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

Dynamical Screening of Local Spin Moments at Metal-Molecule Interfaces

Transition-metal phthalocyanine molecules have attracted considerable interest in the context of spintronics device development due to their amenability to diverse bonding regimes and their intrinsic magnetism. The latter is highly influenced by the quantum fluctuations that arise at the inevitable metal-molecule interface in a device architecture. In this study, we have systematically investigated the dynamical screening effects in phthalocyanine molecules hosting a series of transition-metal ions (Ti, V, Cr, Mn, Fe, Co, and Ni) in contact with the Cu(111) surface. Using comprehensive density functional theory plus Anderson's Impurity Model calculations, we show that the orbital-dependent hybridization and electron correlation together result in strong charge and spin fluctuations. While the instantaneous spin moments of the transition-metal ions are near atomic-like, we find that screening gives rise to considerable lowering or even quenching of these. Our results highlight the importance of quantum fluctuations in metal-contacted molecular devices, which may influence the results obtained from theoretical or experimental probes, depending on their possibly material-dependent characteristic sampling time-scales.

Real-space resolved surface reactions: deprotonation and metalation of phthalocyanine

Upon deposition on a surface, molecules can undergo a plethora of changes, such as reactions with adsorbates and surface atoms and catalytic decomposition. Since different reaction pathways may coexist, spatially averaging techniques can be insufficient for the characterization and distinction of all on-surface products. Here, we present a study of single phthalocyanine molecules on a Cu(111) surface which was performed using high-resolution low-temperature STM. Upon deposition of metal-free H₂Pc, we can identify three distinct molecular species. A thorough investigation reveals that temperature-driven on-surface reactions partially convert H₂Pc into H₀Pc and CuPc. The individual species are differentiated by their topographic appearance and can unambiguously be identified by their STM-induced rotational behavior. While H₂Pc shows a switching between two orientations at low energies, a third orientation can be observed above E > 800 meV, which is induced by tautomerization. Around the Fermi level, the rotational behavior is asymmetric, owing to the excitation of vibrational modes in unoccupied states whereas resonant tunneling occurs in occupied states. A two-step deprotonation of H₂Pc confirms that the second species is H₀Pc. By comparison with CuPc evaporated on Cu(111), we unambiguously reveal that the third species is indeed CuPc, which exhibits an exceptionally low threshold for rotational switching accompanied by an asymmetric behavior around the Fermi level. Varying the post-annealing temperature, we found a sharp threshold for the H₂Pc → CuPc on-surface metalation at around 100 °C. In contrast, the competing process of thermal decomposition from H₂Pc to H₀Pc only increases weakly.

Magnetic Moment Preservation and Emergent Kondo Resonance of Co-Phthalocyanine on Semimetallic Sb(111)

Magnetic molecules on surfaces have been widely investigated to reveal delicate interfacial couplings and for potential technological applications. In these endeavors, one prevailing challenge is how to preserve or recover the molecular spins, especially on highly metallic substrates that can readily quench the magnetic moments of the admolecules. Here, we use scanning tunneling microscopy and spectroscopy to exploit the semimetallic nature of antimony and observe, surprisingly yet pleasantly, that the spin of Co-phthalocyanine is well preserved on Sb(111), as unambiguously evidenced by the emergent strong Kondo resonance across the molecule. Our first-principles calculations further confirm that the optimal density of states near the Fermi level of the semimetal is a decisive factor, weakening the overall interfacial coupling, while still ensuring sufficiently effective electron-spin scattering in the many-body system. Beyond isolated admolecules, we discover that each of the magnetic moments in a molecular dimer or a densely packed island is distinctly preserved as well, rendering such molecular magnets immense potentials for ultrahigh density memory devices.

On-surface synthesis of Mn-phthalocyanines with optically active ligands

The synthesis of novel organic prototypes combining different functionalities is key to achieve operational elements for applications in organic electronics. Here we set the stage towards individually addressable magneto-optical transducers by the on-surface synthesis of optically active manganese-phthalocyanine derivatives (MnPc) obtained directly on a metallic substrate. We created these 2D nanostructures under ultra-high vacuum conditions with atomic precision starting from a simple phthalonitrile precursor with reversible photo-induced reactivity in solution. These precursors maintain their integrity after powder sublimation and coordinate with the Mn ions into tetrameric complexes and then transform into MnPcs on Ag(111) after a cyclotetramerization reaction. Using scanning tunnelling microscopy and spectroscopy together with DFT calculations, we identify the isomeric configuration of two bi-stable structures and show that it is possible to switch them reversibly by mechanical manipulation. Moreover, the robust magnetic moment brought by the central Mn ion provides a feasible pathway towards magneto-optical transducer fabrication. This work should trigger further research confirming such magneto-optical effects in MnPcs both on surfaces and in liquid environments.

