arXiv subjects

C. Ugwumadu

Publications and source records attributed to C. Ugwumadu.

14 recordsLinked to original sources

arXiv2026

Solver-Agnostic Implementation of Atom-Informed Thermal Conductivity Fields in Continuum Heat-Flow Simulations

A recent work introduced the Simulator Collection for Atomic-to-Continuum Scales (SCACS) toolkit, a framework for improving finite element predictions of heat flow by mapping atom-resolved thermal conductivity into the stiffness matrix of the Galerkin finite element formulation [Ugwumadu et al., Phys. Rev. Materials 10, 053804 (2026)]. Here, we demonstrate that SCACS-derived conductivity fields are solver-independent and can be transferred to existing continuum simulation platforms. As a proof of concept, we map SCACS-derived conductivity fields from complex silicon structures onto finite element meshes in Abaqus and compare the resulting heat-flow solutions with that obtained using conventional uniform-conductivity assignment within Abaqus. Comparison of the two implementations shows that atom-informed conductivity fields can be incorporated into existing finite element workflows and improve realistic prediction and the accuracy of its solution. This work supports broader efforts to improve the predictive capability of continuum simulations for efficient materials design and property prediction.

cond-mat.mtrl-sci↗

arXiv2026

Real-Space Mapping of Electronic Conductivity in Complex Materials

We introduce KuboMap, a real-space representation of electronic conductivity derived from the Kubo-Greenwood formula. KuboMap defines a nonnegative conductivity density whose spatial integral recovers the total conductivity and whose form is guided by Mott's picture of transport through spatially overlapping electronic states. This construction provides a direct map of the transport-active regions of a material. Applied to aluminum, KuboMap recovers an extended metallic conduction network. Applied to amorphous silicon, it distinguishes an insulating defect-free network from a defective structure in which localized near-Fermi states form connected hopping-like pathways. In silicon-oxides, it captures the loss of conduction as increasing oxygen content disrupts Silicon-rich transport networks. KuboMap provides a physically transparent route from Kubo--Greenwood conductivity to real-space transport pathways in complex materials.

cond-mat.mtrl-sci↗

arXiv2026

Seamlessly joining length scales: From atomistic thermal graphs to anisotropic continuum conductivity

Thermal transport in complex solids is governed by local structure, defects, and anisotropy, yet most continuum models still rely on oversimplified, homogenized conductivities. Here, we bridge atomistic and continuum descriptions by building finite element (FE) models directly from the site-projected thermal conductivity (SPTC), an atomic-level decomposition of the Green-Kubo thermal conductivity. We introduce a new toolkit, the Simulator Collection for Atomic-to-Continuum Scales (SCACS), which uses a graph neural network to predict SPTC on large atomic structures, coarse-grain these fields into anisotropic conductivity tensors, and embeds them into the heat-flow FE equation with a customized, anisotropy-aware adaptive mesh refinement scheme. Applied to silicon nanostructures, the resulting FE models act as representative volume elements, reproduce bulk conductivities, and capture interfacial and defect-driven anisotropy while maintaining thermodynamic consistency. Additionally, SCACS predicts experimental conductance trends and fields. This innovation demonstrates a novel and general route for transferring atomistic transport information into device-scale thermal simulations with physics-based approximations.

cond-mat.mtrl-sci↗

arXiv2025

Spatially Local Estimates of the Thermal Conductivity of Materials

In this paper we describe a spatial decomposition of the thermal conductivity, what we name "site-projected thermal conductivity", a gauge of the thermal conduction activity at each site. The method is based on the Green-Kubo formula and the harmonic approximation, and requires the force-constant and dynamical matrices and of course the structure of a model sitting at an energy minimum. Throughout the paper, we use high quality models previously tested and compared to many experiments. We discuss the method and underlying approximations for amorphous silicon, carry our detailed analysis for amorphous silicon, then examine an amorphous-crystal silicon interface, and representative carbon materials. We identify the sites and local structures that reduce heat transport, and quantify these (estimate the spatial range) over which these "thermal defects" are effective. Similarities emerge between these filamentary structures in the amorphous silicon network which impact heat transport, electronic structure (the Urbach edge) and electronic transport.

