arXiv subjects

Benjamin A. D. Williamson

Publications and source records attributed to Benjamin A. D. Williamson.

5 recordsLinked to original sources

arXiv2026

Machine-learning-guided exploration of domain walls in the hybrid improper ferroelectric Ca3Ti2O7

Ruddlesden-Popper phases are highly tunable and naturally layered structures, in which polarization can arise via a hybrid improper ferroelectric mechanism. This enables a complex domain wall (DW) structure where multiple order parameters, like octahedral rotations, polar distortions and strain, interact. In this work, we explore the rich set of DW structures in prototypical Ca3Ti2O7, mapping out the DWs in the {100}, {110} and {001} pseudo-tetragonal planes using group theory and machine-learned interatomic potentials (MLIPs). The trained potential reproduces the density functional theory (DFT) order parameter and polarization profiles for all wall types and orientations considered. A charge-aware training framework combined with reference Born effective charges reduces the prediction errors for the DW formation energies by 70%, revealing the importance of including long-range electrostatics to model symmetry-broken interfaces. Finally, the MLIP is used to identify minimum energy pathways at the atomic scale with nearly the precision of DFT calculations, revealing a low-energy antipolar configuration for polarization switching.

cond-mat.mtrl-sci↗

arXiv2026

Electronic Structure and Resonant Circular Dichroism of La$_{0.7}$Sr$_{0.3}$MnO$_3$ from Soft X-ray Angle-Resolved Photoemission

Coupling between spin, orbital, charge, and lattice degrees of freedom in transition-metal oxides produces a variety of electronic and magnetic phenomena of importance for future technologies. Here, we explore the electronic band structure of a (111)-oriented La0.7Sr0.3MnO3 thin film through soft X-ray angle-resolved photoemission spectroscopy (ARPES). The measurements agree with the electronic band structure calculated with density functional theory using Hubbard U correction. Furthermore, we probe the circular dichroism in ARPES, and observe a pronounced momentum- resolved magnetic circular dichroism in resonant photoemission from the Mn L-edge. The approach combines the momentum- and spin-selectivity of ARPES and X-ray magnetic circular dichroism, respectively, which could provide a useful approach for the study of unconventional magnetism.

cond-mat.str-el↗

arXiv2025

Local indirect magnetoelectric coupling at twin walls in CaMnO$_3$

Ferroelastic twin walls in centrosymmetric perovskites can host emergent polar and magnetic properties forbidden in the bulk. We use density functional theory calculations to study the geometry and magnetic properties of ferroelastic domain walls in orthorhombic CaMnO$_3$, which belongs to the most common perovskite space group, $Pnma$. At the wall, the inherent inversion symmetry-breaking induces local polar distortions dependent on the wall geometry, which couple to the magnetic order through the octahedral distortions. Noncollinear calculations reveal enhanced out-of-plane magnetic moments on the Mn atoms and a local, finite magnetization confined to the wall. Strain fields across twin walls thus give rise to coexistence of polarization and magnetization as well as magnetoelectric response that is absent and symmetry-forbidden in bulk CaMnO$_3$. We propose that magnetoelectric coupling and coexisting polarization and magnetization can emerge at twin walls in bulk centrosymmetric antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2025

Domain Walls and Defects in Ferroelectric Inorganic Halide Perovskites CsGeX$_3$ (X = Cl, Br, I)

Among all-inorganic halide perovskites, the only known ferroelectrics are the family of CsGeX$_3$ (X = Cl, Br, I). Here, we study their ferroelectric domain walls (DWs) and common point defects by density functional theory (DFT) calculations and investigate the interplay between DWs and defects. The most stable defects are V$_{\text{X}}$ and V$_{\text{Cs}}$ and the former shows low migration barriers and high mobility. In contrast to oxide ferroelectrics, the affinity between point defects and DWs is negligible, reflecting the subtle structural distortions at CsGeX$_3$ DWs. Concomitantly, the formation energies and migration energy barriers of CsGeX$_3$ DWs are small compared to oxides, and neither V$_{\text{X}}$ nor V$_{\text{Cs}}$ pin migrating DWs. The band gap invariance across DWs and the lack of affinity towards intrinsic charged point defects imply that conducting DWs for nanoelectronics may be challenging to realise in CsGeX$_3$. However, shallow $p$-type defect levels and low hole effective masses suggest that high $p$-type conductivity may be achievable in nominally ferroelectric CsGeX$_3$. The low DW migration energy barriers and insignificant DW pinning by point defects make CsGeX$_3$ promising materials as robust soft ferroelectrics for high-frequency switching applications with low energy dissipation.

cond-mat.mtrl-sci↗

arXiv2025

Finite-Temperature Ferroelectric Phase Transitions from Machine-Learned Force Fields

Simulating finite temperature phase transitions from first-principles is computationally challenging. Recently, molecular dynamics (MD) simulations using machine-learned force fields (MLFFs) have opened a new avenue for finite-temperature calculations with near-first-principles accuracy. Here we use MLFFs, generated using on-the-fly training, to investigate structural phase transitions in four of the most well-studied ferroelectric oxides; BaTiO$_3$, PbTiO$_3$, LiNbO$_3$ and BiFeO$_3$. Only using the 0 K ground state structure as input for the training, the resulting MLFFs can qualitatively predict all the main structural phases and phase transitions, while the quantitative results are sensitive to the choice of exchange correlation functional with PBEsol found to be more robust than LDA and r$^2$SCAN. MD simulations also reproduce the experimentally observed order-disorder character of Ti displacements in BaTiO$_3$, the abrupt first order transitions of BiFeO$_3$ and PbTiO$_3$, and the mixed order-disorder and displacive character of the ferroelectric transition in LiNbO$_3$. Finally, we discuss the potential and limitations of using MLFFs for simulating ferroelectric phase transitions.

cond-mat.mtrl-sci↗