arXiv subjects

Menno Bokdam

Publications and source records attributed to Menno Bokdam.

At least 19 recordsLinked to original sources

arXiv2026

Simulating the influence of stoichiometry on the spectral emissivity of Mo$_x$Si$_y$ thin films

In this work, we simulate the spectral emissivity of various stoichiometric crystal phases of Mo$_x$Si$_y$ compounds using density functional perturbation theory. The dielectric function, including electronic and ionic contributions, is calculated for each phase. We use the bulk properties obtained to simulate the optical absorption spectrum originating from the compound in thin film ($\sim$20 nm) form. We find that most thin films of Mo$_x$Si$_y$ are metallic, however, our results indicate that their emissivity is not simply correlated with the Mo content. For hot metallic films at around 900 K, we predict a maximal emissivity between 5-10 nm thickness. Our results are in good qualitative agreement with experiments, confirming that the emissivity of hexagonal MoSi$_2$ is much lower than in the tetragonal phase. This is related to the small band gap (hexagonal MoSi$_2$) and low density of states at the Fermi level (tetragonal MoSi$_2$). Furthermore, test calculations on defected MoSi$_2$ demonstrate that the infrared emissivity of MoSi$_2$ thin films can be substantially increased by introducing defects.

cond-mat.mtrl-sci↗

arXiv2024

Smooth Overlap of Spin Orientations: Machine Learning Exchange Fields for Ab-initio Spin Dynamics

We add the magnetic degrees of freedom to the widely used Gaussian Approximation Potential of machine learning (ML) and present a model that describes the potential energy surface of a crystal based on the atomic coordinates as well as their noncollinear magnetic moments. Assuming an adiabatic approximation for the spin directions and magnitudes, the ML model depends solely on spin coordinates and orientation, resulting in computational efffciency and enabling ab initio spin dynamics. Leveraging rotational symmetries of magnetic interactions, the ML model can incorporate various magnetic interactions, expanding into two-body, three-body terms, etc., following the spirit of cluster expansion. For simplicity, we implement the ML model with a two-body form for the exchange interaction. Comparing total energies and local fields predicted by the model for noncollinear spin arrangements with explicit results of constrained noncollinear density functional calculations for bcc Fe yields excellent results, within 1 meV/spin for the total energy. Further optimization, including three-body and other terms, is expected to encompass diverse magnetic interactions and enhance the model's accuracy. This will extend the model's applicability to a wide range of materials and facilitate the machine learning ab initio spin dynamics.

cond-mat.mtrl-sci↗

arXiv2024

Tuning Einstein Oscillator Frequencies of Cation Rattlers: A Molecular Dynamics Study of the Lattice Thermal Conductivity of CsPbBr$_3$

The pure CsPbBr$_3$ perovskite is an archetypal example of a strongly anharmonic crystal that poses a major challenge for computational methods to describe its thermodynamic properties. Its lattice dynamics exhibits characteristics of a phonon liquid: mode coupling, low lifetimes and `rattlers'. To study the thermal conduction in this crystal, including the effect of dynamic disorder introduced by the Cs rattlers, we apply large-scale molecular dynamics (MD) simulations combined with machine-learning interatomic potentials. We simulate its ultra-low lattice thermal conductivity in the cubic phase and obtain phonon spectra by measuring velocity autocorrelation functions. The thermal conductivity at 500 K is computed to be $0.53\pm0.04\frac{\rm W}{\rm m K}$, which is similar to that of demineralized water under normal indoor conditions. MD based insight into the heat transport mechanism of halide perovskites is presented. In the analysis the Cs cations are interpreted as damped Einstein oscillators. The phonon bandstructure of a system with artificially raised Cs masses demonstrates an increased interference of the Cs rattling with the acoustic phonon modes. We show that the thermal conductivity of the CsPbBr$_3$ perovskite can still be slightly decreased by tuning the cation rattling frequency into the range of the low lying acoustic mode

cond-mat.mes-hall↗

arXiv2023

Phase transitions of LaMnO$_3$ and SrRuO$_3$ from DFT + U based machine learning force fields simulations