cond-mat.mtrl-sci↗

arXiv2025

Effects of Galactic Irradiation on Thermal and Electronic Transport in Tungsten

The impact of irradiation on the thermal and electronic properties of materials is a persistent puzzle, particularly defect formation at the atomic and nanoscales. This work examines the nanoscale effects of low-energy irradiation on tungsten (W), focusing on defect-induced modifications to thermal and electronic transport. Using the Site-Projected Thermal Conductivity (SPTC) method [A. Gautam et al. PSS-RRL, 2400306, 2024], we analyze bulk and twin-grain boundary W with vacancy defects based on the Norgett-Robinson-Torrens displacements per atom (NRT-dpa) model. SPTC provides a detailed prediction of post-cascade spatial thermal conductivity distribution. We estimate electronic conductivity activity using the "N2 method" [K. Nepal et al. Carbon, 119711, 2025] to explore the consequences of vacancies and grain boundaries, highlighting the defect-dependent nature of charge transport behavior. These findings offer high-resolution insights into irradiation-driven transport phenomena, with implications for space-exposed materials and nanoscale thermal/electronic management.

cond-mat.mtrl-sci↗

arXiv2024

Site-projected Thermal Conductivity: Application to defects, interfaces, and homogeneously disordered materials

With the rapid advance of high-performance computing and electronic technologies, understanding thermal conductivity in materials has become increasingly important. This study presents a novel method: the Site-projected Thermal Conductivity (SPTC) that quantitatively estimates the local (atomic) contribution to heat transport, leveraging the Green-Kubo thermal transport equations. We demonstrate the effectiveness of this approach on disordered and amorphous graphene, amorphous silicon, and grain boundaries in silicon-germanium alloys. Amorphous graphene reveals a percolation behavior for thermal transport. The results highlight the potential of our method to provide new insights into the thermal behavior of materials, offering a promising avenue for materials design and performance optimization.

cond-mat.mtrl-sci↗

arXiv2024

Physical origin of enhanced electrical conduction in aluminum-graphene composites

The electronic and transport properties of aluminum-graphene composite materials were investigated using ab initio plane wave density functional theory. The interfacial structure is reported for several configurations. In some cases, the face-centered aluminum (111) surface relaxes in a nearly ideal registry with graphene, resulting in a remarkably continuous interface structure. The Kubo-Greenwood formula and space-projected conductivity were employed to study electronic conduction in aluminum single- and double-layer graphene-aluminum composite models. The electronic density of states at the Fermi level is enhanced by the graphene for certain aluminum-graphene interfaces, thus, improving electronic conductivity. In double-layer graphene composites, conductivity varies non-monotonically with temperature, showing an increase between 300-400 K at short aluminum-graphene distances, unlike the consistent decrease in single-layer composites.

cond-mat.mtrl-sci↗

arXiv2023

Electronic Conductivity in Metal-Graphene Composites: The Role of Disordered Carbon Structures, Defects, and Impurities

This paper explores the transport properties of aluminum-carbon composite material via ab initio methods. Interfacial and electronic dynamics of the aluminum-graphene interface structure were investigated using models of amorphous graphene added to an aluminum matrix. We examine the impact on electronic conduction caused by the presence of nitrogen impurities within the interfacial amorphous graphene layer. We elucidate the conduction mechanisms by using a projection of the conductivity into space.

cond-mat.mtrl-sci↗

arXiv2023

Improved Photocatalytic Performance via Air-Plasma Modification of Titanium Dioxide: Insights from Experimental and Simulation Investigation

Commercial titanium dioxide is successfully plasma-treated under ambient conditions for different time periods, leading to reduced crystallite size and the creation of oxygen vacancies. Density functional theory-based calculations reveal the emergence of additional localized states close to the conduction band, primarily associated with under-coordinated titanium atoms in non-stoichiometric titanium-oxide systems. The plasma-treated samples exhibit improved photocatalytic performance in the degradation of methylene blue compared to untreated samples. Moreover, the 4-hour plasma-treated photocatalyst demonstrates commendable stability and reusability. This work highlights the potential of cost-effective plasma treatment as a simple modification technique to significantly enhance the photocatalytic capabilities of titanium dioxide.

