arXiv subjects

Gavin Winter

Publications and source records attributed to Gavin Winter.

5 recordsLinked to original sources

arXiv2026

Vibrational, structural, and chemical fingerprints of ion diffusion in crystalline solids

Predicting mobile-ion self-diffusivity $D^*$ from molecular dynamics (MD) simulations is essential for identifying promising solid-state electrolytes, but directly simulating ion diffusion is computationally expensive, particularly with high-accuracy machine learning interatomic potentials (MLIPs). Diffusion is a slow, emergent process that requires long trajectories to converge. Thermodynamic properties, by contrast, converge much faster: the enthalpy $h$, vibrational entropy $s_{vib}$, and 2-body, excess configurational entropy $s^{ex}_{2,config}$ can be extracted from comparatively short MD trajectories, and they encode rich information about the free energy landscape from which transport properties like self-diffusivity ultimately arise. Intuitive correlations are discussed between these thermodynamic properties and ion diffusion, motivating a data-driven approach to exploit this link. A simple neural network was trained to predict diffusivity from features computed over short MD trajectories: a vibrational fingerprint (the vibrational density of states, VDOS) and a structural fingerprint (the radial distribution function, RDF), conditioned on chemistry information encoded in the MLIP embedding. This combination allows the model to predict the converged $\log_{10} D^*$ (cm$^2$/s) \textemdash\ normally obtained from significantly longer MD simulations \textemdash\ with a mean absolute error of 0.398 and a Spearman's rank correlation $\rho$ of 0.844.

cond-mat.mtrl-sci↗

arXiv2024

Flow Matching for Accelerated Simulation of Atomic Transport in Crystalline Materials

Atomic transport underpins the performance of materials in technologies such as energy storage and electronics, yet its simulation remains computationally demanding. In particular, modeling ionic diffusion in solid-state electrolytes (SSEs) requires methods that can overcome the scale limitations of traditional ab initio molecular dynamics (AIMD). We introduce LiFlow, a generative framework to accelerate MD simulations for crystalline materials that formulates the task as conditional generation of atomic displacements. The model uses flow matching, with a Propagator submodel to generate atomic displacements and a Corrector to locally correct unphysical geometries, and incorporates an adaptive prior based on the Maxwell-Boltzmann distribution to account for chemical and thermal conditions. We benchmark LiFlow on a dataset comprising 25-ps trajectories of lithium diffusion across 4,186 SSE candidates at four temperatures. The model obtains a consistent Spearman rank correlation of 0.7-0.8 for lithium mean squared displacement (MSD) predictions on unseen compositions. Furthermore, LiFlow generalizes from short training trajectories to larger supercells and longer simulations while maintaining high accuracy. With speed-ups of up to 600,000$\times$ compared to first-principles methods, LiFlow enables scalable simulations at significantly larger length and time scales.

cond-mat.mtrl-sci↗

arXiv2022

Simulations with machine learning potentials identify the ion conduction mechanism mediating non-Arrhenius behavior in LGPS

Li$_{10}$Ge(PS$_6$)$_2$ (LGPS) is a highly concentrated solid electrolyte, in which Coulombic repulsion between neighboring cations is hypothesized as the underlying reason for concerted ion hopping, a mechanism common among superionic conductors such as Li$_7$La$_3$Zr$_2$O$_{12}$ (LLZO) and Li$_{1.3}$Al$_{0.3}$Ti$_{1.7}$(PO$_4$)$_3$ (LATP). While first principles simulations using molecular dynamics (MD) provide insight into the Li$^+$ transport mechanism, historically, there has been a gap in the temperature ranges studied in simulations and experiments. Here, we used a neural network (NN) potential trained on density functional theory (DFT) simulations, to run up to 40-nanosecond long MD simulations at DFT-like accuracy to characterize the ion conduction mechanisms across a range of temperatures that includes previous simulations and experimental studies. We have confirmed a Li$^+$ sublattice phase transition in LGPS around 400 K, below which the \textit{ab}-plane diffusivity $D^*_{ab}$ is drastically reduced. Concomitant with the sublattice phase transition near 400 K, there is less cation-cation (cross) correlation, as characterized by Haven ratios closer to 1, and the vibrations in the system are more harmonic at lower temperature. Intuitively, at high temperature, the collection of vibrational modes may be sufficient to drive concerted ion hops. However, near room temperature, the vibrational modes available may be insufficient to overcome electrostatic repulsion, thus resulting in less correlated ion motion and comparatively slower ion conduction. Such phenomena of a sublattice phase transition, below which concerted hopping plays a less significant role, may be extended to other highly concentrated solid electrolytes such as LLZO and LATP.