Perovskite oxides are known to exhibit many magnetic, electronic and structural phases as function of doping and temperature. These materials are theoretically frequently investigated by the DFT+U method, typically in their ground state structure at $T=0$. We show that by combining machine learning force fields (MLFFs) and DFT+U based molecular dynamics, it becomes possible to investigate the crystal structure of complex oxides as function of temperature and $U$. Here, we apply this method to the magnetic transition metal compounds LaMnO$_3$ and SrRuO$_3$. We show that the structural phase transition from orthorhombic to cubic in LaMnO$_3$, which is accompanied by the suppression of a Jahn-Teller distortion, can be simulated with an appropriate choice of $U$. For SrRuO$_3$, we show that the sequence of orthorhombic to tetragonal to cubic crystal phase transitions can be described with great accuracy. We propose that the $U$ values that correctly capture the temperature-dependent structures of these complex oxides, can be identified by comparison of the MLFF simulated and experimentally determined structures.

cond-mat.mtrl-sci↗

arXiv2023

Dynamic Distortions of Quasi-2D Ruddlesden-Popper Perovskites at Elevated Temperatures: Influence on Thermal and Electronic Properties

Ruddlesden-Popper hybrid halide perovskite are quasi-two-dimensional materials with a layered structure and structural dynamics that are determined by the interplay between the organic and inorganic layers. While their optical properties are governed by confinement effects, the atomistic origin of thermal and electronic properties of these materials is yet to be fully established. Here we combine computational and experimental techniques to study A$_2$PbI$_4$ (A=butylammonium (BA), phenethylammonium (PEA)) Ruddlesden-Popper perovskites and compare them with the quintessential perovskite CH$_3$NH$_3$PbI$_3$. We use first-principles density functional theory, molecular dynamics simulations based on machine-learned interatomic potentials, thermal measurements, temperature-dependent Raman spectroscopy, and ultraviolet photoelectron spectroscopy, to probe the thermal and electronic properties of these materials at elevated temperatures. Our molecular dynamics simulations demonstrate that dynamic fluctuations in the organic sublattice determine the bulk-average distortions of these materials at room-temperature, explaining significant differences in their electronic density of states close to the Fermi level. Furthermore, by analysing the organic layer dynamics in BA$_2$PbI$_4$ we provide a mechanistic explanation for the phase transition of this material at 274 K and observations from Raman measurements. Our results highlight the role of the organic interlayer for the electronic and thermal transport properties of Ruddlesden-Popper perovskites, paving the way for the design of new hybrid materials for tailored applications.

cond-mat.mtrl-sci↗

arXiv2021

Anharmonic lattice dynamics in large thermodynamic ensembles with machine-learning force fields: CsPbBr$_{3}$ a phonon-liquid with Cs rattlers

The phonon dispersion relations of crystal lattices can often be well-described with the harmonic approximation. However, when the potential energy landscape exhibits more anharmonicity, for instance, in case of a weakly bonded crystal or when the temperature is raised, the approximation fails to capture all crystal lattice dynamics properly. Phonon-phonon scattering mechanisms become important and limit the phonon lifetimes. We take a novel approach and simulate the phonon dispersion of a complex dynamic solid at elevated temperatures with Machine-Learning Force Fields of near-first-principles accuracy. Through large-scale molecular dynamics simulations the projected velocity autocorrelation function (PVACF) is obtained. We apply this approach to the inorganic perovskite CsPbBr$_{3}$. Imaginary modes in the harmonic picture of this perovskite are absent in the PVACF, indicating a dynamic stabilization of the crystal. The anharmonic nature of the potential makes a decoupling of the system into a weakly interacting phonon gas impossible. The phonon spectra of CsPbBr$_{3}$ show the characteristics of a phonon liquid. Rattling motions of the Cs$^{+}$ cations are studied by self-correlation functions and are shown to be nearly dispersionless motions of the cations with a frequency of $\sim$0.8THz within the lead-bromide framework.

