Spin Excitations in a 4f-3d Heterodimer on MgO.

We report on the magnetic properties of HoCo dimers as a model system for the smallest intermetallic compound of a lanthanide and a transition metal atom. The dimers are adsorbed on ultrathin MgO(100) films grown on Ag(100). New for 4f elements, we detect inelastic excitations with scanning tunneling spectroscopy and prove their magnetic origin by applying an external magnetic field. In combination with density functional theory and spin Hamiltonian analysis, we determine the magnetic level distribution, as well as sign and magnitude of the exchange interaction between the two atoms. In contrast to typical 4f-3d bulk compounds, we find ferromagnetic coupling in the dimer.

Magnetic remanence in single atoms.

A permanent magnet retains a substantial fraction of its saturation magnetization in the absence of an external magnetic field. Realizing magnetic remanence in a single atom allows for storing and processing information in the smallest unit of matter. We show that individual holmium (Ho) atoms adsorbed on ultrathin MgO(100) layers on Ag(100) exhibit magnetic remanence up to a temperature of 30 kelvin and a relaxation time of 1500 seconds at 10 kelvin. This extraordinary stability is achieved by the realization of a symmetry-protected magnetic ground state and by decoupling the Ho spin from the underlying metal by a tunnel barrier.

Edge state magnetism in zigzag-interfaced graphene via spin susceptibility measurements.

Development of graphene spintronic devices relies on transforming it into a material with a spin order. Attempts to make graphene magnetic by introducing zigzag edge states have failed due to energetically unstable structure of torn zigzag edges. Here, we report on the formation of nanoridges, i.e., stable crystallographically oriented fluorine monoatomic chains, and provide experimental evidence for strongly coupled magnetic states at the graphene-fluorographene interfaces. From the first principle calculations, the spins at the localized edge states are ferromagnetically ordered within each of the zigzag interface whereas the spin interaction across a nanoridge is antiferromagnetic. Magnetic susceptibility data agree with this physical picture and exhibit behaviour typical of quantum spin-ladder system with ferromagnetic legs and antiferromagnetic rungs. The exchange coupling constant along the rungs is measured to be 450 K. The coupling is strong enough to consider graphene with fluorine nanoridges as a candidate for a room temperature spintronics material.

Transition from Mn(4+) to Mn(3+) induced by surface reconstruction at λ-MnO(2)(001).

Structural and electronic properties of the λ-MnO(2)(001) surface are investigated applying density functional theory approach. The calculations show that all Mn ions at unreconstructed smooth surface preserve the +4 oxidation state observed in the bulk. Upon the λ-MnO(2)(001) reconstruction, one fourth of Mn ions at the surface undergo a change of the oxidation state from +4 to +3, although the reconstruction does not change the Mn coordination number with oxygen. This is accompanied with the filling of initially empty 3d(z(2) ) states localized on cations with one electron denoted by two neighboring O atoms. Although the reconstruction leads to an energy gain of 0.04 eV per surface unit cell, it is not a spontaneous process since it proceeds with an activation energy of 0.12 eV.

Extended atomic hydrogen dimer configurations on the graphite(0001) surface.

We present density functional theory calculations and scanning tunneling microscopy experiments investigating the structures and kinetics of extended hydrogen dimer configurations on the graphite (0001) surface. We identify several hydrogen dimer structures where surface mediated interactions between the two hydrogen atoms lead to increased binding energy even at interatom separations as large as 7 A. By modeling the formation of dimers as sequential adsorption of hydrogen atoms, we find that these dimer configurations exhibit decreased barriers to sticking for the second H atom, compared to the sticking barrier of an H atom on the clean surface. According to our calculations, the activation energies for desorption of a single H atom from any of the experimentally observed extended dimers are higher than the barriers for diffusion to the paradimer configuration. Consequently, molecular hydrogen formation out of the extended dimer structures takes place via diffusion over the paradimer configuration.

Translational energy and state resolved observations of D and D2 thermally desorbing from D clusters chemisorbed on graphite.

