arXiv subjects

Mike Pols

Publications and source records attributed to Mike Pols.

13 recordsLinked to original sources

arXiv2026

Chiral Surface Phonons

We use symmetry arguments combined with density functional theory to demonstrate that all surfaces of crystalline materials host surface phonons that are chiral. As model system, we study slabs of highly symmetric AB rocksalt compounds, and find surface-localized phonons whose atomic displacements exhibit chiral motion. We further show that these chiral surface phonons generate sheets of in-plane magnetism at the surface. Our results reveal that chiral phonons can emerge in all crystalline materials as a result of reduced symmetry at surfaces or interfaces. These findings establish surfaces as a previously overlooked source of chiral phonons and their associated magnetic moments, which could play a role in a broad range of surface-sensitive measurements.

cond-mat.mtrl-sci↗

arXiv2026

Anisotropic Defect Diffusion in Layered CsPbBr$_\mathrm{x}$I$_\mathrm{3-x}$ Perovskites

Mixed-halide perovskites offer a route to enhance phase stability and modify optoelectronic properties. Here, we use large-scale molecular dynamics simulations with a reactive force field to investigate defects in CsPbBr$_\mathrm{x}$I$_\mathrm{3-x}$ perovskites, focusing on how defect mobility can be controlled and the stability of the material improved by layered ordering of Br and I anions. Our results show that layered halide ordering induces strongly anisotropic defect diffusion: migration proceeds readily along the layers, whereas diffusion across them is strongly suppressed. For Cs defects, this anisotropy originates from directional lattice strain and the associated octahedral tilting, whereas halide migration is governed by an interplay between strain and preferential local halide bonding configurations.

cond-mat.mtrl-sci↗

arXiv2026

Halide diffusion in mixed-halide perovskites and heterojunctions

Migration of halide defects guides ion transport in metal halide perovskites and controls the kinetics of halide mixing and phase separation. We study the diffusion of halide vacancies and interstitials in \ce{CsPb(I_{x}Br_{1-x})_{3}} and \ce{CsPbI_{3}}/\ce{CsPbBr_{3}} heterojunctions by molecular dynamics simulations using neural network potentials trained on density functional theory calculations. We observe enhanced diffusion of both vacancies and interstitials in the mixed halide compounds compared to the single halide ones, as well as a difference in mobility between Br and I ions in the mixed compound. Diffusion across heterojunctions is governed by the interface structure, where a Br-rich interface blocks migration of vacancies in particular, but an I-rich interface is permeable.

cond-mat.mtrl-sci↗

arXiv2025

Lattice excitations with finite polarization and magnetization

Ferrons are a type of quasiparticle corresponding to elementary excitations of the ferroelectric order. Analogously to how magnons modulate and transport magnetization, ferrons modulate and transport electric polarization. Here, we introduce multiferrons as elementary excitations with both electric and magnetic character. Multiferrons lead to a tilt and elliptical precession of the polarization and at the same time create a magnetization through the mechanism of dynamical multiferroicity. Using first-principles calculations for LiNbO$_3$, we show that the electric polarization of multiferrons is perpendicular to the equilibrium ferroelectric polarization, whereas the magnetization is parallel to it. Our calculations further demonstrate that multiferrons carry net electric and magnetic quadrupole and octupole moments, which we term multipolons. These multipolons could couple to internal multipolar degrees of freedom, for example in altermagnets, or to external probes such as neutrons, leading to potentially experimentally observable phenomena following coherent or thermal excitation of multiferrons.

cond-mat.mtrl-sci↗

arXiv2025

Impact of Metal Cation on Chiral Properties of 2D Halide Perovskites

Chiral two-dimensional (2D) halide perovskites are formed by embedding chiral organic cations in a perovskite crystal structure. The chirality arises from distortions of the 2D metal halide layers induced by the packing of these organic cations. Sn-based octahedra spontaneously distort, but it remains unclear whether this intrinsic structural instability enhances the chirality. We investigate the effect of the metal cation on structural and phonon chirality in MBA$_{2}$Sn$_{\mathrm{x}}$Pb$_{1-\mathrm{x}}$I$_{4}$ (x = 0, 1/2, and 1). Incorporating Sn does distort the metal halide octehedra, yet it only has a minor impact on the structural chirality. In contrast, the phonons in MBA$_{2}$SnI$_{4}$ are substantially more chiral than in MBA$_{2}$PbI$_{4}$, especially the in-plane acoustic modes. However, this enhanced phonon chirality does not lead to a generation of a larger angular momentum under a temperature gradient, because the contributions of different chiral phonons tend to compensate one another.

