Resolving Ambiguity of the Kondo Temperature Determination in Mechanically Tunable Single-Molecule Kondo Systems.

Determination of the molecular Kondo temperature () poses a challenge in most cases when the experimental temperature cannot be tuned to a sufficient extent. We show how this ambiguity can be resolved if additional control parameters are present, such as magnetic field and mechanical gating. We record the evolution of the differential conductance by lifting an individual molecule from the metal surface with the tip of a scanning tunneling microscope.

Local stiffness and work function variations of hexagonal boron nitride on Cu(111).

Combined scanning tunnelling and atomic force microscopy using a qPlus sensor enables the measurement of electronic and mechanic properties of two-dimensional materials at the nanoscale. In this work, we study hexagonal boron nitride (-BN), an atomically thin 2D layer, that is van der Waals-coupled to a Cu(111) surface. The system is of interest as a decoupling layer for functional 2D heterostructures due to the preservation of the -BN bandgap and as a template for atomic and molecular adsorbates owing to its local electronic trapping potential due to the in-plane electric field.

Free coherent evolution of a coupled atomic spin system initialized by electron scattering.

Full insight into the dynamics of a coupled quantum system depends on the ability to follow the effect of a local excitation in real-time. Here, we trace the free coherent evolution of a pair of coupled atomic spins by means of scanning tunneling microscopy. Rather than using microwave pulses, we use a direct-current pump-probe scheme to detect the local magnetization after a current-induced excitation performed on one of the spins.

Sensing the Spin of an Individual Ce Adatom.

The magnetic moment of rare earth elements originates from electrons in the partially filled 4f orbitals. Accessing this moment electrically by scanning tunneling spectroscopy is hampered by shielding of outerlying orbitals. Here, we show that we can detect the magnetic moment of an individual Ce atom adsorbed on a Cu_{2}N ultrathin film on Cu(100) by using a sensor tip that has its apex functionalized with a Kondo screened spin system.

Long Spin-Relaxation Times in a Transition-Metal Atom in Direct Contact to a Metal Substrate.

Long spin-relaxation times are a prerequisite for the use of spins in data storage or nanospintronics technologies. An atomic-scale solid-state realization of such a system is the spin of a transition-metal atom adsorbed on a suitable substrate. For the case of a metallic substrate, which enables the direct addressing of the spin by conduction electrons, the experimentally measured lifetimes reported to date are on the order of only hundreds of femtoseconds.

Building Complex Kondo Impurities by Manipulating Entangled Spin Chains.

The creation of molecule-like structures in which magnetic atoms interact controllably is full of potential for the study of complex or strongly correlated systems. Here, we create spin chains in which a strongly correlated Kondo state emerges from magnetic coupling of transition-metal atoms. We build chains up to ten atoms in length by placing Fe and Mn atoms on a CuN surface with a scanning tunneling microscope.

A molecular quantum spin network controlled by a single qubit.

Scalable quantum technologies require an unprecedented combination of precision and complexity for designing stable structures of well-controllable quantum systems on the nanoscale. It is a challenging task to find a suitable elementary building block, of which a quantum network can be comprised in a scalable way. We present the working principle of such a basic unit, engineered using molecular chemistry, whose collective control and readout are executed using a nitrogen vacancy (NV) center in diamond.

Potential energy-driven spin manipulation via a controllable hydrogen ligand.

Spin-bearing molecules can be stabilized on surfaces and in junctions with desirable properties, such as a net spin that can be adjusted by external stimuli. Using scanning probes, initial and final spin states can be deduced from topographic or spectroscopic data, but how the system transitions between these states is largely unknown. We address this question by manipulating the total spin of magnetic cobalt hydride complexes on a corrugated boron nitride surface with a hydrogen-functionalized scanning probe tip by simultaneously tracking force and conductance.

Correlation-driven transport asymmetries through coupled spins in a tunnel junction.

