Magnetic impurities in superconductors host Yu-Shiba-Rusinov (YSR) bound states, whereas weak superconducting junctions sustain subharmonic charge transport through multiple Andreev reflections (MAR). We explore these phenomena using a molecular-scale junction created by placing a nickelocene (Nc) molecule atop an Fe adatom on Pb(111) and approaching it with a superconducting scanning-tunneling-microscope tip. Gradually increasing the junction transparency drives the system from conventional YSR spectroscopy in the tunneling regime to resonant MAR transport and, ultimately, to a Josephson-coupled state. The resulting spectra exhibit shifted MAR thresholds, asymmetric line shapes, and odd-even features that identify YSR-assisted MAR. A combined theoretical treatment of junction transparency and exchange coupling accounts for these observations, while density-functional calculations show that the Nc-Fe complex forms an antiferromagnetic ground state with a residual Fe spin-$$\frac{1}{2}$$ generating the YSR resonances. Our results demonstrate that the parameters governing YSR and MAR processes can be continuously tuned within a single superconducting nanojunction. Magnetic impurities and weak links create distinct subgap excitations in superconductors. Here, the authors tune a nickelocene/Fe junction on Pb to show that combined Yu-Shiba-Rusinov (YSR) and multiple-Andreev-reflection (MAR) processes explain shifted subgap features and the onset of Josephson transport.

Self-assembled monolayers and bilayers of the electron acceptor molecule tetracyanoquinodimethane (TCNQ) were investigated on Pb(100) using low-temperature scanning tunneling microscopy (STM) at 4.5 K and density functional theory (DFT) calculations. The monolayer exhibits an unusually low molecular density. In contrast, both the first and second layers in the bilayer adopt a packing density twice that of the monolayer. Our modeling reveals that Pb atoms, apparently released from step edges, stabilize the bilayer structure. The structural transition is accompanied by a pronounced change in the electronic structure: the bilayer hosts a singly occupied molecular orbital (SOMO) and its corresponding unoccupied partner (SUMO), arising from a spin-splitting of the lowest unoccupied molecular orbital (LUMO) observed in the monolayer.

Bilayers of the nonplanar complex indium phthalocyanine (InPc) were studied on superconducting Pb(001) using scanning tunneling microscopy and density functional theory calculations. The structure of the bilayer is drastically different from that of a single layer in terms of the unit cell, the azimuthal orientations of the molecules, and the position of the In ion above (In↑Pc) or below (In↓Pc) the macrocycle. Intriguingly, the presence of the second layer appears to induce a geometric transition of the first layer. Moreover, a Yu-Shiba-Rusinov state reflecting a localized spin is observed above all molecules of the bilayer.

We report the synthesis and purification of erbium bis-phthalocyanine (ErPc2), and the characterization of its single adsorbate on a Au(111) surface. Following a solvothermal synthesis route combined with high-performance liquid chromatography purification, we obtained high-purity ErPc2 molecules suitable for scanning tunneling microscopy/spectroscopy (STM/STS). Constant-current STM reveals intact, flat-lying molecules with well-defined topographic signatures, enabling the unambiguous identification of single adsorbates. Differential conductance measurements uncover a Kondo resonance localized in the molecular ligands, evidencing the persistence of a ligand-centered unpaired electron upon adsorption. In contrast, it shows inelastic electron tunneling features at the molecular center, a signature of discrete vibrational excitations. Density functional theory calculations reproduce the survival of the ligand spin and partial charge transfer to the substrate, whereas many-body transport modeling captures an orbital-selective electron-vibration coupling, predominantly involving nitrogen atoms and Er-N breathing modes. Our results elucidate the interplay between electronic and vibrational channels in lanthanide-based double-decker molecules on metallic substrates, providing insights into the development of molecular spintronic devices and quantum platforms using lanthanide molecules.