cond-mat.mtrl-sci↗

arXiv2020

Fully-Compensated Ferrimagnetic Spin Filter Materials within the Cr$\textit{M}\textit{N}$Al Equiatomic Quaternary Heusler Alloys

XX'YZ equiatomic quaternary Heusler alloys (EQHA's) containing Cr, Al, and select Group IVB elements ($\textit{M}$ = Ti, Zr, Hf) and Group VB elements ($\textit{N}$ = V, Nb, Ta) were studied using state-of-the-art density functional theory to determine their effectiveness in spintronic applications. Each alloy is classified based on their spin-dependent electronic structure as a half-metal, a spin gapless semiconductor, or a spin filter material. We predict several new fully-compensated ferrimagnetic spin filter materials with small electronic gaps and large exchange splitting allowing for robust spin polarization with small resistance. CrVZrAl, CrVHfAl, CrTiNbAl, and CrTiTaAl are identified as particularly robust spin filter candidates with an exchange splitting of $\sim 0.20$ eV. In particular, CrTiNbAl and CrTiTaAl have exceptionally small band gaps of $\sim 0.10$ eV. Moreover, in these compounds, a spin asymmetric electronic band gap is maintained in 2 of 3 possible atomic arrangements they can take, making the electronic properties less susceptible to random site disorder. In addition, hydrostatic stress is applied to a subset of the studied compounds in order to determine the stability and tunability of the various electronic phases. Specifically, we find the CrAlV$\textit{M}$ subfamily of compounds to be exceptionally sensitive to hydrostatic stress, yielding transitions between all spin-dependent electronic phases.

cond-mat.mtrl-sci↗

arXiv2020

Broadband Free Space Impedance in $\mathrm{Co_2Z}$ Hexaferrites by Substitution of Quadrivalent Heavy Transition Metal Ions for Miniaturized RF Devices

Polycrystalline samples of Z-type hexaferrites, having nominal compositions $\mathrm{Ba_3Co_{2+x}Fe_{24-2x}M_xO_{41}}$ where M = $\mathrm{Ir^{4+}, Hf^{4+}, Mo^{4+}}$ and x=0 and 0.05, were processed via ceramic processing protocols in pursuit of low magnetic and dielectric losses as well as equivalent permittivity and permeability. Fine process control was conducted to ensure optimal magnetic properties. Organic dispersants (i.e., isobutylene and maleic anhydride) were employed to achieve maximum densities. Crystallographic structure, characterized by X-ray diffraction, revealed that doping with $\mathrm{Ir^{4+}, Hf^{4+}, Mo^{4+}}$ did not adversely affect the crystal structure and phase purity of the Z-type hexaferrite. The measured microwave and magnetic properties show that the resonant frequency shifts depending on the specific dopant allowing for tunability of the operational frequency and bandwidth. The frequency bandwidth in which permittivity and permeability are very near equal (i.e., ~400 MHz for $\mathrm{Mo^{4+}}$ (x), where x=0.05 doping) is shown to occur at frequencies between 0.2 and 1.0 GHz depending on dopant type. These results give rise to low loss at 650 MHz, with considerable size reduction of an order of magnitude, while maintaining the characteristic impedance of free space (i.e., 377 $\mathrm{\Omega}$). These results allow for miniaturization and optimized band-pass performance of magnetodielectric materials for communication devices such as antenna and radomes that can be engineered to operate over desired frequency ranges using cost effective and volumetric processing methodologies.

physics.app-ph↗