Publications

The structure of the oxygen induced (1×5) reconstruction of V(100)

The adsorption of 1 L oxygen on the V(100) surface at 200°C leads to a (1×5) reconstruction. The structure of this surface was determined by scanning tunneling microscopy (STM), low energy electron diffraction (quantitative LEED) and density functional theory calculations. The STM images show dark lines every fifth vanadium lattice constant. Between these (1×5) lines, oxygen resides in four-fold coordinated hollow sites with a coverage of ≈70%. From the LEED analysis (Pendry R-factor=0.17) it was found that in the dark lines oxygen adsorbs in bridge sites, in agreement with the ab initio calculations. The driving force behind this reconstruction is surface stress induced by the vanadium–oxygen bonds.

Ionization Potentials of Solids: The Importance of Vertex Corrections

The ionization potential is a fundamental key quantity with great relevance to diverse material properties. We find that state of the art methods based on density functional theory and simple diagrammatic approaches as commonly taken in the $GW$ approximation predict the ionization potentials of semiconductors and insulators unsatisfactorily. Good agreement between theory and experiment is obtained only when diagrams resulting from the antisymmetry of the many-electron wave function are taken into account via vertex corrections in the self-energy. The present approach describes both localized and delocalized states accurately, making it ideally suited for a wide class of materials and processes.

Hidden correlation and quasiparticle spectra in valence-skipper Peierls semiconductor $\rm BaBiO_3$ from self-consistent GW

$\rm BaBiO_3$ is characterized by a charge disproportionation with half of the Bi atoms possessing a valence 3+ and half a valence 5+. Because of selfinteraction errors, local and semi-local density functionals fail to describe the charge disproportionation quantitatively, yielding a too small structural distortion and no band gap. Using hybrid functionals we obtain a satisfactory description of the structural, electronic, optical, and vibrational properties of $\rm BaBiO_3$. The results obtained using GW (Green's function G and screened Coulomb potential W) based schemes on top of hybrid functionals, including fully selfconsistent GW calculations with vertex corrections in the dielectric screening, qualitatively confirm the HSE picture but a systematic overestimation of the bandgap by about 0.4 eV is observed.

Small polarons and their interaction with donor centers in Titania

No abstract is provided for this article.

Stressing Pd atoms: Initial oxidation of the Pd(110) surface

We have investigated the oxygen induced structures of the Pd(110) surface in the pressure range of 10 - 5 – 10 - 3 mbar of oxygen, at a sample temperature of around 300°C. These structures, denoted as “ ( 7 × 3 ) ” and “ ( 9 × 3 ) ”, are studied in detail by the use of a combination of low-energy electron diffraction, scanning tunneling microscopy, high-resolution core level spectroscopy, and ab-initio simulations. Based on our data a model is proposed for these structures containing segments of Pd atoms in the [ 1 1 ¯ 0 ] direction, in which the Pd rows are decorated by O atoms in a zig-zag pattern. The segments are periodically separated by displaced Pd atoms. Density functional theory calculations show that the displacements reduce the oxygen induced stress significantly, as compared to a structure with no displacements. The calculations also suggest that the new structures are stabilized by domain formation.

Converged <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>G</mml:mi><mml:mi>W</mml:mi></mml:mrow></mml:math> quasiparticle energies for transition metal oxide perovskites