Anomalous excitations of atomically crafted quantum magnets

High-energy resolution spectroscopic studies of quantum magnets proved extremely valuable in accessing magnetodynamics quantities, such as energy barriers, magnetic interactions, and lifetime of excited states. Here, we investigate a previously unexplored flavor of low-energy spin excitations for quantum spins coupled to an electron bath. In sharp contrast to the usual tunneling signature of two steps symmetrically centered around the Fermi level, we find a single step in the conductance. Combining time-dependent and many-body perturbation theories, magnetic field-dependent tunneling spectra are explained as the result of an interplay between weak magnetic anisotropy energy, magnetic interactions, and Stoner-like electron-hole excitations that are strongly dependent on the magnetic states of the nanostructures. The results are rationalized in terms of a noncollinear magnetic ground state and the dominance of ferro- and antiferromagnetic interactions. The atomically crafted nanomagnets offer an appealing model for the exploration of electrically pumped spin systems.

Iron phthalocyanine on Au(111) is a "non-Landau" Fermi liquid

The paradigm of Landau’s Fermi liquid theory has been challenged with the finding of a strongly interacting Fermi liquid that cannot be adiabatically connected to a non-interacting system. A spin-1 two-channel Kondo impurity with anisotropy D has a quantum phase transition between two topologically different Fermi liquids with a peak (dip) in the Fermi level for D < D_c (D > D_c). Extending this theory to general multi-orbital problems with finite magnetic field, we reinterpret in a unified and consistent fashion several experimental studies of iron phthalocyanine molecules on Au(111) that were previously described in disconnected and conflicting ways. The differential conductance shows a zero-bias dip that widens when the molecule is lifted from the surface (reducing the Kondo couplings) and is transformed continuously into a peak under an applied magnetic field. We reproduce all features and propose an experiment to induce the topological transition.

Single molecules on metal surfaces are paradigmatic systems for the study of many-body phenomena. Here, the authors show that several spectroscopic experiments on iron phthalocyanine on Au(111) surface can be described in a unified way in terms of a strongly interacting topologically non-trivial (non-Landau) Fermi liquid.

Multiple magnetic states of CoPc molecule on a two-dimensional layer of NbSe2

Molecular spintronics hinges on the detailed understanding of electronic and magnetic properties of molecules interfaced with various materials. Here we demonstrate withab initiosimulations that the prototypical Co-phthalocyanine (CoPc) molecule can surprisingly develop multi-spin states once deposited on the two-dimensional 2H-NbSe₂layer. Conventional calculations based on density functional theory (DFT) show the existence of low, regular and high spin states, which reduce to regular and high spins states once correlations are incorporated with a DFT +Uapproach. Depending onU, the ground state is either the low spin or high spin state with energy differences affected by the molecular orientation on top of the substrate. Our results are compared to recent scanning probe measurements and motivate further theoretical and experimental studies on the unveiled rich multi-magnetic behavior of CoPc molecule.

Tuning Molecular Superlattice by Charge-Density-Wave Patterns in Two-Dimensional Monolayer Crystals

Charge density wave (CDW) in two-dimensional (2D) crystals plays a vital role in tuning the interface structures and properties. However, how the CDW tunes the self-assembled molecular superlattice still remains unclear. In this study, we investigated the self-assembled manganese phthalocyanine (MnPc) molecular superlattice on single-layered 1T- and 2H-NbSe₂ crystals under regulation by distinct CDW patterns. We observe that, in low coverage, MnPc molecules preferentially adsorb on 2H-NbSe₂ compared to 1T-NbSe₂. With increasing coverage, MnPc can form a highly ordered superlattice on 2H-NbSe₂; however, it is randomly distributed on 1T-NbSe₂. We reveal a perfect geometric commensurability between the molecular superlattice and intrinsic CDW pattern in 2H-NbSe₂ and a poor commensurability for that of 1T-NbSe₂. We believe that the subtly different geometric commensurability dominates the different adsorption and arrangement of the molecular superlattices on 2D CDW patterns. Our study provides a pioneering approach for tuning the molecular superlattices using the CDW patterns.

