Publications

Competing stabilization mechanism for the polar ZnO(0001)-Zn surface

Density-functional calculations for the (0001)-Zn surface of wurtzite ZnO are reported. Different stabilization mechanisms, such as metallization of the surface layer, adsorption of OH groups or O adatoms, the formation of Zn vacancies, and large scale triangular reconstructions are considered. The calculations indicate that isolated Zn vacancies or O adatoms are unfavorable compared to triangular reconstructions. In the absence of hydrogen, these triangular features are stable under any realistic temperature and pressure. When hydrogen is present, the reconstruction is lifted, and hydroxyl groups stabilize the ideal otherwise unreconstructed surface. The transition between the unreconstructed hydroxyl covered surface and the triangular shaped features occurs abruptly; OH groups lift the reconstruction, but their adsorption is energetically unfavorable on the triangularly reconstructed surface.

Planar Vanadium Oxide Clusters: Two-Dimensional Evaporation and Diffusion on Rh(111)

The formation of novel vanadium oxide cluster molecules by oxidative two-dimensional evaporation from vanadium oxide nanostructures is reported on a Rh(111) metal surface. The structure and stability of the planar ${\mathrm{V}}_{6}{\mathrm{O}}_{12}$ clusters and the physical origin of their 2D evaporation process have been elucidated by high-resolution scanning tunneling microscopy (STM) and ab initio density functional theory calculations. The surface diffusion of the clusters has been followed in elevated-temperature STM experiments, and the diffusion parameters have been extracted, indicating diffusion by hopping of the entire surface stabilized cluster units.

Behavior of Methylammonium Dipoles in MAPbX₃ (X = Br and I)

Dielectric constants of MAPbX3 (X = Br, I) in the 1 kHz-1 MHz range show strong temperature dependence near room temperature, in contrast to the nearly temperature-independent dielectric constant of CsPbBr3. This strong temperature dependence for MAPbX3 in the tetragonal phase is attributed to the MA+ dipoles rotating freely within the probing time scale. This interpretation is supported by ab initio molecular dynamics simulations on MAPbI3 that establish these dipoles as randomly oriented with a rotational relaxation time scale of ∼7 ps at 300 K. Further, we probe the intriguing possibility of transient polarization of these dipoles following a photoexcitation process with important consequences on the photovoltaic efficiency, using a photoexcitation pump and second harmonic generation efficiency as a probe with delay times spanning 100 fs-1.8 ns. The absence of a second harmonic signal at any delay time rules out the possibility of any transient ferroelectric state under photoexcitation.

<i>Ab Initio</i>thermodynamic and elastic properties of alkaline-earth metals and their hydrides

A systematic investigation of the alkaline earth hydrides $\mathrm{Be}{\mathrm{H}}_{2}$, $\mathrm{Mg}{\mathrm{H}}_{2}$, $\mathrm{Ca}{\mathrm{H}}_{2}$, $\mathrm{Sr}{\mathrm{H}}_{2}$, $\mathrm{Ba}{\mathrm{H}}_{2}$, the corresponding deuterides, and their antecedent metals is reported. We calculate lattice parameters, electronic and vibrational energies, enthalpies of formation at 0 and $298\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, components of the elasticity tensor, ${C}_{ij}$, and polycrystalline moduli based on the Hill criteria using density functional theory. Components of the Born effective charge tensors and phonon spectra are also computed for each hydride. We critically compare results obtained via the local density and generalized gradient approximations for the exchange-correlation energy functional. The volume dependence of the zero point energy is also investigated for Be and $\mathrm{Be}{\mathrm{H}}_{2}$.

