First principles studies of the electronic and structural properties of the rutile VO2(110) surface and its oxygen-rich terminations.

We present a density functional theory (DFT) study of the structural and electronic properties of the bare metallic rutile VO2(110) surface and its oxygen-rich terminations. Due to the polyvalent nature of vanadium and abundance of oxide phases, the modelling of this material on the DFT level remains a challenging task. We discuss the performance of various DFT functionals, including PBE, PBE +U(U= 2 eV), SCAN and SCAN + rVV functionals with non-magnetic and ferromagnetic spin ordering, and show that the calculated phase stabilities depend on the chosen functional. We predict the presence of a ring-like termination that is electronically and structurally related to an insulating V2O5(001) monolayer and shows a higher stability than pure oxygen adsorption phases. Our results show that employing the spin-polarized SCAN functional offers a good compromise, as it offers both a reasonable description of the structural and electronic properties of the rutile VO2bulk phase and the enthalpy of formation for oxygen rich vanadium phases present at the surface.

Density functional theory study of structural and thermodynamical stabilities of ferromagnetic MnX (X = P, As, Sb, Bi) compounds.

Density functional theory (DFT) calculations for deriving enthalpies of formation [Formula: see text] H for ferromagnetic MnX (X [Formula: see text] P, As, Sb, Bi) compounds were made for the two competing structures, hexagonal [Formula: see text] and orthorhombic [Formula: see text]. Standard calculations were performed by using pseudopotentials with the generalized-gradient-approximation (PBE) as exchange-correlation functional. Enhanced exchange-correlation interactions were included by making use of a so-called DFT[Formula: see text]U approach which requires [Formula: see text] as a parameter. Application of PBE potentials for all compounds and elementary phases (all-PBE) resulted in negative values of [Formula: see text] H for MnP and MnAs in both structures whereby the result for MnP [Formula: see text] agrees very well with experiment. For MnSb and MnBi the all-PBE calculation gives a positive nonbonding [Formula: see text] H disagreeing with experiment. To overcome this discrepancy for MnSb and MnBi a DFT[Formula: see text]U ansatz was employed for all compounds and elemental Mn. The values for [Formula: see text] ranging between 0.7 for MnBi and 1.4 eV for MnAs were determined by fitting the DFT results to measured data of [Formula: see text] H. As a reference for pure Mn the [Formula: see text]-Mn phase was taken with [Formula: see text] eV by which choice the experimental volume is fitted. Atomic volumes and ionicities were derived applying Bader's concept resulting in ionicities of Mn less than [Formula: see text].

The growth of ultra-thin zirconia films on Pd(3)Zr(0 0 0 1).

Despite its importance in many areas of industry, such as catalysis, fuel cell technology and microelectronics, the surface structure and physical properties of ZrO2 are not well understood. Following the successful growth of ultra-thin zirconia on Pt3Zr(0 0 0 1) (Antlanger et al 2012 Phys. Rev. B 86 035451), we report on recent progress into ZrO2 thin films, which were prepared by oxidation of a Pd3Zr(0 0 0 1) crystal. Results from scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS) as well as density-functional theory (DFT) are presented. Many sputter-annealing cycles are required for preparation of the clean Pd3Zr alloy surface, because oxygen easily dissolves in the bulk. By oxidation and post-annealing, a homogeneous ultra-thin ZrO2 film was obtained. This is an O-Zr-O trilayer based on cubic ZrO2(1 1 1). Using STM images corrected for distortion and creep of the piezo scanner the in-plane lattice parameter was determined as (351.2 ± 0.4) pm, slightly contracted with respect to the cubic ZrO2 bulk phase. The oxide forms an overlayer that is either incommensurate or has a very large superstructure cell (a = 8.3 nm); nevertheless its rotational orientation is always the same. In contrast to ultra-thin zirconia on Pt3Zr(0 0 0 1), where the uppermost substrate layer is pure (but reconstructed) Pt, STM and XPS suggest a stoichiometric Pd3Zr below the oxide. The oxide film binds to the substrate mainly via bonds between oxygen and the Zr atoms in the substrate. The ultra-thin oxide shows large buckling in STM, confirmed by DFT calculations, where the buckling of the Zr layer can exceed 100 pm. Compared to the ZrO2 film on Pt3Zr(0 0 0 1), the oxide on Pd3Zr(0 0 0 1) has the advantage that the substrate below does not reconstruct, leading to a homogeneous oxide film.

Experimental observation of defect pair separation triggering phase transitions.

First-order phase transitions typically exhibit a significant hysteresis resulting for instance in boiling retardation and supercooling. The hysteresis arises, because nucleation of the new phase is activated. The free-energy change is positive until the nucleus reaches a critical size beyond which further growth is downhill. In practice, the barrier is often circumvented by the presence of heterogeneous nucleation centres, e.g. at vessel walls or seed crystals. Recently, it has been proposed that the homogeneous melting of ice proceeds via separation of defect pairs with a substantially smaller barrier as compared to the mere aggregation of defects. Here we report the observation of an analogous mechanism catalysing a two-dimensional homogeneous phase transition. A similar process is believed to occur in spin systems. This suggests that separation of defect pairs is a common trigger for phase transitions. Partially circumventing the activation barrier it reduces the hysteresis and may promote fluctuations within a temperature range increasing with decreasing dimensionality.

p-electron magnetism in CdS doped with main group elements.

