. The method combines Kohn-Sham density functional theory for treating the weakly interacting electrons on a static mean-field level with non-perturbative many-body methods for the strongly interacting electrons in the open d- and f-shells. An effective description of the strongly interacting electrons in terms of a multi-orbital Anderson impurity model is obtained by projection onto the strongly correlated subspace properly taking into account the non-orthogonality of the atomic basis set. A special focus lies on the ab initio calculation of the effective screened interaction matrix U for the Anderson model. Solution of the effective Anderson model with the one-crossing approximation or other impurity solver techniques yields the dynamic correlations within the strongly correlated subspace giving rise e.g. to the Kondo effect. As an example the method is applied to the case of a Co adatom on the Cu(0 0 1) surface. The calculated low-bias tunnel spectra show Fano-Kondo lineshapes similar to those measured in experiments. The exact shape of the Fano-Kondo feature as well as its width depend quite strongly on the filling of the Co 3d-shell. Although this somewhat hampers accurate quantitative predictions regarding lineshapes and Kondo temperatures, the overall physical situation can be predicted quite reliably.

Magnetic moments of chromium-doped gold clusters: the Anderson impurity model in finite systems

The magnetic moment of a single impurity atom in a finite free electron gas is studied in a combined x-ray magnetic circular dichroism spectroscopy, charge transfer multiplet calculation, and density functional theory study of size-selected free chromium-doped gold clusters. The observed size dependence of the local magnetic moment can be understood as a transition from a local moment to a mixed valence regime. This shows that the Anderson impurity model essentially describes finite systems even though the discrete density of states introduces a significant deviation from a bulk metal, and the free electron gas is only formed by less than 10 electrons. Electronic shell closure in the gold host minimizes the interaction of localized impurity states with the confined free electron gas and preserves the magnetic moment of 5 μ_{B} fully in CrAu_{2}^{+} and almost fully in CrAu_{6}^{+}. Even for open-shell species, large local moments are observed that scale with the energy gap of the gold cluster. This indicates that an energy gap in the free electron gas stabilizes the local magnetic moment of the impurity atom.

Relevance of hybridization and filling of 3d orbitals for the Kondo effect in transition metal phthalocyanines

Magnetic organic molecules, such as 3d transition metal phthalocyanines (TMPc), exhibit properties which make them promising candidates for future applications in magnetic data storage or spin-based data processing. Due to their small size, however, TMPc molecules are prone to quantum effects. For example, the interaction of uncompensated molecular spins with conduction electrons of the substrate may lead to the formation of a many-body singlet state, which gives rise to the so-called Kondo effect. Although the Kondo effect of TMPc molecules has been the object of several investigations, a consistent picture to describe under which conditions a Kondo state is formed is still missing. Here, we study the Kondo properties of MnPc on Ag(001) by means of the low-temperature scanning tunneling spectroscopy (LT-STS) measurements. Differential conductance dI/dU spectra reveal a zero-bias peak that is localized on the Mn ion site. Ab initio calculations combined with a many-body treatment of the multiorbital interaction show that the local Hund coupling favors the high-spin configuration on the 3d shell of the central TM atom. Therefore, each orbital gets close to its individual half-filling creating the necessary condition for many of the 3d orbitals to contribute to the observed Kondo resonance. This, however, happens only for the 3dz(2) orbital, whose hybridization to the substrate is much stronger than for the other orbitals thanks to its shape and its orientation.

Excitation spectra of transition-metal atoms on the Ag (100) surface controlled by Hund's exchange

We report photoemission experiments revealing the valence electron spectral function of Mn, Fe, Co, and Ni atoms on the Ag (100) surface. The series of spectra shows splittings of higher energy features which decrease with the filling of the 3d shell and a highly nonmonotonic evolution of spectral weight near the Fermi edge. First principles calculations demonstrate that two manifestations of Hund's exchange J are responsible for this evolution. First, there is a monotonic reduction of the effective exchange splittings with increasing filling of the 3d shell. Second, the amount of charge fluctuations and, thus, the weight of quasiparticle peaks at the Fermi level varies nonmonotonically through this 3d series due to a distinct occupancy dependence of effective charging energies U(eff).

Conductance-driven change of the Kondo effect in a single cobalt atom

A low-temperature scanning tunneling microscope is employed to build a junction comprising a Co atom bridging a copper-coated tip and a Cu(100) surface. An Abrikosov-Suhl-Kondo resonance is evidenced in the differential conductance and its width is shown to vary exponentially with the ballistic conductance for all tips employed. Using a theoretical description based on the Anderson model, we show that the Kondo effect and the total conductance are related through the atomic relaxations affecting the environment of the Co atom.