The ab initio calculation of quasiparticle (QP) energies is a technically and computationally challenging problem. In condensed matter physics, the most widely used approach to determine QP energies is the $GW$ approximation. Although the $GW$ method has been widely applied to many typical semiconductors and insulators, its application to more complex compounds such as transition metal oxide perovskites has been comparatively rare, and its proper use is not well established from a technical point of view. In this work, we have applied the single-shot ${G}_{0}{W}_{0}$ method to a representative set of transition metal oxide perovskites including $3d$ (${\mathrm{SrTiO}}_{3}, {\mathrm{LaScO}}_{3}, {\mathrm{SrMnO}}_{3}, {\mathrm{LaTiO}}_{3}, {\mathrm{LaVO}}_{3}, {\mathrm{LaCrO}}_{3}, {\mathrm{LaMnO}}_{3}$, and ${\mathrm{LaFeO}}_{3}$), $4d$ (${\mathrm{SrZrO}}_{3}, {\mathrm{SrTcO}}_{3}$, and ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$), and $5d$ (${\mathrm{SrHfO}}_{3}, {\mathrm{KTaO}}_{3}$, and ${\mathrm{NaOsO}}_{3}$) compounds with different electronic configurations, magnetic orderings, structural characteristics, and band gaps ranging from 0.1 to 6.1 eV. We discuss the proper procedure to obtain well-converged QP energies and accurate band gaps within single-shot ${G}_{0}{W}_{0}$ by comparing the conventional approach based on an incremental variation of a specific set of parameters (number of bands, energy cutoff for the plane-wave expansion and number of k points) and the basis-set extrapolation scheme [J. Klime\ifmmode \check{s}\else \v{s}\fi{} et al., Phys. Rev. B 90, 075125 (2014)]. Although the conventional scheme is not supported by a formal proof of convergence, for most cases it delivers QP energies in reasonably good agreement with those obtained by the basis-set correction procedure and it is by construction more useful for calculating band structures. In addition, we have inspected the difference between the adoption of norm-conserving and ultrasoft potentials in $GW$ calculations and found that the norm violation for the $d$ shell can lead to less accurate results in particular for charge-transfer systems and late transition metals. A minimal statistical analysis indicates that the correlation of the $GW$ data with the density functional theory gap is more robust than the correlation with the experimental gaps; moreover, we identify the static dielectric constant as alternative useful parameter for the approximation of $GW$ gap in high-throughput automatic procedures. Finally, we compute the QP band structure and spectra within the random phase approximation and compare the results with available experimental data.

Density functional theory calculations of adsorption of water at calcium oxide and calcium fluoride surfaces

Electronic structure calculations using the density functional theory (DFT) within the generalized-gradient approximation and ultra-soft pseudopotentials have been used to investigate the adsorption of water on the main cleavage planes of CaO and CaF2. The calculated structural parameters are found to be in good agreement with experiment. The unhydrated surfaces show negligible ionic relaxation from bulk termination due to the minimal distortion of the electron density in the surface layer. Electron density plots show both crystals to be strongly ionic. We found on both mineral surfaces that associative adsorption of water is energetically preferred. Water molecules which were initially dissociatively adsorbed, recombined to form associatively adsorbed species. The water molecules are adsorbed by their oxygen ion to surface calcium ions, but electron density plots show strong interactions between surface anions and hydrogen atoms. The calculated hydration energies of approximately 70kJmol−1 on the CaO {100} surface and 41–53kJmol−1 on the CaF2 {111} surface indicate physisorption on both surfaces.

The Pdð 100 Þ-ð ffiffiffi pffiffiffi p ÞR27 -O surface oxide: A LEED, DFT and STM study

Using low energy electron diffraction (LEED), density functional theory (DFT) and scanning tunneling microscopy (STM), we have re-analyzed the Pdð 100 Þ-ð ffiffiffi

Pathways to dissociation of O2 on Cu (110) surface: first principles simulations

First principles simulations based on density functional theory have been used to determine absorption energies and pathways to dissociation of oxygen at finite temperature on the Cu (110) surface. Our results, which are in accord with recent experimental studies, suggest that adsorption kinetics play an important role in determining the mechanism of dissociation. Energy minimisation studies confirm that the HL site within the fourfold hollow is energetically most favourable for molecular adsorption where the O2 is peroxo like. Studies using the NEB method show that asymmetric rather than symmetric dissociation is preferred at this site and that the energy barrier for dissociation is virtually zero along the [001] direction and 120meV for the [11̄0] direction. We have identified the [11̄0] orientation at this site as a possible precursor state but only at very low temperatures. Ab initio dynamic simulations were used to examine dissociation on a substrate initially at 300K for collisions normal to the surface with a kinetic energy of either 50 or 500meV. Particularly for the lower impact energy molecules carry out complex motions once at the surface and significant steering is observed. In some cases molecules are observed to navigate several atomic distances across the surface before arriving at a fourfold hollow and adopting a favourable orientation prior to dissociation. On the time scale of the simulations (up to 5ps) some molecules were observed to become trapped on short-bridge sites with the [11̄0] orientation; in this case the electronic structure is superoxo like.