Trioxatriangulenium was functionalized with a rotatable pyridyl moiety (pyridyl-TOTA), which exhibits a large electrostatic dipole moment, and investigated on the Ag(111) surface using low-temperature scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM). The molecules arrange themselves in a complex, well-ordered long-range pattern, namely, a chiral hexagonal mesh of rings composed of six molecules. Submolecularly resolved STM images reveal the orientations of the molecular dipoles, which are confirmed by NC-AFM measurements. Density functional theory (DFT) calculations reproduce the STM images and their intriguing dependence on the junction bias and confirm that the experimental results allow us to fully determine the orientation of the functional units. Comparison with phenyl-TOTA reveals that the dipole-dipole interaction of the pyridyl unit is strong enough to change the structure from a hexagonal close-packed pattern to a chiral hexagonal mesh of supramolecular hexagons. These results show that dipole functionalization may serve to control the formation of molecular assemblies on the surfaces.

Single atomic adsorbates on ultrathin insulating films provide a promising route towards building bottom-up quantum architectures based on atomically identical yet individually addressable spin qubits on solid surfaces. A key challenge in engineering quantum-coherent spin nanostructures lies in understanding and controlling the spin state of individual adsorbates. In this work, we investigate single titanium (Ti) atoms adsorbed on MgO/Ag(100) surfaces using a combined scanning tunneling microscopy and electron spin resonance. Our measurements reveal two distinct spin states, S = 1/2 and S = 1, depending on the local adsorption site and the thickness of the MgO film. Density functional theory calculations suggest a Ti+ configuration for the Ti adsorbates with approximately 3 electrons in the 4s and 3d valence shells. Using multi-orbital magnetic multiplet calculations the site dependence of the spin can be rationalized as a charge redistribution between spin-polarizing and depolarizing orbitals. These findings underscore the potential of surface-supported single atoms as spin qubits with tunable spin and charge states, enabling atom-by-atom control in the realization of a versatile quantum platform on surfaces. The study shows that the spin state of a single titanium atom on an insulating surface can be reversibly switched by its local environment, without chemical changes (no hydrogenation), using advanced microscopy, spectroscopy, and theory.

N-Heterocyclic carbenes are highly effective ligands for anchoring functional organic molecules to metal surfaces and nanoparticles, facilitating the formation of self-assembled monolayers. However, their adsorption on surface is difficult to predict and control, and there is an ongoing debate on the geometry of NHC derivatives on gold surfaces and on the role of gold adatoms. We present two single molecules based on a benzimidazole NHC, one equipped with a thiophene substituent, and the other ending with a Br atom. By low temperature scanning tunneling microscopy we show that both molecules adsorb planar on Au(111) and are chiral on the surface. Our results indicate that in both cases a complex between NHC and a gold adatom is formed. Upon voltage pulses with the STM tip, both complexes move excited by inelastic tunneling electrons. For the derivative with thiophene, we observe a stepwise 60° unidirectional rotation around the S atom. The direction of rotation is determined by both the chirality and the position of the applied pulse. On the contrary, the NHC derivative without thiophene moves laterally on the surface. Adsorption, binding to gold atoms, and motion are discussed with the support of density functional theory calculations and image simulations.

We investigated trioxatriangulenium functionalized with phenyl (phenyl-TOTA) on the (111) surfaces of Ag and Au using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). On Ag(111), the molecules form hexagonal arrays, and on Au(111), honeycomb patterns are also observed. The orientations of the phenyl moieties are resolved on both substrates. On Ag(111), the orientations are parallel within a row and they differ by approximately 60° between adjacent molecular rows, and STM images suggest dimerization of the molecules. DFT calculations for Ag(111) reveal that van der Waals interactions dominate this system. The optimized structure matches the experimental pattern, and the simulated STM images exhibit apparent dimerization. The dimerization results from an asymmetry of the phenyl wave function, which reflects intramolecular hydrogen bonding between the ligand and an oxygen atom within the triangulenium platform. The orientation of the phenyl moieties is explained by the interaction of each phenyl subunit with its triangulenium platform combined with the direct long-range interaction between phenyl moieties across molecules.