Spin-spin correlations can be the driving force that favours certain ground states and are key in numerous models that describe the behaviour of strongly correlated materials. While the sum of collective correlations usually lead to a macroscopically measurable change in properties, a direct quantification of correlations in atomic scale systems is difficult. Here we determine the correlations between a strongly hybridized spin impurity on the tip of a scanning tunnelling microscope and its electron bath by varying the coupling to a second spin impurity weakly hybridized to the sample surface.

Mass Spectrometry as a Preparative Tool for the Surface Science of Large Molecules.

Measuring and understanding the complexity that arises when nanostructures interact with their environment are one of the major current challenges of nanoscale science and technology. High-resolution microscopy methods such as scanning probe microscopy have the capacity to investigate nanoscale systems with ultimate precision, for which, however, atomic scale precise preparation methods of surface science are a necessity. Preparative mass spectrometry (pMS), defined as the controlled deposition of m/z filtered ion beams, with soft ionization sources links the world of large, biological molecules and surface science, enabling atomic scale chemical control of molecular deposition in ultrahigh vacuum (UHV).

Quantum engineering of spin and anisotropy in magnetic molecular junctions.

Single molecule magnets and single spin centres can be individually addressed when coupled to contacts forming an electrical junction. To control and engineer the magnetism of quantum devices, it is necessary to quantify how the structural and chemical environment of the junction affects the spin centre. Metrics such as coordination number or symmetry provide a simple method to quantify the local environment, but neglect the many-body interactions of an impurity spin coupled to contacts.

Spin polarization of the split Kondo state.

Spin-resolved scanning tunneling microscopy is employed to quantitatively determine the spin polarization of the magnetic field-split Kondo state. Tunneling conductance spectra of a Kondo-screened magnetic atom are evaluated within a simple model taking into account inelastic tunneling due to spin excitations and two Kondo peaks positioned symmetrically around the Fermi energy. We fit the spin state of the Kondo-screened atom with a spin Hamiltonian independent of the Kondo effect and account for Zeeman splitting of the Kondo peak in the magnetic field.

Tracking temperature-dependent relaxation times of ferritin nanomagnets with a wideband quantum spectrometer.

We demonstrate the tracking of the spin dynamics of ensemble and individual magnetic ferritin proteins from cryogenic up to room temperature using the nitrogen-vacancy color center in diamond as a magnetic sensor. We employ different detection protocols to probe the influence of the ferritin nanomagnets on the longitudinal and transverse relaxation of the nitrogen-vacancy center, which enables magnetic sensing over a wide frequency range from Hz to GHz. The temperature dependence of the observed spectral features can be well understood by the thermally induced magnetization reversals of the ferritin and enables the determination of the anisotropy barrier of single ferritin molecules.

Lateral and vertical stiffness of the epitaxial h-BN monolayer on Rh(111).

The response to strain in covalently bound single layers has a large impact on the growth and properties. We investigate the quasi-two-dimensional hexagonal boron nitride on Rh(111), which is interesting due to its high intrinsic corrugation. We use combined atomic force and scanning tunneling microscopy to measure the response of this monolayer to probing forces.

Temperature and magnetic field dependence of a Kondo system in the weak coupling regime.

The Kondo effect arises due to the interaction between a localized spin and the electrons of a surrounding host. Studies of individual magnetic impurities by scanning tunneling spectroscopy have renewed interest in Kondo physics; however, a quantitative comparison with theoretical predictions remained challenging. Here we show that the zero-bias anomaly detected on an organic radical weakly coupled to a Au (111) surface can be described with astonishing agreement by perturbation theory as originally developed by Kondo 60 years ago.

The quantum magnetism of individual manganese-12-acetate molecular magnets anchored at surfaces.

The high intrinsic spin and long spin relaxation time of manganese-12-acetate (Mn(12)) makes it an archetypical single molecular magnet. While these characteristics have been measured on bulk samples, questions remain whether the magnetic properties replicate themselves in surface supported isolated molecules, a prerequisite for any application. Here we demonstrate that electrospray ion beam deposition facilitates grafting of intact Mn(12) molecules on metal as well as ultrathin insulating surfaces enabling submolecular resolution imaging by scanning tunneling microscopy.