On the basis of ab initio supercell calculations employing density functional theory (DFT) and post-DFT methods, we investigate the behavior of main group element impurities (B, C, N, Al, Si, P, Ga, Ge) in wurtzite (w) and zincblende (zb) CdS lattices. It is found that the impurities prefer the sulfur position and most of them, depending on the concentration, exhibit magnetic order. We find that for small concentrations (64zb and 72w supercells) a half-metallic behavior is found. For a 16-atom supercell for both the zb- and w-structure partly also unsaturated magnetic moments occur. The field dependence of the magnetic moments in these materials may lead to new technological applications of these magnetic semiconductors as tunable spin injection materials.

Strain and structure driven complex magnetic ordering of a CoO overlayer on Ir(100).

We investigate the magnetic ordering in the ultrathin c(10×2) CoO(111) film supported on Ir(100) on the basis of ab initio calculations. We find a close relationship between the local structural properties of the oxide film and the induced magnetic order, leading to alternating ferromagnetically and antiferromagnetically ordered segments. While the local magnetic order is directly related to the geometric position of the Co atoms, the mismatch between the CoO film and the Ir substrate leads to a complex long-range order of the oxide.

FLAPW: applications and implementations.

Modern material design involves a close collaboration between experimental and computational materials scientists. To be useful, the theory must be able to accurately predict the stability and properties of new materials, describe the physics of the experiments, and be applicable to new and complex structures-the all-electron full-potential linearized augmented plane wave (FLAPW) is one such method that provides the requisite level of numerical accuracy, albeit at the cost of complexity. Technical aspects and modifications related to the choice of basis functions (energy parameters, core-valence orthogonality, extended local orbitals) that affect the applicability and accuracy of the method are described, as well as an approach for obtaining k-independent matrix elements. The inclusion of external electric fields is illustrated by results for the induced densities at the surfaces of both magnetic and non-magnetic metals, and the relationship to image planes and to nonlinear effects such as second harmonic generation. The magnetic coupling of core hole excitations in Fe, the calculation of intrinsic defect formation energies, the concentration-dependent chemical potentials, entropic contributions, and the relative phase stability of Zr-rich Zr-Al alloys are also discussed.

The phase diagram of halogens on Pt(110): structure of the (4 × 1)-Br/Pt(110) phase.

When adsorbed on the strongly anisotropic Pt(110) surface Br forms a sequence of (n × 1) structures. In the present study we investigate the (4 × 1) structure by scanning tunneling microscopy, quantitative low-energy electron diffraction and density-functional calculations. We show that the optimal structural model contains essentially the same adsorption sites as the (3 × 1) structure, but with a different preference. The positions of the substrate atom are consistent with a frozen surface phonon of fourfold periodicity, suggesting that the phase diagram can be understood on the basis of a tunable charge density wave (Swamy et al 2001 Phys. Rev. B 86 1299). The structure could also be explained by assuming short-range interactions only, but evidence is presented that adsorbate-adsorbate interactions mediated by quasi-one-dimensional surface resonances play a major role in both cases.

YCo2: intrinsic magnetic surface of a paramagnetic bulk material.

Here we report on results of a spin-resolved photoelectron spectroscopic (SRPES) study of YCo2 thin films (150 A-thick) grown on a W(110) substrate. The films were prepared by co-deposition of stoichiometric amounts of Y and Co onto a clean W surface followed by thermal annealing leading to (2x2) overstructure with respect to W(110) in the low-energy electron diffraction pattern indicated formation of a structurally ordered YCo2(111) surface. While no clear spin asymmetry was observed for bulk-sensitive SRPES data taken at hnu=1253.6 eV, the more surface-sensitive SRPES data obtained at hnu=21.2 eV photon energy revealed a clear spin-asymmetry probing the validity of the recent theoretical prediction.

Magnetism on the surface of the bulk paramagnetic intermetallic compound YCo2.

Using full-potential electronic structure calculations, we predict that the (111) surface of the cubic Laves phase Pauli paramagnet YCo2 is ferromagnetic. The magnetism of the (111) surface is independent of the termination of the surface, does not extend beyond two Co layers, and is related to the field-induced metamagnetism of the bulk. YCo2 appears to be a prominent candidate to demonstrate the phenomenon of surface-induced itinerant magnetism localized in two dimensions.

Adsorption sites and ligand effect for CO on an alloy surface: a direct view.

CO adsorption on a PtCo(111) surface was studied by scanning tunneling microscopy. Comparison of images with chemical contrast of Pt and Co and images showing the CO molecules indicates that CO resides exclusively on top of Pt sites and never on Co. CO bonding is highly sensitive to the chemical environment. The probability to find CO on a Pt atom increases drastically with the number of its Co nearest neighbors. Ab initio calculations show that this ligand effect is due to different positions of the center of the Pt d band.

Influence of impurities on localized transition metal surface states: scanning tunneling spectroscopy on V(001).

The first scanning tunneling spectroscopy measurements on V(001) are reported. A strong surface state is detected which is very sensitive to the presence of segregated carbon impurities. The surface state energy shifts from 0.03 eV below the Fermi level at clean areas towards higher energies (up to approximately 0.2 eV) at contaminated areas. Because of the negative dispersion of this state, the upward shift cannot be described in a simple confinement picture. Rather, the surface state energy is governed by vanadium surface s- d interactions which are altered by carbon coverage.

Bulk terminated NaCl(111) on aluminum: a polar surface of an ionic crystal?

Atomically resolved scanning tunneling microscopy reveals the existence of triangular (111) bulk terminated NaCl islands. The islands can be grown by subsequent adsorption of Na and Cl2 on Al(111) and Al(100) or by conversion of stoichiometric NaCl(100) islands to NaCl(111) via additional Na adsorption. The NaCl(111) islands have Na-Cl-Na sandwich structure. Ab initio calculations of the electronic structure of these islands show that each of the Na atoms carries half a positive elementary charge, leaving the islands neutral and explaining the existence of an otherwise unstable surface.