The ability to control molecular adsorption and transformation on surfaces is key to advancing nanoscale fabrication, catalysis, and quantum materials engineering. Transition-metal metallocenes, such as nickelocene (NiCp2), offer intriguing opportunities due to their well-defined electronic and magnetic properties, making them ideal candidates for studying surface interactions at the atomic level. We investigate the adsorption and transformation of NiCp2, a nickel atom coordinated by two cyclopentadienyl (Cp) rings, on a Au(111) surface using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. At low temperatures, NiCp2 preferentially adsorbs at herringbone elbows and step edges, forming ordered assemblies. Upon heating, NiCp2 molecules dissociate into NiCp complexes and Cp radicals. The NiCp fragments self-assemble into one-dimensional chains, which further arrange into triangular structures due to the underlying Au(111) substrate, while Cp radicals exhibit low diffusion barriers on the surface. The dissociated NiCp fragments are non-magnetic, contrasting with the magnetic properties of intact NiCp2 molecules. The formation of one type of dimer of the NiCp fragment is rendered possible by the stabilization granted by gold atoms. This study highlights the controlled formation and assembly of surface-confined nanostructures via temperature-driven molecular dissociation.

The influence of hydrogen on magnetization is of substantial interest to spintronics. Understanding and controlling this phenomenon at the atomic scale, in particular in nanoscale systems, is crucial. In this study, we used scanning tunneling microscopy (STM) combined with a nickelocene molecule to sense the spin of a hydrogen-loaded nanoscale Co island grown on Cu(111). Magnetic exchange maps obtained from the molecular tip revealed the presence of a hydrogen superstructure and a 90° rotation of the magnetization compared to the pristine island. Ab initio calculations corroborate these observations, indicating that hydrogen hybridization with Co atoms on the island surface drives the spin reorientation of the island. This reorientation is further reinforced by hydrogen penetration into the island that locates at the Co/Cu interface. However, the subsurface sensitivity of the magnetic exchange maps indicates that this effect is limited. Our study provides valuable microscopic insights into the chemical control of magnetism at the nanoscale.

Scanning tunneling microscope data from a dinuclear Co(II) complex adsorbed on Au(111) are analyzed using density functional theory calculations. We find that the interaction with the substrate substantially changes the geometry of the nonplanar molecule. Its electronic states, however, remain fairly similar to those calculated for a gas-phase molecule. The calculations reproduce intriguing contrasts observed in experimental maps of the differential conductance dI/dV and reveal the relative importance of geometric and electronic factors that impinge on the image contrasts. For a meaningful comparison, it is important that the calculations closely mimic the experimental mode of measurement.

The norbornadiene-quadricyclane photoswitch is a promising example of molecular solar thermal systems capable of energy conversion, storage and release. We present a scanning tunneling microscopy and spectroscopy study of a symmetric NBD-COOMe derivative adsorbed on the Au(1 1 1) surface, supported by density functional theory simulations. We observe the formation of ordered self-assembled islands of NBD-COOMe on the gold surface, driven by intermolecular van der Waals interaction, which become disordered and gradually desorb upon increasing the surface temperature. After annealing at 200 °C, a few adsorbed molecules show a different appearance and electronic structure suggesting the formation of single metalorganic complexes of NBD-COOMe with gold adatoms.

Surface-adsorbed rare-earth nanostructures are ideal platforms to investigate the interplay between intra-atomic interactions and multiorbital spin configurations. However, addressing these properties has posed severe experimental and theoretical challenges. Here, we use the orbital selectivity offered by x-ray absorption spectroscopy to quantify the Coulomb integrals of Nd atoms on conductive surfaces directly from experimental quantities, as well as the variation of individual orbital occupation upon cluster nucleation. Using x-ray magnetic circular dichroism we identify magnetic moments of the order of 0.1-0.2  μ_{B} at the 5d orbitals and their magnetic coupling with the 4f spins. Our results validate orbital-resolved x-ray spectroscopy as a reliable method for quantifying complex multiorbital interactions in surface-adsorbed lanthanides.