cond-mat.mtrl-sci↗

arXiv2024

Chiral Phonons in 2D Halide Perovskites

Phonons in chiral crystal structures can be circularly polarized, making them chiral. Chiral phonons carry angular momentum, which is observable in heat currents, and, via coupling to electron spin, in spin currents. Two-dimensional (2D) halide perovskites, versatile direct band gap semiconductors, can easily form chiral structures by incorporating chiral organic cations. As a result, they exhibit phenomena such as chirality-induced spin selectivity (CISS) and the spin Seebeck effect, although the underlying mechanisms remain unclear. Using on-the-fly machine-learning force fields trained against density functional theory calculations, we confirm the presence of chiral phonons, a potential key factor for these effects. Our analysis reveals that low-energy phonons, originating from the inorganic framework, primarily exhibit chirality. Under a temperature gradient, these chiral phonons generate substantial angular momentum, leading to experimentally observable effects. These findings position chiral 2D perovskites as a promising platform for exploring the interplay between phononic, electronic, spintronic, and thermal properties.

cond-mat.mtrl-sci↗

arXiv2024

Tracing Ion Migration in Halide Perovskites with Machine Learned Force Fields

Halide perovskite optoelectronic devices suffer from chemical degradation and current-voltage hysteresis induced by migration of highly mobile charged defects. Atomic scale molecular dynamics simulations can capture the motion of these ionic defects, but classical force fields are too inflexible to describe their dynamical charge states. Using CsPbI3 as a case study, we train machine learned force fields from density functional theory calculations and study the diffusion of charged halide interstitial and vacancy defects in bulk CsPbI3. We find that negative iodide interstitials and positive iodide vacancies, the most stable charge states for their respective defect type, migrate at similar rates at room temperature. Neutral interstitials are faster, but neutral vacancies are one order of magnitude slower. Oppositely charged interstitials and vacancies, as they can occur in device operation or reverse bias conditions, are significantly slower and can be considered relatively immobile.

cond-mat.mtrl-sci↗

arXiv2024

Temperature-Dependent Chirality in Halide Perovskites

With the use of chiral organic cations in two-dimensional metal halide perovskites, chirality can be induced in the metal halide layers, which results in semiconductors with intriguing chiral optical and spin-selective transport properties. The chiral properties strongly depend upon the temperature, despite the basic crystal symmetry not changing fundamentally. We identify a set of descriptors that characterize the chirality of metal halide perovskites such as MBA$_{2}$PbI$_{4}$, and study their temperature dependence using molecular dynamics simulations with on-the-fly machine-learning force fields obtained from density functional theory calculations. We find that, whereas the arrangement of organic cations remains chiral upon increasing the temperature, the inorganic framework loses this property more rapidly. We ascribe this to the breaking of hydrogen bonds that link the organic with the inorganic substructures, which leads to a loss of chirality transfer.

cond-mat.mtrl-sci↗

arXiv2023

Mixing I and Br in Inorganic Perovskites: Atomistic Insights from Reactive Molecular Dynamics Simulations

All-inorganic halide perovskites have received a lot of attention as attractive alternatives to overcome the stability issues of hybrid halide perovskites that are commonly associated with organic cations. To find a compromise between the optoelectronic properties of CsPbI$_{3}$ and CsPbBr$_{3}$, perovskites with CsPb(Br$_{\rm{x}}$I$_{\rm{1-x}}$)$_{3}$ mixed compositions are commonly used. An additional benefit is that, without sacrificing the optoelectronic properties for applications such as solar cells or LEDs, small amounts of Br in CsPbI$_{3}$ can prevent the inorganic perovskite from degrading to a photoinactive nonperovskite yellow phase. Despite indications that strain in the perovskite lattice plays a role in the stabilization of the material, a full understanding of such strain is lacking. Here we develop a reactive force field (ReaxFF) for perovskites starting from our previous work for CsPbI$_{3}$, we extend this force field to CsPbBr$_{3}$ and mixed CsPb(Br$_{\rm{x}}$I$_{\rm{1-x}}$)$_{3}$ compounds. This force field is used in large-scale molecular dynamics simulations to study perovskite phase transitions and the internal ion dynamics associated with the phase transitions. We find that an increase of the Br content lowers the temperature at which the perovskite reaches a cubic structure. Specifically, by substituting Br for I, the smaller ionic radius of Br induces a strain in the lattice that changes the internal dynamics of the octahedra. Importantly, this effect propagates through the perovskite lattice ranging up to distances of 2 nm, explaining why small concentrations of Br in CsPb(Br$_{\rm{x}}$I$_{\rm{1-x}}$)$_{3}$ (x $\leq$ 1/4) have a significant impact on the phase stability of mixed halide perovskites.

cond-mat.mtrl-sci↗

arXiv2022

What Happens at Surfaces and Grain Boundaries of Halide Perovskites: Insights from Reactive Molecular Dynamics Simulations of CsPbI$_{3}$

