arXiv subjects

David A. Drabold

Publications and source records attributed to David A. Drabold.

At least 19 recordsLinked to original sources

arXiv2026

A Defect-Free Model of Amorphous Silicon with Pristine Electronic Structure

Amorphous silicon (a-Si) is understood to be the canonical continuous random network material, ideally defined by fully fourfold coordination. Here, we show that a defect-free ('ideal') model of a-Si from machine-learning-driven molecular-dynamics simulations [L. A. M. Rosset et al., Nat. Commun. 16, 2360 (2025)], subsequently evaluated with hybrid-level density-functional theory computations, can accurately reproduce the experimentally observed electronic bandgap. We compare this model with one resulting from the Wooten-Winer-Weaire (WWW) bond-switching approach and with other recent approximants to ideal a-Si. More broadly, our work provides a platform for studies of band tails, optical properties, and transport in a-Si.

cond-mat.mtrl-sci↗

arXiv2025

Electronic and thermal properties of the phase-change memory material, Ge2Sb2Te5, and results from spatially resolved transport calculations

We report new insights into the electronic, structural, and transport (heat and charge) properties of the phase-change memory material Ge2Sb2Te5. Using realistic structural models of Konstantinou et. al. [Nat. Commun. 10, 3065 (2019)], we analyze the topology, electronic states, and lattice dynamics with density functional methods, including hybrid-functional calculations and machine-learned interatomic potentials. The Kohn-Sham orbitals near the Fermi level display a strong electron-phonon coupling, and exhibit large energy fluctuations at room temperature. The conduction tail states exhibit larger phonon-induced fluctuations than the valence tail states. To resolve transport at the atomic scale, we employ space-projected electronic conductivity and site-projected thermal conductivity methods. Local analysis of heat transport highlights the role of filamentary networks dominated by Te, with Sb and Ge making progressively smaller contributions.

cond-mat.mtrl-sci↗

arXiv2024

Signatures of paracrystallinity in amorphous silicon

The structure of amorphous silicon (a-Si) has been studied for decades. The two main theories are based on a continuous random network and on a `paracrystalline' model, respectively -- the latter being defined as showing localized structural order resembling the crystalline state whilst retaining an overall amorphous network. However, the extent of this local order has been unclear, and experimental data have led to conflicting interpretations. Here we show that signatures of paracrystallinity in an otherwise disordered network are indeed compatible with the existing body of experimental observations for a-Si. We use quantum-mechanically accurate, machine-learning-driven simulations to systematically sample the configurational space of quenched a-Si, thereby allowing us to elucidate the boundary between amorphization and crystallization. We analyze our dataset using structural and local-energy descriptors to show that paracrystalline models are consistent with experiments in both regards. Our work provides a unified explanation for seemingly conflicting theories in one of the most widely studied amorphous networks.

cond-mat.mtrl-sci↗

arXiv2023

Understanding defects in amorphous silicon with million-atom simulations and machine learning

The structure of amorphous silicon is widely thought of as a fourfold-connected random network, and yet it is defective atoms, with fewer or more than four bonds, that make it particularly interesting. Despite many attempts to explain such "dangling-bond" and "floating-bond" defects, respectively, a unified understanding is still missing. Here, we show that atomistic machine-learning methods can reveal the complex structural and energetic landscape of defects in amorphous silicon. We study an ultra-large-scale, quantum-accurate structural model containing a million atoms, and more than ten thousand defects, allowing reliable defect-related statistics to be obtained. We combine structural descriptors and machine-learned local atomic energies to develop a universal classification of the different types of defects in amorphous silicon. The results suggest a revision of the established floating-bond model by showing that fivefold-coordinated atoms in amorphous silicon exhibit a wide range of local environments, and it is shown that fivefold (but not threefold) coordination defects tend to cluster together. Our study provides new insights into one of the most widely studied amorphous solids, and has general implications for modelling and understanding defects in disordered materials beyond silicon alone.

cond-mat.mtrl-sci↗

arXiv2019

Structural transitions in dense disordered silicon from quantum-accurate ultra-large-scale simulations