The norbornadiene-quadricyclane photoswitch is a promising example of molecular solar thermal systems capable of energy conversion, storage and release. We present a scanning tunneling microscopy and spectroscopy study of a symmetric NBD-COOMe derivative adsorbed on the Au(1 1 1) surface, supported by density functional theory simulations. We observe the formation of ordered self-assembled islands of NBD-COOMe on the gold surface, driven by intermolecular van der Waals interaction, which become disordered and gradually desorb upon increasing the surface temperature. After annealing at 200 degrees C, a few adsorbed molecules show a different appearance and electronic structure suggesting the formation of single metalorganic complexes of NBD-COOMe with gold adatoms.

Precise control of spin states and spin-spin interactions in atomic-scale magnetic structures is crucial for spin-based quantum technologies. A promising architecture is molecular spin systems, which offer chemical tunability and scalability for larger structures. An essential component, in addition to the qubits themselves, is switchable qubit-qubit interactions that can be individually addressed. In this study, we present an electrically controlled single-molecule spin switch based on a bistable complex adsorbed on an insulating magnesium oxide film. The complex, which consists of an Fe adatom coupled to an iron phthalocyanine (FePc) molecule, can be reversibly switched between two stable states using bias voltage pulses locally via the tip of a scanning tunnelling microscope. Inelastic electron tunnelling spectroscopy measurements and density functional theory calculations reveal a distinct change between a paramagnetic and a non-magnetic spin configuration. Lastly, we demonstrate the functionality of this molecular spin switch by using it to modify the electron spin resonance frequency of a nearby target FePc spin within a spin-spin coupled structure. Thus, we showcase how individual molecular machines can be utilized to create scalable and tunable quantum devices.

MnTe has recently emerged as a canonical altermagnet, a newly identified class of magnetism characterized by compensated antiferromagnetic order coexisting with spin-split electronic bands, traditionally considered exclusive to ferromagnets. However, the extent to which altermagnetism persists as altermagnets are thinned to the two-dimensional (2D) limit remains unexplored. Here, we investigate the magnetic behaviour of 2D MnTe, specifically atomically-thin monolayers (MLs) and bilayers (BLs) grown on graphene/Ir(111) substrate, by combining experimental scanning tunneling microscopy, x-ray photoelectron spectroscopy, x-ray absorption spectroscopy and x-ray magnetic circular dichroism with density functional theory calculations. We find that while ML and BL MnTe adopt atomic structures with symmetries incompatible with altermagnetism, they exhibit intriguing magnetic phases: the BL forms a highly-robust layered antiferromagnet with in-plane spin anisotropy, whereas the ML exhibits a spin-glass-like behavior below its freezing temperature, a phenomenon not previously observed in an atomically thin material. These findings highlight how reduced dimensionality can promote the emergence of unusual magnetic structures distinct from those of their three-dimensional counterparts, providing new insights into low-dimensional magnetism.

Molecular assemblies based on porphyrins (Pors), specifically Por nanotapes (NTs) containing magnetic metal ions, offer a versatile platform to explore magnetic interactions arising from the electronic interplay between -conjugated ligands and transition metal d -orbitals. Using on-surface synthesis under ultra-high vacuum, we synthesized -extended PorNTs of different lengths incorporating magnetic metal ions such as Fe and Co on Au(111). We resolved their atomic structure using scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). Differential conductance ( ) measurements, interpreted by extensive density functional theory calculations and theoretical modeling, reveal two distinct magnetic behaviors for the Fe- and Co-based systems. In FePorNTs, the magnetic interactions are dominated by strong Fe–ligand ferromagnetic coupling and weak antiferromagnetic Fe–Fe coupling. By contrast, CoPorNTs exhibit stronger Co–Co antiferromagnetic exchange and weaker Co–ligand coupling, with Kondo screening evident at the ligand sites. Our findings underscore the profound influence of metal centers, ligands, and substrate interactions on the magnetic and electronic properties of PorNTs, establishing these assemblies as interesting building blocks for low-dimensional magnetism and future spintronic or quantum-material applications.