Direct D atom desorption, as well as associative desorption of D(2) molecules are observed in thermal desorption from D atoms chemisorbed on a C(0001) surface by combining laser induced T-jumps with resonance enhanced multiphoton ionization detection. Bleaching curves suggest that different classes of chemisorbed D atom clusters are present on the initial surface. The energy resolved atomic desorption flux, obtained via time of flight techniques, compares favorably (via detailed balance) with theoretical calculations of atomic sticking. Density functional theory calculations of chemical processes (atomic desorption, atomic diffusion/cluster annealing, and associative desorption) on an extensive set of four atom H(D) clusters chemisorbed on C(0001) provide a good interpretation of the experiments. State and energy resolved D(2) desorption fluxes are compared with previous state averaged results. In combination with density functional theory calculations these measurements reveal a substantial energy loss (>1 eV) to the surface in the associative desorption.

Probing enantioselectivity with x-ray photoelectron spectroscopy and density functional theory.

The enantioselectivity of gold is investigated by x-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Cysteine molecules on a chiral Au(17 11 9);{S} surface show enantiospecific core level binding energies in the amino and in the thiol group. The sign and order of magnitude of the XPS core level shifts is reproduced by DFT. Identical preparations of D- and L-cysteine layers lead to D-cysteine molecules in the pure NH2 form, while a small portion of L-cysteine molecules maintains a hydrogen rich amino group (NH3). This implies enantiospecific adsorption reaction pathways and is consistent with DFT that indicates an activated hydrogen abstraction reaction from the amino group, which is downhill for D-cysteine.

Clustering of chemisorbed H(D) atoms on the graphite (0001) surface due to preferential sticking.

We present scanning tunneling microscopy experiments and density functional theory calculations which reveal a unique mechanism for the formation of hydrogen adsorbate clusters on graphite surfaces. Our results show that diffusion of hydrogen atoms is largely inactive and that clustering is a consequence of preferential sticking into specific adsorbate structures. These surprising findings are caused by reduced or even vanishing adsorption barriers for hydrogen in the vicinity of already adsorbed H atoms on the surface and point to a possible novel route to interstellar H2 formation.

Metastable structures and recombination pathways for atomic hydrogen on the graphite (0001) surface.

We present scanning tunneling microscopy results which reveal the existence of two distinct hydrogen dimer states on graphite basal planes. Density functional theory calculations allow us to identify the atomic structure of these states and to determine their recombination and desorption pathways. Direct recombination is only possible from one of the two dimer states. This results in increased stability of one dimer species and explains the puzzling double peak structure observed in temperature programmed desorption spectra for hydrogen on graphite.

Chiral recognition of organic molecules by atomic kinks on surfaces.

Two distinct non-mirror-symmetric conformations of D- and L-cysteine were found after adsorption on Au(17 11 9)S. This demonstrates chiral heterorecognition, i.e., enantioselectivity of S kinks on vicinal Au(111). The structures as determined by angle scanned x-ray photoelectron diffraction agree well with those from density functional theory calculations. The calculations predict adsorption energies of approximately 2 eV where D-cysteine binds 140 meV stronger than L-cysteine. The classical three point contact model for molecular recognition fails to explain these findings.

Supported fe nanoclusters: evolution of magnetic properties with cluster size.

Using a density functional approach, we study structural and magnetic properties of small Fe(n) clusters (n

Distinct reaction mechanisms in the catalytic oxidation of carbon monoxide on Rh(110): scanning tunneling microscopy and density functional theory studies.

By means of scanning tunneling microscopy measurements and density functional theory calculations, we identify the reaction mechanism for the oxidation of carbon monoxide to carbon dioxide on the Rh(110) surface at 160 K, which appears to be completely different than the one active at room temperature. The reasons for these different behaviors are determined. Our results demonstrate that even for a very simple catalytic reaction, the microscopic mechanism can dramatically change with temperature, following pathways that differ for nucleation sites and surface propagation and involve different surface moieties.

Oxygen dissociation at Pt steps.

Using scanning tunneling microscopy, thermal energy atom scattering, and density functional theory we have characterized O (2) dissociation on Pt(111) stepped surfaces at the atomic scale. The most reactive site is at the top of the Pt steps. In both the molecular precursor state (MPS) and the transition state (TS), the O (2) has its axis aligned parallel to the step edge. Controlled step decoration with Ag monatomic chains was used to locally tune the reactivity of Pt step sites. The enhanced reactivity at the Pt step sites is not caused by a decrease of the local dissociation barriers from the MPS but is related to a stabilization of both the MPS and TS.