Structurally disordered materials continue to pose fundamental questions, including that of how different disordered phases ("polyamorphs") can coexist and transform from one to another. As a widely studied case, amorphous silicon (a-Si) forms a fourfold-coordinated, covalent random network at ambient conditions, but much higher-coordinated, metallic-like phases under pressure. However, a detailed mechanistic understanding of the liquid-amorphous and amorphous-amorphous transitions in silicon has been lacking, due to intrinsic limitations of even the most advanced experimental and computational techniques. Here, we show how machine-learning (ML)-driven simulations can break through this long-standing barrier, affording a comprehensive, quantum-accurate, and fully atomistic description of all relevant liquid and amorphous phases of silicon. Combining a model system size of 100,000 atoms (ten-nanometre length scale) with a prediction accuracy of a few meV per atom, our simulations reveal a remarkable, three-step transformation sequence for a-Si under increasing external pressure. First, up to 10-11 GPa, polyamorphic low- and high-density amorphous (LDA and HDA) regions are found to coexist, rather than appearing sequentially. Then, we observe a structural collapse into a distinct, very-high-density amorphous (VHDA) phase at 12-13 GPa, reminiscent of the dense liquid but being formed at a much lower temperature. Finally, our simulations indicate the transient nature of this VHDA phase: it rapidly nucleates crystallites at 13-16 GPa, ultimately leading to the formation of a poly-crystalline, simple-hexagonal structure, consistent with experiments but not seen in earlier simulations.

cond-mat.mtrl-sci↗

arXiv2019

Ab initio inversion of structure and the lattice dynamics of a metallic glass: The case of Pd40Ni40P20

In this paper we infer the structure of Pd40Ni40P20 from experimental diffraction data and ab initio interactions using Force Enhanced Atomic Refinement (FEAR). Our model accurately reproduces known experimental signatures of the system and is more efficient than conventional melt-quench schemes. We critically evaluate the local order, carry out detailed comparisons to extended X-ray absorption fine structure (EXAFS) experiments and also discuss the electronic structure. We thoroughly explore the lattice dynamics of the system, and describe a vibrational localized-to-extended transition and discuss the special role of P dynamics. At low energies P is fully contributing to extended modes, but at higher frequencies executes local motion reminiscent of a rattler inside a cage of metal atoms. These highly localized vibrational states suggest a possible utility of these materials for thermoelectric applications.

cond-mat.mtrl-sci↗

arXiv2018

Quantifying Chemical Structure and Atomic Energies in Amorphous Silicon Networks

Amorphous materials are coming within reach of realistic computer simulations, but new approaches are needed to fully understand their intricate atomic structures. Here, we show how machine-learning (ML)-based techniques can give new, quantitative chemical insight into the atomic-scale structure of amorphous silicon (a-Si). Based on a similarity function ("kernel"), we define a structural metric that unifies the description of nearest- and next-nearest-neighbor environments in the amorphous state. We apply this to an ensemble of a-Si networks, generated in melt-quench simulations with an ML-based interatomic potential, in which we tailor the degree of ordering by varying the quench rates down to $10^{10}$ K/s (leading to a structural model that is lower in energy than the established WWW network). We then show how "machine-learned" atomic energies permit a chemical interpretation, associating coordination defects in a-Si with distinct energetic stability regions. The approach is straightforward and inexpensive to apply to arbitrary structural models, and it is therefore expected to have more general significance for developing a quantitative understanding of the amorphous state.

cond-mat.mtrl-sci↗

arXiv2018

First principles modeling of the structural, electronic, and vibrational properties of Ni$_{40}$Pd$_{40}$P$_{20}$ bulk metallic glass

The structural, vibrational, and electronic properties of Ni$_{40}$Pd$_{40}$P$_{20}$ bulk metallic glass have been studied using ${\it ab\,initio}$ molecular-dynamics simulations and total-energy optimization. Structural analyses of the resulting ${\it ab\,initio}$ models show the presence of few to no P-P bonds and two main building blocks, consisting of tricapped trigonal prism (TTP) and capped square anti-prism (CSAP) with P as the center of these blocks. The computed Pd and Ni K-edge spectra of extended x-ray absorption fine structure (EXAFS) are found to be in good agreement with experimental data. The configurational-average static structure factor and the generalized vibrational density of states are also observed to be in good agreement with experimental data.

