arXiv subjects

Harrison A. Walker

Publications and source records attributed to Harrison A. Walker.

4 recordsLinked to original sources

arXiv2026

PySlice: Routine Vibrational Electron Energy Loss Spectroscopy Prediction with Universal Interatomic Potentials

Vibrational spectroscopy in the electron microscope can reveal phonon excitations with nanometer spatial resolution, yet routine prediction remains out of reach due to fragmented workflows requiring specialized expertise. Here we introduce PySlice, the first publicly available implementation of the Time Autocorrelation of Auxiliary Wavefunction (TACAW) method, providing an automated framework that produces momentum- and energy-resolved vibrational electron energy-loss spectra directly from atomic structures. By integrating universal machine learning interatomic potentials with TACAW, PySlice eliminates the bottleneck of per-system potential development. Users input atomic structures and obtain phonon dispersions, spectral diffraction patterns, and spectrum images through a unified workflow spanning molecular dynamics, GPU-accelerated electron scattering, and frequency-domain analysis. We outline the formulation behind the code, demonstrate its application to canonical systems in materials science, and discuss its use for advanced analysis and materials exploration. The modular Python architecture additionally supports conventional electron microscopy simulations, providing a general-purpose platform for imaging and diffraction calculations. PySlice makes vibrational spectroscopy prediction routine rather than specialized, enabling computational screening for experimental design, systematic exploration of phonon physics across materials families, and high-throughput generation of simulated data for training of future machine learning models.

cond-mat.mtrl-sci↗

arXiv2026

Phonon selection and interference in momentum-resolved electron energy loss spectroscopy

As momentum-resolved Electron Energy Loss Spectroscopy (q-EELS) becomes more widely used for phonon measurements, better understanding of the intricacies of the acquired signal is necessary. Selection rules limit the allowed scattering, which may prohibit the appearance of specific phonon branches in some measurements. Simultaneous sampling of the lattice across all basis atoms also warrants a coherent treatment of phonons, which yields a reciprocal-space repeating unit cell that is larger than the standard Brillouin zone. We thus introduce the concept of the ``interferometric Brillouin zone'' where phonons are observed, which is closely related to the Dynamic Structure Factor. These effects follow from our new mathematical formalism for waves and vibrations, and we demonstrate calculations of q-EELS experiments using molecular dynamics and lattice dynamics. Results are compared to established q-EELS simulation methods in well-studied material systems, including the utilization of scattering selection rules to acquire a polarization-selective vibrational density of states. Finally, we note the analysis involved is directly applicable to any wave phenomena, such as plasmons or polaritons.

cond-mat.mtrl-sci↗

arXiv2025

Hydrogen bonding in water under extreme confinement

Fluids under extreme confinement exhibit unique structures and intermolecular bonding, distinct from their bulk analogs, driving innovative applications at the water-energy nexus. Probing confined water experimentally at the length scale of intermolecular and surface forces has, however, remained a challenge. Here, we report direct molecular-level observations of hydrogen bonding in water confined inside individual carbon nanotubes, enabled by in-situ vibrational electron energy-loss spectroscopy with nanoscale resolution. Hydrogen bonding is probed via the intramolecular O-H stretching frequency, which serves as a sensitive spectral signature of the local intermolecular bonding environment. Water in larger carbon nanotubes exhibit the bonded O-H vibrations of bulk water, but at smaller diameters, the frequency blueshifts to near the free O-H stretch found in water vapor and hydrophobic surfaces, indicating a highly dispersed, non-H-bonded environment. Theoretical analysis based on quantum vibrational oscillators indicates that enhanced damping rates, corresponding to rapid hydrogen-bond fluctuations, leads the bimodal spectral peaks to merge into a single broad feature, matching the experimental observation. Furthermore, cryogenic experiments provide insights into complex structural phase transitions of confined water. This research reveals the quantum and dynamic nature of hydrogen bonds under confinement and the potential impact of unveiling molecular-level structure and bonding in confined fluids.

physics.chem-ph↗

arXiv2025

Topology-Driven Vibrations in a Chiral Polar Vortex Lattice

The ordering of magnetic or electric dipoles leading to real-space topological structures is at the forefront of materials research as their quantum mechanical nature often lends itself to emergent properties. Atomic lattice vibrations (phonons) are often a key contributor to the formation of long-range dipole textures based on ferroelectrics and impact the properties of the emergent phases. Here, using monochromated, momentum-resolved electron energy-loss spectroscopy (qEELS) with nanometer spatial resolution and meV-spectral-precision, we demonstrate that polar vortex lattices in PbTiO$_3$ spatially modulate the material's vibrational spectrum in patterns that directly reflect the overlying symmetry of the topological patterns. Moreover, by combining experiments with molecular dynamics simulations using machine learned potentials we reveal how these structures modify phonon modes across the vibrational spectrum. Beyond simple intensity modulation, we find that the chirality of the vortex topology imparts its unique symmetry onto phonons, producing a distinctive asymmetrical spectral shift across the vortex unit cell. Finally, the high spatial resolution of the technique enables topological defects to be probed directly, demonstrating a return to trivial PbTiO$_3$ modes at vortex dislocation cores. These findings establish a fundamental relationship between ferroelectric-ordering-induced topologies and phonon behavior, opening new avenues for engineering thermal transport, electron-phonon coupling, and other phonon-mediated properties in next-generation nanoscale devices.

cond-mat.mtrl-sci↗