Local decomposition of hybridization functions: Chemical insight into correlated molecular adsorbates

Hybridization functions are an established tool for investigating the coupling between a correlated subsystem (often a single transition metal atom) and its uncorrelated environment (the substrate and any ligands present). The hybridization function can provide valuable insight into why and how strong correlation features such as the Kondo effect can be chemically controlled in certain molecular adsorbates. To deepen this insight, we introduce a local decomposition of the hybridization function, based on a truncated cluster approach, enabling us to study individual effects on this function coming from specific parts of the systems (e.g., the surface, ligands, or parts of larger ligands). It is shown that a truncated-cluster approach can reproduce the Co 3d and Mn 3d hybridization functions from periodic boundary conditions in Co(CO)₄/Cu(001) and MnPc/Ag(001) qualitatively well. By locally decomposing the hybridization functions, it is demonstrated at which energies the transition metal atoms are mainly hybridized with the substrate or with the ligand. For the Kondo-active 3d_x²-y² orbital in Co(CO)₄/Cu(001), the hybridization function at the Fermi energy is substrate-dominated, so we can assign its enhancement compared with ligand-free Co to an indirect effect of ligand-substrate interactions. In MnPc/Ag(001), the same is true for the Kondo-active orbital, but for two other orbitals, there are both direct and indirect effects of the ligand, together resulting in such strong screening that their potential Kondo activity is suppressed. A local decomposition of hybridization functions could also be useful in other areas, such as analyzing the electrode self-energies in molecular junctions.

Long-Range Surface-Assisted Molecule-Molecule Hybridization

Metalated phthalocyanines (Pc's) are robust and versatile molecular complexes, whose properties can be tuned by changing their functional groups and central metal atom. The electronic structure of magnesium Pc (MgPc)-structurally and electronically similar to chlorophyll-adsorbed on the Ag(100) surface is investigated by low-temperature scanning tunneling microscopy and spectroscopy, non-contact atomic force microscopy, and density functional theory. Single, isolated MgPc's exhibit a flat, fourfold rotationally symmetric morphology, with doubly degenerate, partially populated (due to surface-to-molecule electron transfer) lowest unoccupied molecular orbitals (LUMOs). In contrast, MgPc's with neighbouring molecules in proximity undergo a lift of LUMOs degeneracy, with a near-Fermi local density of states with reduced twofold rotational symmetry, indicative of a long-range attractive intermolecular interaction. The latter is assigned to a surface-mediated two-step electronic hybridization process. First, LUMOs interact with Ag(100) conduction electrons, forming hybrid molecule-surface orbitals with enhanced spatial extension. Then, these delocalized molecule-surface states further hybridize with those of neighbouring molecules. This work highlights how the electronic structure of molecular adsorbates-including orbital degeneracies and symmetries-can be significantly altered via surface-mediated intermolecular hybridization, over extended distances (beyond 3 nm), having important implications for prospects of molecule-based solid-state technologies.

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.

Tunable giant magnetoresistance in a single-molecule junction

Controlling electronic transport through a single-molecule junction is crucial for molecular electronics or spintronics. In magnetic molecular devices, the spin degree-of-freedom can be used to this end since the magnetic properties of the magnetic ion centers fundamentally impact the transport through the molecules. Here we demonstrate that the electron pathway in a single-molecule device can be selected between two molecular orbitals by varying a magnetic field, giving rise to a tunable anisotropic magnetoresistance up to 93%. The unique tunability of the electron pathways is due to the magnetic reorientation of the transition metal center, resulting in a re-hybridization of molecular orbitals. We obtain the tunneling electron pathways by Kondo effect, which manifests either as a peak or a dip line shape. The energy changes of these spin-reorientations are remarkably low and less than one millielectronvolt. The large tunable anisotropic magnetoresistance could be used to control electronic transport in molecular spintronics.

Molecular electronics or spintronics relies on manipulating the electronic transport through microscopic molecule structures. Here the authors demonstrate the selective electron pathway in single-molecule device by magnetic field which enables a tunable anisotropic magnetoresistance up to 93%.