cond-mat.mtrl-sci↗

arXiv2021

Exploring librational pathways with on-the-fly machine-learning force fields: Methylammonium molecules in MAPbX$_3$ (X=I, Br, Cl) perovskites

Two seemingly similar crystal structures of the low-temperature (~100 K) MAPbX$_3$ (X=I,Br,Cl) perovskites, but with different relative Methylammonium (MA) ordering, have appeared as representatives of this orthorhombic phase. Distinguishing them by X-ray diffraction experiments is difficult and conventional first-principles based molecular-dynamics approaches are often too computationally intensive to be feasible. Therefore, to determine the thermodynamically stable structure, we use a recently introduced on-the-fly Machine-Learning Force Field method, which reduces the computation time from years to days. The molecules exhibit a large degree of anharmonic motion depending on temperature: i.e. rattling, twisting and tumbling. We observe the crystal's 'librational pathways' while slowly heating it in isothermal-isobaric simulations. Marked differences in the thermal evolution of structural parameters allow us to determine the real structure of the system via a comparison with experimentally determined crystal structures.

cond-mat.mtrl-sci↗

arXiv2021

Optical and excitonic properties of transition metal oxide perovskites by the Bethe-Salpeter equation

We present a systematic investigation of the role and importance of excitonic effects on the optical properties of transitions metal oxide perovskites. A representative set of fourteen compounds has been selected, including 3$d$ (SrTiO$_3$, LaScO$_3$, LaTiO$_3$, LaVO$_3$, LaCrO$_3$, LaMnO$_3$, LaFeO$_3$ and SrMnO$_3$), 4$d$ (SrZrO$_3$, SrTcO$_3$ and Ca$_2$RuO$_4$) and 5$d$ (SrHfO$_3$, KTaO$_3$ and NaOsO$_3$) perovskites, covering a band gap ranging from 0.1 eV to 6.1 eV and exhibiting different electronic, structural and magnetic properties. Optical conductivities and optical transitions including electron-hole interactions are calculated through the solution of the Bethe-Salpeter equation (BSE) with quasi-particle energies evaluated by single-shot $G_0W_0$ approximation. The exciton binding energies are computed by means of a model-BSE (mBSE), carefully benchmarked against the full BSE method, in order to obtain well-converged results in terms of k-point sampling. The predicted results are compared with available measured data, with an overall satisfactory agreement between theory and experiment.

cond-mat.mtrl-sci↗

arXiv2019

Long-range order imposed by short-range interactions in methylammonium lead iodide: Comparing point-dipole models to machine-learning force fields

The crystal structure of the MAPbI$_3$ hybrid perovskite forms an intricate electrostatic puzzle with different ordering patterns of the MA molecules at elevated temperatures. For this perovskite three published model Hamiltonians based on the point-dipole (pd) approximation combined with short-range effective interactions are compared to a recently developed machine-learning force field. A molecular order parameter is used to consistently compare the transformation of the anti-ferroelectric ordering in the orthorhombic phase upon raising the temperature. We show that the ground states and the order-disorder transition of the three models are completely different. Our analysis indicates that the long-range order in the low-temperature orthorhombic phase can be captured by pd-based models with a short cutoff radius, including the nearest and next-nearest neighbor molecules. By constructing effective atomic interactions the ordering can already be described within 6 A radius. By extracting the coupling energetics of the molecules from density functional theory calculations on MA$_x$Cs$_{1-x}$PbI$_3$ test systems, we show that the pd-approximation holds at least for static structures. To improve the accuracy of the pd-interaction an Ewald summation is applied combined with a distance dependent electronic screening function.