Oxygen-induced step bunching and faceting of Rh(553): Experiment and<i>ab initio</i>calculations

Using a combined experimental and theoretical approach, we show that the initial oxidation of a Rh(553) surface, a surface vicinal to (111), undergoes step bunching when exposed to oxygen, forming lower-index facets. At a pressure of about ${10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}\mathrm{mbar}$ and a temperature of $380\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ this leads to (331) facets with one-dimensional oxide chains along the steps, coexisting with (111) facets. Further increase of the pressure and temperature results in (111) facets only, covered by an O-Rh-O surface oxide. Our density functional theory calculations provide an atomistic understanding of the observed behavior.

Outside Back Cover: Spin Frustration Determines the Stability and Reactivity of Metal–Organic Frameworks with Triangular Iron(III)–Oxo Clusters (Angew. Chem. Int. Ed. 41/2025)

Low-Scaling MP2 for Solids

Making free-energy calculations routine: Combining first principles with machine learning

The chemical potentials of atoms and molecules in condensed matter are fundamental properties that allow one to predict a wide variety of thermodynamic properties. However, predictions using first principles are challenging. Here, an efficient and accurate method using machine-learned force fields is presented. A key point is that it requires training only at the end points of the thermodynamic pathway, rendering the training simple and efficient. Applications to liquid Si, and Li and F ions hydrated by water show that the method can predict accurate chemical potentials at low computational cost.

Hard antiphase domain boundaries in strontium titanate unravelled using machine-learned force fields

We investigate the properties of hard antiphase boundaries in ${\mathrm{SrTiO}}_{3}$ using machine-learned force fields. In contrast to earlier findings based on standard ab initio methods, for all pressures up to $120\phantom{\rule{0.16em}{0ex}}\mathrm{kbar}$ the observed domain wall pattern maintains an almost perfect N\'eel character in quantitative agreement with Landau-Ginzburg-Devonshire theory, and the in-plane polarization ${P}_{3}$ shows no tendency to decay to zero. Together with the switching properties of ${P}_{3}$ under reversal of the N\'eel order parameter component, this provides hard evidence for the presence of rotopolar couplings. The present approach overcomes the severe limitations of ab initio simulations of wide domain walls and opens avenues toward concise atomistic predictions of domain-wall properties even at finite temperatures.

Copernicium: A Relativistic Noble Liquid

The chemical nature and aggregate state of superheavy copernicium (Cn) have been subject of speculation for many years. While strong relativistic effects render Cn chemically inert, which led Pitzer to suggest a noble‐gas‐like behavior in 1975, Eichler and co‐workers in 2008 reported substantial interactions with a gold surface in atom‐at‐a‐time experiments, suggesting a metallic character and a solid aggregate state. Herein, we explore the physicochemical properties of Cn by means of first‐principles free‐energy calculations, which confirm Pitzer's original hypothesis: With predicted melting and boiling points of 283±11 K and 340±10 K, Cn is indeed a volatile liquid and exhibits a density very similar to that of mercury. However, in stark contrast to mercury and the lighter Group 12 metals, we find bulk Cn to be bound by dispersion and to exhibit a large band gap of 6.4 eV, which is consistent with a noble‐gas‐like character. This non‐group‐conforming behavior is eventually traced back to strong scalar‐relativistic effects, and in the non‐relativistic limit, Cn appears as a common Group 12 metal.

Band structure and effective mass calculations for III-V compound semiconductors using hybrid functionals and optimized local potentials

No abstract is provided for this article.

Transport coefficients of liquids from first principles

The use of first-principles molecular dynamics to calculate the viscosity of liquid metals using the Green–Kubo relations is described. The first-principles techniques are based on density functional theory, the pseudopotential approximation, and plane-wave basis sets. The statistical-mechanical basis of the Green–Kubo relations is summarised, and extensive first-principles molecular dynamics simulations of liquid aluminium are presented to demonstrate that the method works in practice. Calculated viscosity results are reported for two important systems: liquid iron at Earth's core conditions, and liquid selenium at states on the liquid–vapour curve. The significance of the viscosity results for an understanding of these systems is discussed.