Observation of Coexistence of Yu-Shiba-Rusinov States and Spin-Flip Excitations

We investigate the spectral evolution in different metal phthalocyanine molecules on NbSe₂ surface using scanning tunnelling microscopy (STM) as a function of the coupling with the substrate. For manganese phthalocyanine (MnPc), we demonstrate a smooth spectral crossover from Yu-Shiba-Rusinov (YSR) bound states to spin-flip excitations. This has not been observed previously and it is in contrast to simple theoretical expectations. We corroborate the experimental findings using numerical renormalization group calculations. Our results provide fundamental new insight on the behavior of atomic scale magnetic/SC hybrid systems, which is important, for example, for engineered topological superconductors and spin logic devices.

Molecular attachment to a microscope tip: inelastic tunneling, Kondo screening, and thermopower

The vibrational excitation related transport properties of a manganese phthalocyanine molecule suspended between the tip of a scanning tunneling microsope (STM) and a surface are investigated by combining the local manipulation capabilities of the STM with inelastic electron tunneling spectroscopy. By attachment of the molecule to the probe tip, the intrinsic physical properties similar to those exhibited by a free standing molecule become accessible. This technique allows one to study locally the magnetic properties, as well as other elementary excitations and their mutual interaction. In particular a clear correlation is observed between the Kondo resonance and the vibrations with a strong incidence of the Kondo correlation on the thermopower measured across the single-molecule junction.

Imprinting Directionality into Proton Transfer Reactions of an Achiral Molecule

Directionality is key for the functionality of molecular machines, which is often achieved by built-in structural chiralities. Here, we present a scanning tunneling microscopy study of achiral H₂Pc and HPc molecules that acquire chirality by adsorption onto a Ag(100) surface. The adsorption-geometry-induced chirality is caused by a -29° (+29°) rotation of the molecules with respect to the [011] substrate direction, resulting in tautomerization processes that preferentially occur in a clockwise (counterclockwise) direction. The directionality is found to be independent of the particular energy and location of charge carrier injection. In contrast to built-in structural chiralities that are fixed by the molecular structure, the direction of proton motion in HPc on Ag(100) can be inverted by a rotation of the molecule on the substrate.

Electronic effects and fundamental physics studied in molecular interfaces

Scanning probe instruments in conjunction with a very low temperature environment have revolutionized the ability of building, functionalizing, and analysing two dimensional interfaces in the last twenty years. In addition, the availability of fast, reliable, and increasingly sophisticated methods to simulate the structure and dynamics of these interfaces allow us to capture even very small effects at the atomic and molecular level. In this review we shall focus largely on metal surfaces and organic molecular compounds and show that building systems from the bottom up and controlling the physical properties of such systems is no longer within the realm of the desirable, but has become day to day reality in our best laboratories.

Ensemble Control of Kondo Screening in Molecular Adsorbates

Switching the magnetic properties of organic semiconductors on a metal surface has thus far largely been limited to molecule-by-molecule tip-induced transformations in scanned probe experiments. Here we demonstrate with molecular resolution that collective control of activated Kondo screening can be achieved in thin-films of the organic semiconductor titanyl phthalocyanine on Cu(110) to obtain tunable concentrations of Kondo impurities. Using low-temperature scanning tunneling microscopy and spectroscopy, we show that a thermally activated molecular distortion dramatically shifts surface-molecule coupling and enables ensemble-level control of Kondo screening in the interfacial spin system. This is accompanied by the formation of a temperature-dependent Abrikosov-Suhl-Kondo resonance in the local density of states of the activated molecules. This enables coverage-dependent control over activation to the Kondo screening state. Our study thus advances the versatility of molecular switching for Kondo physics and opens new avenues for scalable bottom-up tailoring of the electronic structure and magnetic texture of organic semiconductor interfaces at the nanoscale.

High temperature ferromagnetism in π-conjugated two-dimensional metal-organic frameworks