cond-mat.mtrl-sci↗

arXiv2019

Phase transitions of hybrid perovskites simulated by machine-learning force fields trained on-the-fly with Bayesian inference

Realistic finite temperature simulations of matter are a formidable challenge for first principles methods. Long simulation times and large length scales are required, demanding years of compute time. Here we present an on-the-fly machine learning scheme that generates force fields automatically during molecular dynamics simulations. This opens up the required time and length scales, while retaining the distinctive chemical precision of first principles methods and minimizing the need for human intervention. The method is widely applicable to multi-element complex systems. We demonstrate its predictive power on the entropy driven phase transitions of hybrid perovskites, which have never been accurately described in simulations. Using machine learned potentials, isothermal-isobaric simulations give direct insight into the underlying microscopic mechanisms. Finally, we relate the phase transition temperatures of different perovskites to the radii of the involved species, and we determine the order of the transitions in Landau theory.

cond-mat.mtrl-sci↗

arXiv2018

The Finite Temperature Structure of the MAPbI3 Perovskite: Comparing Density Functional Approximations and Force Fields to Experiment

Determining the finite temperature structure of the hybrid perovskite MAPbI3 is a challenge for both experimental and theoretical methods. A very powerful computational method that can resolve the atomic structure is molecular dynamics (MD). The resulting structure depends on the density functional approximation (DFA) in the case of ab initio MD and the force field in classical MD. We compare the structure between 250K and 400K obtained with different DFAs and force fields in one consistent manner. The symmetry of the PbI3 framework is analyzed as well as the relative ordering of the neighboring organic molecules inside the framework. The distribution function of the molecules is used to map out an effective energy surface for the rotation of a single molecule. This surface is accurately modeled by a pair of cubic harmonics. Available experimental data in literature are discussed and compared to the structure obtained with the different methods. The spread in these data is still too large to uniquely determine the method that 'best' describes the perovskite, however promising candidates and outliers have been identified.

cond-mat.mtrl-sci↗

arXiv2017

Assessing density functionals using many body theory for hybrid perovskites

Which density functional is the "best" for structure simulations of a particular material? A concise, first principles, approach to answer this question is presented. The random phase approximation (RPA)--- an accurate many body theory--- is used to evaluate various density functionals. To demonstrate and verify the method, we apply it to the hybrid perovskite MAPbI$_3$, a promising new solar cell material. The evaluation is done by first creating finite temperature ensembles for small supercells using RPA molecular dynamics, and then evaluating the variance between the RPA and various approximate density functionals for these ensembles. We find that, contrary to recent suggestions, van der Waals functionals do not improve the description of the material, whereas hybrid functionals and the SCAN (strongly constrained appropriately normed) density functional yield very good agreement with the RPA. Finally, our study shows that in the room temperature tetragonal phase of MAPbI$_3$, the molecules are preferentially parallel to the shorter lattice vectors but reorientation on ps timescales is still possible.

cond-mat.mtrl-sci↗

arXiv2017

Room temperature dynamic correlation between methylammonium molecules in lead-iodine based perovskites: An ab-initio molecular dynamics perspective

The high efficiency of lead organo-metal-halide perovskite solar cells has raised many questions about the role of the methylammonium (MA) molecules in the Pb-I framework. Experiments indicate that the MA molecules are able to 'freely' spin around at room temperature even though they carry an intrinsic dipole moment. We have performed large supercell (2592 atoms) finite temperature ab-initio molecular dynamics calculations to study the correlation between the molecules in the framework. An underlying long range anti-ferroelectric ordering of the molecular dipoles is observed. The dynamical correlation between neighboring molecules shows a maximum around room temperature in the mid-temperature phase. In this phase, the rotations are slow enough to (partially) couple to neighbors via the Pb-I cage. This results in a collective motion of neighboring molecules in which the cage acts as the mediator. At lower and higher temperatures the motions are less correlated.