Small polarons and their interaction with donor centers in Titania

<i>GW</i>100: A Plane Wave Perspective for Small Molecules

In a recent work, van Setten and co-workers have presented a carefully converged G0W0 study of 100 closed shell molecules [ J. Chem. Theory Comput. 2015 , 11 , 5665 - 5687 ]. For two different codes they found excellent agreement to within a few 10 meV if identical Gaussian basis sets were used. We inspect the same set of molecules using the projector augmented wave method and the Vienna ab initio simulation package (VASP). For the ionization potential, the basis set extrapolated plane wave results agree very well with the Gaussian basis sets, often reaching better than 50 meV agreement. In order to achieve this agreement, we correct for finite basis set errors as well as errors introduced by periodically repeated images. For positive electron affinities differences between Gaussian basis sets and VASP are slightly larger. We attribute this to larger basis set extrapolation errors for the Gaussian basis sets. For quasi particle (QP) resonances above the vacuum level, differences between VASP and Gaussian basis sets are, however, found to be substantial. This is tentatively explained by insufficient basis set convergence of the Gaussian type orbital calculations as exemplified for selected test cases.

Electronic State Unfolding for Plane Waves: Energy Bands, Fermi Surfaces, and Spectral Functions

Modern computing facilities grant access to first-principles density-functional theory study of complex physical and chemical phenomena in materials, that require large supercell to properly model the system. However, supercells are associated to small Brillouin zones in the reciprocal space, leading to folded electronic eigenstates that make the analysis and interpretation extremely challenging. Various techniques have been proposed and developed in order to reconstruct the electronic band structures of super cells, unfolded into the reciprocal space of an ideal primitive cell. Here, we propose an efficient unfolding scheme embedded directly in the Vienna Ab-initio Simulation Package (VASP), that requires modest computational resources and allows for an automatized mapping from the reciprocal space of the supercell to primitive cell Brillouin zone. This algorithm can computes band structures, Fermi surfaces and spectral functions, by using an integrated post-processing tool (bands4vasp). The method is here applied to a selected variety of complex physical situations: the effect of doping on the band dispersion in the BaFe$_{\rm 2(1-x)}$Ru$_{\rm 2x}$As$_2$ superconductor, the interaction between adsorbates and polaronic states on the TiO$_2$(110) surface, and the band splitting induced by non-collinear spin fluctuations in EuCd$_2$As$_2$.

Modeling cation ordering in A(B[sub 1/3][sup ʹ]B[sub 2/3][sup ʺ])O[sub 3] perovskites

Ionic model (SSCAD) calculations were performed for 36 different cation ordered supercell configurations in the pseudobinary perovskite related system BaTaO3−BaZnO3. Ba(Zn1/3Ta2/3)O3 (BZT) in a P3̄m1 ordered structure is the only single phase compound that is observed experimentally, and the one that SSCAD calculations predict as having lowest formation energy. It is therefore the presumed ground state at the BZT composition. The SSCAD results are supported by first principles VASP pseudopotential calculations which were performed for BaTaO3, BaZnO3, Ba(Zn1/3Ta2/3)O3, and Ba(Zn1/2Ta1/2)O3 with NaCl type ordering of Zn and Ta. Finite temperature calculations that are based on the SSCAD results, predict a strongly first-order P3̄m1→Pm3̄m transition, but the predicted TC appears to be about an order of magnitude too high.

Raman spectroscopy of template grown single wall carbon nanotubes in zeolite crystals

Single wall carbon nanotubes with diameter 0.4 nm grown in the channels of AlPO4-5 crystals were studied by Raman spectroscopy and ab initio density functional calculations. In the experiment up to 19 different laser lines were used to characterize vibrational properties. Spectra depend strongly on the energy of the laser line used for excitation. Even though the observed Raman spectra were very rich on lines only two types of nanotubes with different chiralities, (5,0) and (4,2), were found to be responsible for the observed response. The frequencies of the radial breathing modes were reliably assigned. Even though the (5,0) is metallic, the A1g mode does not couple to the electronic continuum and the Peierls-type mechanism does not shift the mode toward lower frequencies. A strong response was also observed for frequencies around 1250 cm−1. The positions of two peaks assigned to the (5,0) do not depend on the laser energy whereas only one peak was observed for the (4,2) nanotube. Its frequency shifts with the laser energy like the D line of large diameter nanotubes, but the rate of the shift is only one half of the value known for the latter. These unexpected results could be traced back to the phonon dispersion of the narrow tubes.