The commercialization of perovskite solar cells is hindered by the poor long-term stability of the metal halide perovskite (MHP) light absorbing layer. Solution processing, the common fabrication method for MHPs, produces polycrystalline films with a wide variety of defects, such as point defects, surfaces, and grain boundaries. Although the optoelectronic effects of such defects have been widely studied, the evaluation of their impact on the long-term stability remains challenging. In particular, an understanding of the dynamics of degradation reactions at the atomistic scale is lacking. In this work, using reactive force field (ReaxFF) molecular dynamics simulations, we investigate the effects of defects, in the forms of surfaces, surface defects and grain boundaries, on the stability of the inorganic halide perovskite CsPbI$_{3}$. Our simulations establish a stability trend for a variety of surfaces, which correlates well with the occurrence of these surfaces in experiments. We find that a perovskite surface degrades by progressively changing the local geometry of PbI$_{\mathrm{x}}$ octahedra from corner- to edge- to face-sharing. Importantly, we find that Pb dangling bonds and the lack of steric hindrance of I species are two crucial factors that induce degradation reactions. Finally, we show that the stability of these surfaces can be modulated by adjusting their atomistic details, either by creating additional point defects or merging them to form grain boundaries. While in general additional defects, particularly when clustered, have a negative impact on the material stability, some grain boundaries have a stabilizing effect, primarily because of the additional steric hindrance.

cond-mat.mtrl-sci↗

arXiv2021

A Reparameterized Density Functional Tight-Binding Method for Engineering phase-stable CsPbX\textsubscript{3} Perovskites

Halide perovskites are a promising class of materials for optoelectronic applications, due to their excellent optoelectronic performance. However, they suffer several dynamical degradation problems, the characterization of which is challenging in experiments. Atomic scale simulations can provide valuable insights, however, the high computational cost of traditional quantum mechanical methods such as DFT makes it difficult to model dynamical processes in large perovskite systems. In this work, we present a re-parameterized GFN1-xTB method for the accurate description of structural and dynamical properties of CsPbBr\textsubscript{3}, CsPbI\textsubscript{3}, and CsPb(I\textsubscript{1-x}Br\textsubscript{x})\textsubscript{3}. Our molecular dynamics simulations show that the phase stability is strongly correlated to the displacement of ions in the perovskites. In the low temperature orthorhombic phase, the directional movement of the Cs cations decreases contact with the surrounding halides, initiating a transition to the non-perovskite phase. However, this loss of contact can be compensated by increased halide displacement, once enough thermal energy is available, resulting in a transition to the tetragonal or cubic phases. Furthermore, we find the mixing of halides increase halide displacement over a significant range of temperatures, resulting in lower phase transition temperatures and therefore improved phase stability.

cond-mat.mtrl-sci↗

arXiv2021

A ReaxFF molecular dynamics study of hydrogen diffusion in ruthenium -- the role of grain boundaries

Ruthenium thin films can serve as protective caps for multi-layer extreme ultraviolet mirrors exposed to atomic hydrogen. Hydrogen permeation through ruthenium is problematic as it leads to blisters on the mirrors. H has been shown to exhibit low solubility in bulk Ru, and rapidly diffuses in and out of Ru. Therefore, the underlying mechanisms of the blistering effect remains unknown. This work makes use of reactive molecular dynamics simulations to study the influence of imperfections in a Ru film on the behaviour of H. For the Ru/H system, a ReaxFF force field was parametrised which reproduces structures and energies obtained from quantum-mechanical calculations. Molecular dynamics simulations have been performed with the newly-developed force field, to study the effect of tilt and twist grain boundaries on the overall diffusion behaviour of H in Ru. Our simulations show the tilt and twist grain boundaries provide energetically favourable sites for hydrogen atoms and act as sinks and highways for H. They therefore block H transport across their planes, and favour diffusion along their planes. This results in the accumulation of hydrogen at the grain boundaries. The strong effect of the grain boundaries on the hydrogen diffusion suggests tailoring the morphology of ruthenium thin films as a means to curb the rate of hydrogen permeation.

cond-mat.mtrl-sci↗

arXiv2021

Atomistic insights into the degradation of halide perovskites: a reactive force field molecular dynamics study

Halide perovskites make efficient solar cells due to their exceptional optoelectronic properties, but suffer from several stability issues. The characterization of the degradation processes is challenging because of the limitations in the spatio-temporal resolution in experiments and the absence of efficient computational methods to study the reactive processes. Here, we present the first effort in developing reactive force fields for large scale molecular dynamics simulations of the phase instability and the defect-induced degradation reactions in inorganic CsPbI$_{3}$. We find that the phase transitions are driven by a combination of the anharmonicity of the perovskite lattice with the thermal entropy. At relatively low temperatures, the Cs cations tend to move away from the preferential positions with good contacts with the surrounding metal halide framework, potentially causing its conversion to a non-perovskite phase. Our simulations of defective structures reveal that, although both iodine vacancies and interstitials are very mobile in the perovskite lattice, the vacancies have a detrimental effect on the stability, initiating the decomposition reactions of perovskites to PbI$_{2}$. Our work puts ReaxFF forward as an effective computational framework to study reactive processes in halide perovskites.

cond-mat.mtrl-sci↗