Interplay of conductance, force, and structural change in metallic point contacts.

The coupling between two atomically sharp nanocontacts provides tunable access to a fundamental underlying interaction: the formation of the bond between two atoms as they are brought into contact. Here we report a detailed experimental and theoretical analysis of the relation between the chemical force and the tunneling current during bond formation in atom-scale metallic junctions and their dependence on distance, junction structure, and material. We found that the short-range force as well as the conductance in two prototypical metal junctions depend exponentially on the distance and that they have essentially the same exponents.

Spectroscopic manifestations of the Kondo effect on single adatoms.

The present topical review focuses on recent advances concerning an intriguing phenomenon in condensed matter physics, the scattering of conduction electrons at the localized spin of a magnetic impurity: the Kondo effect. Spectroscopic signatures of this effect have been observed in the past by high-resolution photoemission which, however, has the drawback of averaging over a typical surface area of 1 mm(2). By combining the atomic-scale spatial resolution of the scanning tunneling microscope (STM) with an energy resolution of a few tens of µeV achievable nowadays in scanning tunneling spectroscopy (STS), and by exposing the magnetic adatom to external magnetic fields, our understanding of the interaction of a single magnetic impurity with the conduction electrons of the nonmagnetic host has been considerably deepened.

The force needed to move an atom on a surface.

Manipulation of individual atoms and molecules by scanning probe microscopy offers the ability of controlled assembly at the single-atom scale. However, the driving forces behind atomic manipulation have not yet been measured. We used an atomic force microscope to measure the vertical and lateral forces exerted on individual adsorbed atoms or molecules by the probe tip.

Diatomic molecular switches to enable the observation of very-low-energy vibrations.

Using low-temperature scanning tunneling microscopy and spectroscopy, we found that the coadsorption of atomic hydrogen to single transition-metal and rare-earth-metal atoms on a Ag(100) surface gives rise to surprising phenomena, a bias dependent switching from a large to a small apparent size of the diatomic molecules and a concomitant appearance of very low-energy vibrational features of 3 to 7 meV in the differential conductance spectra. These phenomena, which have until now escaped observation, may be of general relevance for low-temperature adsorption.

Large magnetic anisotropy of a single atomic spin embedded in a surface molecular network.

Magnetic anisotropy allows magnets to maintain their direction of magnetization over time. Using a scanning tunneling microscope to observe spin excitations, we determined the orientation and strength of the anisotropies of individual iron and manganese atoms on a thin layer of copper nitride. The relative intensities of the inelastic tunneling processes are consistent with dipolar interactions, as seen for inelastic neutron scattering.

Scanning-tunneling spectroscopy of surface-state electrons scattered by a slightly disordered two-dimensional dilute "solid": Ce on Ag(111).

Low temperature (3.9 K) scanning-tunneling spectroscopy on a hexagonal superlattice of Ce adatoms on Ag(111) reveals site-dependent characteristic features in differential conductance spectra and in spectroscopic images at atomic-scale spatial resolution. Using a tight-binding model, we relate the overall spectral structures to the scattering of Ag(111) surface-state electrons by the Ce adatoms, the site dependence to the disorder induced by imperfections of the superlattice, and the opening of a gap in the local density of states to the observed stabilization of superlattices with adatom distances in the range of 2.

Creation of an atomic superlattice by immersing metallic adatoms in a two-dimensional electron sea.

Cerium adatoms, deposited on a Ag(111) surface, are found by low-temperature scanning tunneling microscopy to self-assemble into large ordered hexagonal arrays covering macroscopically the entire surface. We show that the 32 A periodicity of the superlattice is caused by the interaction of surface-state electrons with Ce adatoms and that the large-scale formation of the superlattice is governed by a subtle balance between the sample temperature, the surface diffusion barrier, and the concentration-dependent adatom interaction potential.