N ‐Heterocyclische Carbene sind hocheffektive Liganden zur Verankerung funktioneller organischer Moleküle auf Metalloberflächen und Nanopartikeln, wodurch die Bildung selbstorganisierter Monoschichten erleichtert wird. Ihre Adsorption auf Oberflächen ist jedoch schwer vorherzusagen und zu kontrollieren, und die Geometrie von NHC‐Derivaten auf Goldoberflächen sowie die Rolle von Gold‐Adatomen stehen im Zentrum wissenschaftlicher Diskussionen. Hier präsentieren wir zwei Moleküle welche auf einem Benzimidazol‐NHC Motiv basieren, eines mit einem Thiophen‐Substituenten und das andere mit einem Br‐Atom. Mittels Tieftemperatur‐Rastertunnelmikroskopie (RTM) zeigen wir, dass beide Moleküle planar auf Au(111) adsorbieren und auf der Oberfläche chiral sind. Unsere Ergebnisse deuten darauf hin, dass in beiden Fällen ein Komplex zwischen NHC und einem Gold‐Adatom gebildet wird. Nach Spannungspulsen mit der RTM‐Spitze bewegen sich beide Komplexe durch inelastische Tunnel‐Elektronen. Für das Derivat mit Thiophen beobachten wir eine schrittweise 60°‐Rotation um das S‐Atom in eine Richtung. Die Drehrichtung wird sowohl durch die Chiralität als auch durch die Position des angewendeten Impulses bestimmt. Im Gegensatz dazu bewegt sich das NHC‐Derivat ohne Thiophen lateral auf der Oberfläche. Adsorption, Bindung an Goldatome und Bewegung werden durch Dichtefunktionaltheorie‐Berechnungen und Bildsimulationen unterstützt und diskutiert.

The properties of 2D materials are strongly influenced by their substrate, leading to a variety of "proximity effects" like screening, charge transfer, and hybridization. Surprisingly, there is a dearth of theoretical studies on these effects. Particularly, previous theoretical research on the Star of David (SOD) structure in 1T-NbSe$_2$ has focused on single-layer configurations or stacking with the same 1T phase without any real substrate. Here, we depart from these approaches and explore how these proximity effects shape the electronic and magnetic properties of the 1T-NbSe$_2$ phase when it is grown on the metallic 1H-NbSe$_2$ substrate. Using Density Functional Calculations, we establish a common framework to define the key characteristics of both free-standning 1T-NbSe$_2$ and 1H-NbSe$_2$. We then identify the optimal stacking arrangement for these two layers, revealing a transfer from the 1T to the 1H phase and a reorganization of charge within each layer. Our findings indicate that the magnetic moment of the SOD structure is still robust; however, is diminished due to a reduction in the on-site Coulomb interaction of the Hubbard bands. Additionally, the interlayer coupling induces metallicity in the 1T phase and increases the decoupling of the lower Hubbard band from the valence band.

Spin-crossover compounds can be switched between two stable states with different magnetic moments, conformations, electronic, and optical properties, which opens appealing perspectives for technological applications including miniaturization down to the scale of single molecules. Although control of the spin states is crucial their direct identification is challenging in single-molecule experiments. Here we investigate the spin-crossover complex [Fe(HB(1,2,4-triazol-1-yl)3)2] on a Cu(111) surface with scanning tunneling microscopy and density functional theory calculations. Spin crossover of single molecules in dense islands is achieved via electron injection. Spin-flip excitations are resolved in scanning tunneling spectra in a magnetic field enabling the direct identification of the molecular spin state, and revealing the existence of magnetic anisotropy in the HS molecules.