Atomic and electronic structure of icosahedral Al-Pd-Mn alloys and approximant phases

The electronic structures of the stable face-centered-icosahedral alloy ${\mathrm{Al}}_{70}$${\mathrm{Pd}}_{20}$${\mathrm{Mn}}_{10}$ and of a hierarchy of rational approximants to the icosahedral phase have been calculated using ab initio pseudopotential, linear-muffin-tin-orbital (LMTO), and tight-binding (TB)-LMTO techniques. The description of the aotmic structure is based on a projection from six-dimensional space, with acceptance volumes chosen such as to reproduce the observed diffraction data. For the lowest-order approximants (1/1 and 2/1 with 128 and 544 atoms in the periodically repeated cell), the electronic eigenvalues and eigenfunctions have been calculated self-consistently using LMTO and ab initio pseudopotential techniques. For the 1/1 approximant, we have also performed a relaxation of the idealized structure using the Hellmann-Feynman forces. For the higher-order approximants (we go up to the 8/5 approximants with 41 068 atoms), the electronic densities of states and the spectral functions have been calculated from the TB-LMTO Hamiltonian via a real-space recursion technique. The electronic density of states (DOS) of the higher-order approximants is characterized by a structure-induced minimum at the Fermi level, indicating the possibility of a Hume-Rothery-type electronic mechanism for the stabilization of the icosahedral phase. However, in the lowest-order approximants the DOS minimum is either flattened or shifted away from the Fermi energy. This is in contrast to the simple icosahedral alloys such as Al-Cu-Li, where the DOS minimum exists in the quasiperiodic phase and in the crystalline approximants. Hence it appears that for the face-centered icosahedral alloys, the structure-induced DOS minimum may be not only a generic, but also a specific property of the quasicrystalline phase. In addition to the spectral properties, we have also studied the character of the electronic states via a calculation of the participation ratio. We find that the states in the vicinity of the Fermi level tend to be more localized than in the rest of the valence band and that the localization is related to certain aspects of the quasicrystalline structure. This is important for understanding the anomalous transport properties of these alloys.

Minimax Isometry Method.

First-principles study of the adsorption of atomic H on Ni (111), (100) and (110)

The adsorption of atomic H on Ni (111), (100) and (110) surfaces is studied using spin-polarized gradient-corrected density-functional theory. Full H-coverage and 1/4 H-coverage are considered. Trends in the adsorption energy, repulsion between adsorbed hydrogen atoms, and diffusion barriers are discussed in detail. In addition, calculations are presented for the experimentally observed Ni(111) (2×2)-2H superstructure and for the Ni(110) (1×2)-3H pairing-row reconstruction. The results are compared with experiments, showing a good agreement with the structural models derived from experimental data. We use a simulated annealing approach to show the global stability of the pairing-row reconstruction.

The first principles calculations of Fermi level pinning in FUSI-PtSi/HfO2/Si system induced by local distortion of HfO2

No abstract is provided for this article.

Thermal transport and phase transitions of zirconia by on-the-fly machine-learned interatomic potentials

Machine-learned interatomic potentials enable realistic finite temperature calculations of complex materials properties with first-principles accuracy. It is not yet clear, however, how accurately they describe anharmonic properties, which are crucial for predicting the lattice thermal conductivity and phase transitions in solids and, thus, shape their technological applications. Here we employ a recently developed on-the-fly learning technique based on molecular dynamics and Bayesian inference in order to generate an interatomic potential capable to describe the thermodynamic properties of zirconia, an important transition metal oxide. This machine-learned potential accurately captures the temperature-induced phase transitions below the melting point. We further showcase the predictive power of the potential by calculating the heat transport on the basis of Green-Kubo theory, which allows to account for anharmonic effects to all orders. This study indicates that machine-learned potentials trained on the fly offer a routine solution for accurate and efficient simulations of the thermodynamic properties of a vast class of anharmonic materials.

Electron-Phonon Interactions Using Wannier Functions and the Projector-Augmented-Wave Method