cond-mat.dis-nn↗

arXiv2018

${\it Ab \: initio}$ density-functional studies of 13-atom Cu and Ag clusters

The putative ground-state structures of 13-atom Cu and Ag clusters have been studied using ${\it ab \: initio}$ molecular-dynamics (AIMD) simulations based on the density-functional theory (DFT). An ensemble of low-energy configurations, collected along the AIMD trajectory and optimized to nearest local minimum-energy configurations, were studied. An analysis of the results indicates the existence of low-symmetric bilayer structures as strong candidates for the putative ground-state structure of Cu$_{13}$ and Ag$_{13}$ clusters. These bilayer structures are markedly different from a buckled bi-planar (BBP) configuration and energetically favorable, by about 0.4$-$0.5 eV, than the latter proposed earlier by others. Our study reveals that the structure of the resulting putative global-minimum configuration is essentially independent of the nature of basis functions (i.e., plane waves versus pseudoatomic orbitals) employed in the calculations, for a given exchange-correlation functional. The structural configurations obtained from plane-wave-based DFT calculations show a slightly tighter or dense first-shell of Cu and Ag atoms than those from local-basis functions. A comparison of our results with recent full-potential DFT simulations is presented.

cond-mat.dis-nn↗

arXiv2018

Effect of low-temperature annealing on the void-induced microstructure in amorphous silicon: A computational study

We present a computational study of the void-induced microstructure in amorphous silicon ($\it a$-Si) by generating ultra-large models of $\it a$-Si with a void-volume fraction of 0.3$\%$, as observed in small-angle x-ray scattering (SAXS) experiments. The relationship between the morphology of voids and the intensity of scattering in SAXS has been studied by computing the latter from the Fourier transform of the reduced pair-correlation function and the atomic-form factor of amorphous silicon. The effect of low-temperature ($\le$ 600 K) annealing on the scattering intensities and the microstructure of voids has been addressed, with particular emphasis on the shape and size of the voids, by studying atomic rearrangements on the void surfaces and computing the average radius of gyration of the voids from the spatial distribution of surface atoms and the intensity plots in the Guinier approximation. The study suggests that low-temperature annealing can lead to considerable restructuring of void surfaces, which is clearly visible from the three-dimensional shape of the voids but it may not necessarily reflect in one-dimensional scattering-intensity plots.

cond-mat.dis-nn↗

arXiv2018

Small-angle X-ray scattering in amorphous silicon: A computational study

We present a computational study of small-angle X-ray scattering (SAXS) in amorphous silicon ($a$-Si) with particular emphasis on the morphology and microstructure of voids. The relationship between the scattering intensity in SAXS and the three-dimensional structure of nanoscale inhomogeneities or voids is addressed by generating ultra-large high-quality $a$-Si networks with 0.1-0.3% volume concentration of voids, as observed in experiments using SAXS and positron annihilation spectroscopy. A systematic study of the variation of the scattering intensity in the small-angle scattering region with the size, shape, number density, and the spatial distribution of the voids in the networks is presented. Our results suggest that the scattering intensity in the small-angle region is particularly sensitive to the size and the total volume-fraction of the voids, but the effect of the geometry or shape of the voids is less pronounced in the intensity profiles. A comparison of the average size of the voids obtained from the simulated values of the intensity, using the Guinier approximation and Kratky plots, with those from the spatial distribution of the atoms in the vicinity of void surfaces is presented.

cond-mat.dis-nn↗

arXiv2016

Inversion of diffraction data for amorphous materials

The general and practical inversion of diffraction data-producing a computer model correctly representing the material explored - is an important unsolved problem for disordered materials. Such modeling should proceed by using our full knowledge base, both from experiment and theory. In this paper, we describe a robust method to jointly exploit the power of ab initio atomistic simulation along with the information carried by diffraction data. The method is applied to two very different systems: amorphous silicon and two compositions of a solid electrolyte memory material silver-doped GeSe3 . The technique is easy to implement, is faster and yields results much improved over conventional simulation methods for the materials explored. By direct calculation, we show that the method works for both poor and excellent glass forming materials. It offers a means to add a priori information in first principles modeling of materials, and represents a significant step toward the computational design of non-crystalline materials using accurate interatomic interactions and experimental information.

cond-mat.mtrl-sci↗

arXiv2011

Temperature Coefficient of Resistivity in Amorphous Semiconductors

By invoking the microscopic response method in conjunction with a reasonable set of approximations, we obtain new explicit expressions for the electrical conductivity and temperature coefficient of resistivity (TCR) in amorphous semiconductors, especially a-Si:H and a-Ge:H. The predicted TCR for n-doped a-Si:H and a-Ge:H is in agreement with experiments. The conductivity from the transitions from a localized state to an extended state (LE) is comparable to that from the transitions between two localized states (LL). This resolves a long-standing anomaly, a "kink" in the experimental $\log_{10}σ$ vs. T$^{-1}$ curve.