Metastable surface oxide on CoGa(100): Structure and stability

We investigated the structure and formation of a surface oxide and bulk $\ensuremath{\beta}{\text{-Ga}}_{2}{\text{O}}_{3}$ on CoGa(100) from ultrahigh vacuum to 1 bar oxygen pressure in a temperature range from 300 to 1040 K. We combined in situ surface x-ray diffraction with scanning tunneling microscopy, atomic force microscopy, and density-functional theory calculations. We find that the two-dimensional epitaxial surface oxide layer exhibits a $p2mm$ symmetry with an additional mirror plane as compared to the bulk oxide. The surface oxide layer is found to form under metastable conditions at an oxygen chemical potential $\ensuremath{\sim}1.6\text{ }\text{eV}$ above the stability limit for bulk $\ensuremath{\beta}{\text{-Ga}}_{2}{\text{O}}_{3}$. The formation of the bulk oxide is kinetically hindered by the presence of the oxygen-terminated surface oxide, which most likely hampers dissociative oxygen chemisorption. We observe that below 620 K, the surface oxide is surprisingly stable at 1 bar oxygen pressure. Substrate faceting accompanies the bulk oxide formation at temperatures higher than 1020 K.

Toward Large-Scale AFQMC Calculations: Large Time Step Auxiliary-Field Quantum Monte Carlo

We report modifications of the ph-AFQMC algorithm that allow the use of large time steps and reliable time step extrapolation. Our modified algorithm eliminates size-consistency errors present in the standard algorithm when large time steps are employed. We investigate various methods to approximate the exponential of the one-body operator within the AFQMC framework, distinctly demonstrating the superiority of Krylov methods over the conventional Taylor expansion. We assess various propagators within AFQMC and demonstrate that the Split-2 propagator is the optimal method, exhibiting the smallest time-step errors. For the HEAT set molecules, the time-step extrapolated energies deviate on average by only 0.19 kcal/mol from the accurate small time-step energies. For small water clusters, we obtain accurate complete basis-set binding energies using time-step extrapolation with a mean absolute error of 0.07 kcal/mol compared to CCSD(T). Using large time-step ph-AFQMC for the N2 dimer, we show that accurate bond lengths can be obtained while reducing CPU time by an order of magnitude.

How to verify the precision of density-functional-theory implementations via reproducible and universal workflows

In the past decades many density-functional theory methods and codes adopting periodic boundary conditions have been developed and are now extensively used in condensed matter physics and materials science research. Only in 2016, however, their precision (i.e., to which extent properties computed with different codes agree among each other) was systematically assessed on elemental crystals: a first crucial step to evaluate the reliability of such computations. We discuss here general recommendations for verification studies aiming at further testing precision and transferability of density-functional-theory computational approaches and codes. We illustrate such recommendations using a greatly expanded protocol covering the whole periodic table from Z=1 to 96 and characterizing 10 prototypical cubic compounds for each element: 4 unaries and 6 oxides, spanning a wide range of coordination numbers and oxidation states. The primary outcome is a reference dataset of 960 equations of state cross-checked between two all-electron codes, then used to verify and improve nine pseudopotential-based approaches. Such effort is facilitated by deploying AiiDA common workflows that perform automatic input parameter selection, provide identical input/output interfaces across codes, and ensure full reproducibility. Finally, we discuss the extent to which the current results for total energies can be reused for different goals (e.g., obtaining formation energies).

Atomic and electronic structure of diamond (111) surfaces: III. Electronic structure of the clean and hydrogen-covered three-dangling-bond surfaces

We present ab initio local-density functional calculations of the electronic structure of clean and hydrogen-covered three-dangling-bond (3db) diamond (111) surfaces, extending our earlier work on the atomic structure of the three-dangling-bond surface. Our results show that the three photoelectron spectra measured on the C(111) surface at different degrees of hydrogenation can be attributed to the 1db-(2 × 1), 1db-(1 × 1):H and 3db-(2 × 1):2H, in agreement with the predicted stabilities as a function of the chemical potential of hydrogen above the surface.