Publications

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.

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 $\mathrm{Be}{\mathrm{H}}_{2}$.

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.

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.

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.

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.