We use first-principles calculations to show that the square symmetry of two-dimensional (2D) metal-organic frameworks (MOFs) made from octaamino-substituted phthalocyanines and square planar Ni2+ ions, which enable strong conjugation of π electrons, has a critical impact on the magnetic properties of the lattice. In particular, we predict the unexpected emergence of a rare high-temperature ferromagnetic half-metallic ground state in one case. Among charge neutral MOFs made from (2,3,9,10,16,17,23,24)-octaiminophthalocyanine (OIPc) metallated with divalent first-row transition metal ions (M-OIPc; M = Cr2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) and connected through square planar Ni-bisphenylenediimine moieties, NiMn-OIPc exhibits a half-metallic and ferromagnetic ground state with a large exchange energy resulting from the unique strong hybridization between the d/π orbitals of Mn, the Pc ring, and the Ni-bisphenylenediimine nodes. Notably, we show that for NiMn-OIPc there is a considerable difference between the ferromagnetic ordering temperature (Tc) predicted by a 2D Ising model, which exceeds 600 K, and a Tc of 170 K predicted by our more realistic Monte Carlo simulation that includes magnetic anisotropy. Critically, our simulations adopt two spin models that incorporate magnetic anisotropy in the form of exchange anisotropy and single-ion anisotropy. We further show that in the bulk, 2D layers of NiMn-OIPc adopt a slipped-parallel stacking configuration, and exhibit interlayer magnetic coupling that is sensitive to the relative in-plane displacement between adjacent layers. These results highlight the critical role of magnetic anisotropy in modeling the properties of 2D magnetic systems. More generally, it demonstrates that strong hybridization between open-shell ions and delocalized aromatic π systems with appropriate symmetry, combined with large magnetic anisotropy, will be an effective design strategy to realize ferromagnetic 2D MOFs with high Tc.

Two-Orbital Kondo Screening in a Self-Assembled Metal-Organic Complex

Iron atoms adsorbed on a Cu(111) surface and buried under polyphenyl dicarbonitrile molecules exhibit strongly spatial anisotropic Kondo features with directionally dependent Kondo temperatures and line shapes, as evidenced by scanning tunneling spectroscopy. First-principles calculations find nearly full polarization for the half-filled Fe 3d_xz and 3d_yz orbitals, which therefore can give rise to Kondo screening with the experimentally observed directional dependence and distinct Kondo temperatures. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements confirm that the spin in both channels is effectively Kondo-screened. At ideal Fe coverage, these two-orbital Kondo impurities are arranged in a self-assembled honeycomb superlattice.

Controlling electronic access to the spin excitations of a single molecule in a tunnel junction

Spintronic phenomena underpin new device paradigms for data storage and sensing. Scaling these down to the single molecule level requires controlling the properties of current-carrying molecular orbitals to enable access to spin states through phenomena such as inelastic electron tunnelling. Here we show that the spintronic properties of a tunnel junction containing a single molecule can be controlled using the local environment as a pseudo-gate. For tunnelling through iron phthalocyanine (FePc) on an insulating copper nitride (Cu₂N) monolayer above Cu(001), we find that spin transitions may be strongly excited depending on the binding site of the central Fe atom. Different interactions between the Fe and the underlying Cu or N atoms shift the Fe d orbitals with respect to the Fermi energy and control the relative strength of the spin excitations; this effect is captured in a simple co-tunnelling model. This work demonstrates the importance of the atomic-scale environment for the development of single molecule spintronic devices.

A Ru–Ru pair housed in ruthenium phthalocyanine: the role of a “cage” architecture in the molecule coupling with the Ag(111) surface

A combined experimental and theoretical study on the Ru–Pc dimer on Ag(111) proves surface-to-molecule charge transfer and predicts magnetic moment quenching.

Effects of valence, geometry and electronic correlations on transport in transition metal benzene sandwich molecules

We study the impact of the valence and the geometry on the electronic structure and transport properties of different transition metal-benzene sandwich molecules bridging the tips of a Cu nanocontact. Our density-functional calculations show that the electronic transport properties of the molecules depend strongly on the molecular geometry which can be controlled by the nanocontact tips. Depending on the valence of the transition metal center certain molecules can be tuned in and out of half-metallic behaviour facilitating potential spintronics applications. We also discuss our results in the framework of an Anderson impurity model, indicating cases where the inclusion of local correlations alters the ground state qualitatively. For Co and V centered molecules we find indications of an orbital Kondo effect.

Sub-molecular modulation of a 4f driven Kondo resonance by surface-induced asymmetry

Coupling between a magnetic impurity and an external bath can give rise to many-body quantum phenomena, including Kondo and Hund's impurity states in metals, and Yu-Shiba-Rusinov states in superconductors. While advances have been made in probing the magnetic properties of d-shell impurities on surfaces, the confinement of f orbitals makes them difficult to access directly. Here we show that a 4f driven Kondo resonance can be modulated spatially by asymmetric coupling between a metallic surface and a molecule containing a 4f-like moment. Strong hybridization of dysprosium double-decker phthalocyanine with Cu(001) induces Kondo screening of the central magnetic moment. Misalignment between the symmetry axes of the molecule and the surface induces asymmetry in the molecule's electronic structure, spatially mediating electronic access to the magnetic moment through the Kondo resonance. This work demonstrates the important role that molecular ligands have in mediating electronic and magnetic coupling and in accessing many-body quantum states.