cond-mat.stat-mech↗

arXiv2011

The electronic activity of boron and phosphorus impurities in a-Si and a-Si:H

In amorphous materials, acceptor and donor impurities rarely dope the system (shift the Fermi level). We find out why in a-Si:H. We report simulations on B and P doping of a-Si:H and a-Si. We analyze the Electronic Density of States (EDOS) with concentrations ranging from 1.6% to 12.5% of B or P in a-Si. The results indicate that tetrahedral B and P are effective doping configurations in a-Si, but high impurity concentrations introduce defect states. Clustered B or P also introduced mid-gap states. For a-Si:H, we report that both B(3,1) and P(3,1) (B or P atom bonded with three Si atoms and one H atom) are effective doping configurations. We investigate H passivation in both cases. There exists a "hydrogen poison range" for which H can modify the dopant configuration and suppress doping. For B doping, nearby H prefers to stay at the bond-center of Si-Si, leaves B four-fold and neutralizes the doping configuration; for P doping, nearby H spoils the doping by making tetrahedral P three-fold.

cond-mat.dis-nn↗

arXiv2011

The properties of amorphous GaN

In this paper, we present three amorphous GaN models obtained from the first principles simulation. We find that a chemically ordered continuous random network is the ideal structure for a-GaN. If we exclude the tail states, we predict a 3.0eV optical gap for 64-atom model and 2.3eV for 250-atom models. We observe a highly localized valence tail and a remarkably delocalized exponential conduction tail which we associate with different hybridization in the two tails. Based upon these results, we speculate on potential differences in n and p type doping. The structural origin of tail and defect states is discussed. The vibrational density of states and dielectric function are computed, and are consistent with experiment.

cond-mat.dis-nn↗

arXiv2007

Experimentally Constrained Molecular Relaxation: The case of hydrogenated amorphous silicon

We have extended our experimentally constrained molecular relaxation technique (P. Biswas {\it et al}, Phys. Rev. B {\bf 71} 54204 (2005)) to hydrogenated amorphous silicon: a 540-atom model with 7.4 % hydrogen and a 611-atom model with 22 % hydrogen were constructed. Starting from a random configuration, using physically relevant constraints, {\it ab initio} interactions and the experimental static structure factor, we construct realistic models of hydrogenated amorphous silicon. Our models confirm the presence of a high frequency localized band in the vibrational density of states due to Si-H vibration that has been observed in a recent vibrational transient grating measurements on plasma enhanced chemical vapor deposited films of hydrogenated amorphous silicon.

cond-mat.mtrl-sci↗

arXiv2003

Enhanced Radiative Transition in Si_nGe_m Nanoclusters

Using an ab-initio molecular dynamics scheme (the Fireball scheme), we determined the equilibrium structure of intermediate size Si_nGe_m (n+m=71) nanoclusters with/without hydrogen passivation on the surface. Due to the strong surface distortion, defect states are found to permeate the energy gap of Si_nGe_m clusters. However, the defect states are removed by adding H atoms on the surface of Si_nGe_m clusters, and the gap opens up to a few eV, indicating a blueshift for photoluminescence. It is also found that the radiative transition between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) states is enhanced by one to two orders of magnitude for Si_nGe_m nanoclusters with respect to the corresponding pure Si clusters. This significant increase of the emission probability is attributed to the strong overlap of HOMO and LUMO wavefunctions that are centered mostly on the Ge atoms.

cond-mat.mtrl-sci↗

arXiv2003

Physical properties of a GeS2 glass using approximate ab initio molecular dynamics

With the use of {\em ab initio} based molecular dynamics simulations we study the structural, dynamical and electronic properties of glassy g-GeS$_2$ at room temperature. From the radial distribution function we find nearest neighbor distances almost identical to the experimental values and the static structure factor is close to its experimental counterpart. From the Ge-S-Ge bond angle distribution we obtain the correct distribution of corner and edge-sharing GeS$_4$ tetrahedra. Concerning the dynamical characteristics we find in the mean square displacement of the atoms discontinuous variations corresponding either to the removal of coordination defects around a single particle or to structural rearrangements involving a larger number of atoms. Finally we calculate the vibrational density of states, which exhibits two well separated bands as well as some features characteristic of the amorphous state, and the electronic density of states showing an optical gap of 3.27 eV.

cond-mat.dis-nn↗