physics.app-ph↗

arXiv2023

Computer Simulation of Carbonization and Graphitization of Coal

This study describes computer simulations of carbonization and graphite formation, including the effects of hydrogen, nitrogen, oxygen, and sulfur. We introduce a novel technique to simulate carbonization, "Simulation of Thermal Emission of Atoms and Molecules (STEAM)," designed to elucidate the removal of volatiles and density variations in carbonization residue. The investigation extensively analyzes the functional groups that endure through high-temperature carbonization and examines the graphitization processes in carbon-rich materials containing non-carbon "impurity elements". The physical, vibrational, and electronic attributes of impure amorphous graphite are analyzed, and the impact of nitrogen on electronic conduction is investigated, revealing its substitutional integration into the sp$^2$ layered network.

cond-mat.mtrl-sci↗

arXiv2022

Simulation of multi-shell fullerenes using Machine-Learning Gaussian Approximation Potential

Multi-shell fullerenes "buckyonions" were simulated, starting from initially random configurations, using a density-functional-theory (DFT)-trained machine-learning carbon potential within the Gaussian Approximation Potential (ML-GAP) Framework [Volker L. Deringer and Gabor Csanyi, Phys. Rev. B 95, 094203 (2017)]. A large set of such fullerenes were obtained with sizes ranging from 60 ~ 3774 atoms. The buckyonions are formed by clustering and layering starts from the outermost shell and proceed inward. Inter-shell cohesion is partly due to interaction between delocalized $π$ electrons into the gallery. The energies of the models were validated ex post facto using density functional codes, VASP and SIESTA, revealing an energy difference within the range of 0.02 - 0.08 eV/atom after conjuagte gradient energy convergence of the models were achieved with both methods.

cond-mat.dis-nn↗

arXiv2022

Electronic transport in copper-graphene composites

We investigate the electronic transport properties of copper-graphene composites using a density-functional framework. Conduction in composites by varying the interface distance of a copper/graphene/copper (Cu/G/Cu) interface models was studied. The electronic density of states reveals increasing contributions from both copper and carbon atoms near the Fermi level with decreasing Cu-G interfacial distance. Electronic conductivity of the models computed using the Kubo-Greenwood formula showed the conductivity increases with decreasing Cu-G distance. We also find that the conductivity saturates below a threshold Cu-G distance. By computing the space-projected conductivity of the Cu/G/Cu models, we show that the graphene forms a bridge to the electronic conduction at small copper-graphene distances, thereby enhancing the conductivity.

cond-mat.mtrl-sci↗

arXiv2022

Atomistic Nature of Amorphous Graphite

This paper focuses on the structural, electronic, and vibrational features of amorphous graphite [R. Thapa $\textit{et. al.}$, Phys. Rev. Lett. 128, 236402 (2022)]. The structure order in amorphous graphite is discussed and compared with graphite and amorphous carbon. The electronic density of states and localization in these phases were analyzed. Spatial projection of charge densities in the $π$ bands showed a high charge concentration on participating atoms in connecting hexagons. A vibrational density of states was computed and is potentially an experimentally testable fingerprint of the material. An analysis of the vibrational modes was carried out using the phase quotient, and the mode stretching character. The average thermal conductivity calculated for aG was 0.85 Wcm$^{-1}$K$^{-1}$ and 0.96 Wcm$^{-1}$K$^{-1}$ at room temperature and 1000 K respectively.

cond-mat.mtrl-sci↗

arXiv2022

Formation of amorphous carbon multi-walled nanotubes from random initial configurations

Amorphous carbon nanotubes (a-CNT) with up to four walls and sizes ranging from 200 to 3200 atoms have been simulated, starting from initial random configurations and using the Gaussian Approximation Potential [Phys. Rev. B 95, 094203 (2017)]. The important variables (like density, height, and diameter) required to successfully simulate a-CNTs, were predicted with a machine learning random forest technique. The models were validated $\textit{ex post facto}$ using density functional codes. The a-CNT models ranged from 0.55 nm - 2 nm wide with an average inter-wall spacing of 0.31 nm. The topological defects in a-CNTs were discussed and new defect configurations were observed. The electronic density of states and localization in these phases were discussed and delocalized electrons in the $π$ subspace were identified as an important factor for inter-layer cohesion. Spatial projection of the electronic conductivity favors axial transport along connecting hexagons, while non-hexagonal parts of the network either hinder or bifurcate the electronic transport. A vibrational density of states was calculated and is potentially an experimentally testable fingerprint of the material and the appearance of a low-frequency radial breathing mode was discussed. The thermal conductivity at 300 K was calculated using the Green-Kubo formula.

cond-mat.mtrl-sci↗