In the Kondo effect, a bath of conduction electrons screens a localized magnetic moment. Here, the authors demonstrate Kondo screening of a normally isolated 4f-like moment in a magnetic molecule on a Cu(001) surface that is modulated by strong ligand-mediated coupling.

Superstructures and Electronic Properties of Manganese-Phthalocyanine Molecules on Au(110) from Submonolayer Coverage to Ultrathin Molecular Films

The adsorption of manganese-phthalocyanine molecules on Au(110) was investigated using a low-temperature scanning tunneling microscope. A rich variety of commensurate superstructures was observed upon increasing the molecule coverage from submonolayers to ultrathin films. All structures were associated with reconstructions of the Au(110) substrate. Molecules adsorbed in the second molecular layer exhibited negative differential conductance occurring symmetrically around zero bias voltage. A double-barrier tunneling model rationalized this observation in terms of a peaked molecular resonance at the Fermi energy together with a voltage drop across the molecular film.

Switching Molecular Kondo Effect via Supramolecular Interaction

We apply supramolecular assembly to control the adsorption configuration of Co-porphyrin molecules on Au(111) and Cu(111) surfaces. By means of cryogenic scanning tunneling microscopy, we reveal that the Kondo effect associated with the Co center is absent or present in different supramolecular systems. We perform first-principles calculations to obtain spin-polarized electronic structures and compute the Kondo temperatures using the Anderson impurity model. The switching behavior is traced to varied molecular adsorption heights in different supramolecular structures. These findings unravel that a competition between intermolecular interactions and molecule-substrate interactions subtly regulates the molecular Kondo effect in supramolecular systems.

Magnetic anisotropy in Shiba bound states across a quantum phase transition

The exchange coupling between magnetic adsorbates and a superconducting substrate leads to Shiba states inside the superconducting energy gap and a Kondo resonance outside the gap. The exchange coupling strength determines whether the quantum many-body ground state is a Kondo singlet or a singlet of the paired superconducting quasiparticles. Here we use scanning tunnelling spectroscopy to identify the different quantum ground states of manganese phthalocyanine on Pb(111). We observe Shiba states, which are split into triplets by magnetocrystalline anisotropy. Their characteristic spectral weight yields an unambiguous proof of the nature of the quantum ground state. Our results provide experimental insights into the phase diagram of a magnetic impurity on a superconducting host and shine light on the effects induced by magnetic anisotropy on many-body interactions.

The exchange coupling strength between magnetic adsorbates and a superconducting surface determines the nature of the system's quantum ground state. Here, the authors use scanning tunnelling microscopy to explore the ground state and excited state properties of manganese phthalocyanine adsorbed on a Pb(111) surface.

Controlled Complete Suppression of Single-Atom Inelastic Spin and Orbital Cotunneling

The inelastic portion of the tunnel current through an individual magnetic atom grants unique access to read out and change the atom's spin state, but it also provides a path for spontaneous relaxation and decoherence. Controlled closure of the inelastic channel would allow for the latter to be switched off at will, paving the way to coherent spin manipulation in single atoms. Here, we demonstrate complete closure of the inelastic channels for both spin and orbital transitions due to a controlled geometric modification of the atom's environment, using scanning tunneling microscopy (STM). The observed suppression of the excitation signal, which occurs for Co atoms assembled into chains on a Cu2N substrate, indicates a structural transition affecting the dz(2) orbital, effectively cutting off the STM tip from the spin-flip cotunneling path.

Shifting the Voltage Drop in Electron Transport Through a Single Molecule

A Mn-porphyrin was contacted on Au(111) in a low-temperature scanning tunneling microscope (STM). Differential conductance spectra show a zero-bias resonance that is due to an underscreened Kondo effect according to many-body calculations. When the Mn center is contacted by the STM tip, the spectrum appears to invert along the voltage axis. A drastic change in the electrostatic potential of the molecule involving a small geometric relaxation is found to cause this observation.

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.