Multi-orbital non-crossing approximation from maximally localized Wannier functions: the Kondo signature of copper phthalocyanine on Ag(100)

We have developed a multi-orbital approach to compute the electronic structure of a quantum impurity using the non-crossing approximation. The calculation starts with a mean-field evaluation of the system's electronic structure using a standard quantum chemistry code; here we use density functional theory (DFT). We transformed the one-electron structure into an impurity Hamiltonian by using maximally localized Wannier functions. Hence, we have developed a method to study the Kondo effect in systems based on an initial one-electron calculation. We have applied our methodology to a copper phthalocyanine molecule chemisorbed on Ag(100), and we have described its spectral function for three different cases where the molecule presents a single spin or two spins with ferro- and anti-ferromagnetic exchange couplings. We find that the use of broken-symmetry mean-field theories such as Kohn-Sham DFT cannot deal with the complexity of the spin of open-shell molecules on metal surfaces and extra modeling is needed.

Two-site Kondo effect in atomic chains

Linear CoCu(n)Co clusters on Cu(111) fabricated by atomic manipulation represent a two-site Kondo system with tunable interaction. Scanning tunneling spectroscopy reveals oscillations of the Kondo temperature T(K) with the number n of Cu atoms for n≥3. Density functional calculations show that the Ruderman-Kittel-Kasuya-Yosida interaction mediated by the Cu chains causes the oscillations. Calculations find ferromagnetic and antiferromagnetic interaction for n=1 and 2, respectively. Both interactions lead to a decrease of T(K) as experimentally observed.

Ammonia adsorption on iron phthalocyanine on Au(111): influence on adsorbate-substrate coupling and molecular spin

The adsorption of ammonia on Au(111)-supported monolayers of iron phthalocyanine has been investigated by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, and density functional theory calculations. The ammonia-induced changes of the x-ray photoemission lines show that a dative bond is formed between ammonia and the iron center of the phthalocyanine molecules, and that the local spin on the iron atom is quenched. This is confirmed by density functional theory, which also shows that the bond between the iron center of the metalorganic complex and the Au(111) substrate is weakened upon adsorption of ammonia. The experimental results further show that additional adsorption sites exist for ammonia on the iron phthalocyanine monolayer.

Single-molecule chemistry of metal phthalocyanine on noble metal surfaces

To develop new functional materials and nanoscale electronics, researchers would like to accurately describe and precisely control the quantum state of a single molecule on a surface. Scanning tunneling microscopy (STM), combined with first-principles simulations, provides a powerful technique for acquiring this level of understanding. Traditionally, metal phthalocyanine (MPc) molecules, composed of a metal atom surrounded by a ligand ring, have been used as dyes and pigments. Recently, MPc molecules have shown great promise as components of light-emitting diodes, field-effect transistors, photovoltaic cells, and single-molecule devices. In this Account, we describe recent research on the characterization and control of adsorption and electronic states of a single MPc molecule on noble metal surfaces. In general, the electronic and magnetic properties of a MPc molecule largely depend on the type of metal ion within the phthalocyanine ligand and the type of surface on which the molecule is adsorbed. However, with the STM technique, we can use on-site molecular "surgery" to manipulate the structure and the properties of the molecule. For example, STM can induce a dehydrogenation reaction of the MPc, which allows us to control the Kondo effect, which describes the spin polarization of the molecule and its interaction with the complex environment. A specially designed STM tip can allow researchers to detect certain molecule-surface hybrid states that are not accessible by other techniques. By matching the local orbital symmetry of the STM tip and the molecule, we can generate the negative differential resistance effect in the formed molecular junction. This orbital symmetry based mechanism is extremely robust and does not critically depend on the geometry of the STM tip. In summary, this simple model system, a MPc molecule absorbed on a noble metal surface, demonstrates the power of STM for quantum characterization and manipulation of single molecules, highlighting the potential of this technique in a variety of applications.

Kondo versus magnetic coupling of cobalt dimers in a Cu-O (2 × 1) reconstruction

Co atoms and dimers embedded in a Cu(110)(2 × 1)-O surface oxide are investigated via low-temperature STM/STS experiments and first-principles simulations. It is found that Co dimers incorporated into adjacent rows of the Cu(110)(2 × 1)-O reconstruction show Kondo resonances decoupled from each other, whereas they are antiferromagnetically coupled (and do not exhibit a Kondo effect) when they are aligned on the same row. This shows that it is possible to decouple single carriers of the Kondo effect via a proper choice of the adsorption and host geometry. It is also shown that an oxidic environment presents features remarkably different from those of pure metal substrates.

Kondo effect and conductance of nanocontacts with magnetic impurities

We study the impact of dynamical correlations on the electronic structure and coherent transport properties of Cu nanocontacts hosting a single magnetic impurity (Ni, Co, Fe) in the contact region. The strong dynamical correlations of the impurity 3d electrons are fully taken into account by combining density-functional calculations with a dynamical treatment of the impurity 3d shell in the one-crossing approximation. We find that dynamical correlations give rise to the Kondo effect and lead to Fano features in the coherent transport characteristics similar to those observed in related experiments.