arXiv subjects

Michael O. Atambo

Publications and source records attributed to Michael O. Atambo.

5 recordsLinked to original sources

arXiv2026

Parity-controlled electron-hole interference in exciton-phonon coupling

We demonstrate that exciton-phonon coupling in polar semiconductors is governed by a parity-controlled interference selection rule. By performing an exact multipole expansion of the excitonic form factor and validating it against numerical integration of hydrogenic envelope functions, we show that the long-range infrared divergence of the Fr\"ohlich interaction is exactly canceled for elastic scattering between excitonic states of the same parity. The ground-state exciton is thereby protected from long-wavelength polar phonons by destructive electron-hole interference. In contrast, transitions between states of opposite parity exhibit constructive interference, preserving a finite, robust coupling to macroscopic polar fields independent of band-structure details. Mass asymmetry between the electron and hole activates higher-order multipole terms in the elastic channel but leaves the constructive inelastic channel essentially unaffected. The selection rule is dimensionally invariant, applying to bulk and two-dimensional systems alike, and naturally explains the anomalously weak phonon dressing observed in halide perovskites as well as the strong phonon sidebands in transition-metal dichalcogenides. Our framework provides a universal, analytically exact criterion for exciton-phonon coupling strength, offering a design principle for engineering excitonic materials with tailored phonon interactions.

cond-mat.mtrl-sci↗

arXiv2026

Accidental accuracy and vertex corrections in $GW$: Exact benchmarks for the extended Hubbard model

The $GW$ approximation is the standard tool for quasiparticle predictions in materials, yet its regime of validity in correlated systems remains poorly quantified, because \textit{ab initio} vertex corrections are computationally prohibitive. Using exact diagonalization of the half-filled extended Hubbard model on finite rings as a numerically exact reference, we construct the corresponding model-space $GW$ theory on the identical Hilbert space and quantify its error as a function of local ($U$) and non-local ($V$) interaction strength. We find that the vertex correction changes character across the phase diagram: in the weak-coupling regime the effective vertex $\Gamma_{\rm eff} < 1$, reflecting the suppression of RPA charge fluctuations by exact short-range correlations, whereas in the Mott regime $\Gamma_{\rm eff}$ grows monotonically (to $\sim 3$ for $N=6$, reflecting the local vertex required to open the Hubbard gap. Vertex corrections in the electron-hole (polarizability) channel are shown to \emph{worsen} the gap error, indicating that the Mott gap resides in the self-energy channel. For $V=0$, static $COHSEX$ is accidentally exact at a single crossover point $U^* \approx 3.5\,t$; finite $V$, through non-local Fock exchange, splits this point into a double-crossover window that collapses toward weak coupling. These results yield quantitative diagnostics for the reliability of $GW$ in correlated materials.

cond-mat.str-el↗

arXiv2025

On the Automation of High Throughput Modeling of Adsorption In Porous Zeolitic Imidazolate Frameworks

The zeolitic imidazolate frameworks (ZIF) have emerged as a promising candidate for catalysis, carbon-dioxide (CO$_{2}$) capture and storage as well as flue gas separation due to their tunable porosity and chemical stability. ZIFs consists of transition metals in a tetrahedral coordination with imidazolate linkers, allowing for structural modifications that can enhance CO$_{2}$ adsorption and storage. To systematically design and optimize ZIFs for superior CO$_{2}$ capture performance, theoretical modelling provides an effective approach, though hindered by the sheer size of the search space, with hundred to thousands of possible sites per ZIF, and thousands of possible ZIF that can be synthesized. In this work, we employ a high-throughput techniques underpinned by first-principle Density Functional Theory (DFT) to develop a systematic automated method for characterizing adsorption energetics in ZIFs, guiding the rational design and selection of desirable ZIF structures. Using high-throughput computing tools, we perform pore and pore size analysis, and implement automated CO$_{2}$ molecule placement algorithm, accounting for positional symmetry, crystallographic orientation, and collision detection. The results obtained are in good agreement with previous studies, demonstrating the reliability of the approach in accelerating the discovery of next-generation ZIFs for CO$_{2}$ capture and storage.

cond-mat.mtrl-sci↗

arXiv2023

High Throughput Screening of Ternary Nitrides with Convolutional Neural Networks

The development of new materials is a core aspect of advancement in synthesis and application for industry. There is a vast number of possible chemical permutations of the basic elements that can be explored to synthesize materials that possess attractive catalytic, mechanical and electrical properties that may not be easily accessible to traditional experimental methods for various reasons, including cost and time considerations. Nitrides, as examples, require very stringent and precise conditions to successfully synthesize making their experimental exploration very slow. In this paper, we employ the use of machine learning algorithms to predict the bulk properties of Ternary Metal Nitrides (TMN), specifically their bulk modulus which is correlated with the hardness of the material. We were able to develop a consistent model with encouraging accuracy, that was able to predict the bulk moduli of materials that previously did not have computed values. The model was trained on $10^3$ ternary materials with known elastic properties and defined structures, and was able to predict the bulk modulus of $\thickapprox 1,000$ Ternary Metal Nitrides (TMNs) to $\thickapprox 80\%$ accuracy. This approach is orders of magnitude faster than the traditional computational approaches like density functional theory (DFT)\cite{dft-paper} which makes exploratory identification of materials with promising properties fast. We propose that such models be used to select interesting candidates for high throughput computation from first principles.

cond-mat.mtrl-sci↗

arXiv2019

Electronic and optical properties of doped TiO2 by many-body perturbation theory

Doping is one of the most common strategies for improving the photocatalytic and solar energy conversion properties of TiO$_2$, hence an accurate theoretical description of the electronic and optical properties of doped TiO$_2$ is of both scientific and practical interest. In this work we use many-body perturbation theory techniques to investigate two typical n-type dopants, Niobium and Hydrogen, in TiO$_2$ rutile. Using the GW approximation to determine band edges and defect energy levels, and the Bethe Salpeter equation for the calculation of the absorption spectra, we find that the defect energy levels form non-dispersive bands %associated with localized states lying $\simeq 2.2 eV$ above the top of the corresponding valence bands ($\simeq 0.9 eV$ below the conduction bands of the {\it pristine} material). The defect states are also responsible for the appearance of low energy absorption peaks that enhance the solar spectrum absorption of rutile. The spatial distributions of the excitonic wavefunctions associated with these low energy excitations are very different for the two dopants, suggesting a larger mobility of photoexcited electrons in Nb-TiO$_2$.

cond-mat.mtrl-sci↗