cond-mat.mtrl-sci↗

arXiv2016

Role of Polar Phonons in the Photo Excited State of Metal Halide Perovskites

The development of high efficiency perovskite solar cells has sparked a multitude of measurements on the optical properties of these materials. For the most studied methylammonium(MA)PbI$_3$ perovskite, a large range (6-55 meV) of exciton binding energies has been reported by various experiments. The existence of excitons at room temperature is unclear. For the MAPb$X_3$ perovskites we report on relativistic $GW$-BSE calculations. This method is capable to directly calculate excitonic properties from first-principles. At low temperatures it predicts exciton binding energies in agreement with the reported 'large' values. For MAPbI$_3$, phonon modes present in this frequency range have a negligible contribution to the ionic screening. By calculating the polarisation in time from finite temperature molecular dynamics, we show that at room temperature this does not change. We therefore exclude ionic screening as an explanation for the experimentally observed reduction of the exciton binding energy at room temperature.

cond-mat.mtrl-sci↗

arXiv2016

$\mathbb{Z}_2$ invariance of Germanene on MoS$_2$ from first principles

We present a low energy Hamiltonian generalized to describe how the energy bands of germanene ($\rm \overline{Ge}$) are modified by interaction with a substrate or a capping layer. The parameters that enter the Hamiltonian are determined from first-principles relativistic calculations for $\rm \overline{Ge}|$MoS$_2$ bilayers and MoS$_2|\rm \overline{Ge} |$MoS$_2$ trilayers and are used to determine the topological nature of the system. For the lowest energy, buckled germanene structure, the gap depends strongly on how germanene is oriented with respect to the MoS$_2$ layer(s). Topologically non-trivial gaps for bilayers and trilayers can be almost as large as for a free-standing germanene layer.

cond-mat.mes-hall↗

arXiv2014

Large potential steps at weakly interacting metal-insulator interfaces

Potential steps exceeding 1 eV are regularly formed at metal|insulator interfaces, even when the interaction between the materials at the interface is weak physisorption. From first-principles calculations on metal|h-BN interfaces we show that these potential steps are only indirectly sensitive to the interface bonding through the dependence of the binding energy curves on the van der Waals interaction. Exchange repulsion forms the main contribution to the interface potential step in the weakly interacting regime, which we show with a simple model based upon a symmetrized product of metal and h-BN wave functions. In the strongly interacting regime, the interface potential step is reduced by chemical bonding.

cond-mat.mes-hall↗

arXiv2014

Schottky barriers at hexagonal boron nitride/metal interfaces: a first principles study

The formation of a Schottky barrier at the interface between a metal and hexagonal boron nitride (h-BN) is studied using density functional theory. For metals whose work functions range from 4.2 to 6.0 eV, we find Schottky barrier heights for holes between 1.2 and 2.3 eV. A central role in determining the Schottky barrier height is played by a potential step of between 0.4 and 1.8 eV that is formed at the metal|h-BN interface and effectively lowers the metal work function. If h-BN is physisorbed, as is the case on fcc Cu, Al, Au, Ag and Pt(111) substrates, the interface potential step is described well by a universal function that depends only on the distance separating h-BN from the metal surface. The interface potential step is largest when h-BN is chemisorbed, which is the case for hcp Co and Ti (0001) and for fcc Ni and Pd (111) substrates.

cond-mat.mes-hall↗

arXiv2014

Band gaps in incommensurable graphene on hexagonal boron nitride

Devising ways of opening a band gap in graphene to make charge-carrier masses finite is essential for many applications. Recent experiments with graphene on hexagonal boron nitride (h-BN) offer tantalizing hints that the weak interaction with the substrate is sufficient to open a gap, in contradiction of earlier findings. Using many-body perturbation theory, we find that the small observed gap is what remains after a much larger underlying quasiparticle gap is suppressed by incommensurability. The sensitivity of this suppression to a small modulation of the distance separating graphene from the substrate suggests ways of exposing the larger underlying gap.

cond-mat.mes-hall↗