arXiv subjects

David A. Strubbe

Publications and source records attributed to David A. Strubbe.

At least 19 recordsLinked to original sources

arXiv2026

Multi-scale calculation of light-induced structural changes in low-angle twisted bilayer WSe$_2$

Exciton-phonon interactions in transition metal dichalcogenides (TMD) are strong and lead to phenomena such as coherent phonon generation. When stacked and twisted, their properties can be tuned by the twisting angle. In experiments with 1.1$^\circ$ twisted 2L WSe$_2$, a change of 0.1 {\AA} in the interlayer distance was observed when light was shone on this material, and here we explain the microscopic mechanism behind this. Theoretical works to study such systems are limited because the Moir\'e unit cell is too large. To overcome this, we combined classical force field relaxations with our implementation of ab initio GW/Bethe-Salpeter excited state forces (ESF). From the relaxations we found that the low-angle twisting induced an in-plane strain field, the AB regions are large enough to be simulated as periodic AB stacked 2L WSe2, and the interlayer force constant becomes softer in relation to the perfect AB stacking. From the ab initio ESF we obtained that the in-plane strain increases the out of plane ESFs. Those two effects combined, the weakening of the interlayer force constant and strain dependence of the ESF, make light-induced changes in the interlayer distance of twisted 2L WSe2 stronger than in the perfectly stacked case, in agreement with experimental observations. Therefore, our results show that the exciton-phonon interactions can be tuned in twisted 2L TMDs and can be observed experimentally, which makes those materials excellent platforms to study light-induced changes in materials.

cond-mat.mtrl-sci↗

arXiv2026

Analyzing Band Gaps in Ensemble Density Functional Theory using Thermodynamic Limits of Finite One-Dimensional Model Systems

Ensemble Density Functional Theory (EDFT) is a promising extension to Density Functional Theory (DFT) for calculating excited states. While Kohn-Sham eigenvalue differences underestimate gaps, EDFT has been shown to provide more accurate excitation energies in atoms, molecules and isolated model systems. However, it is unclear whether EDFT is capable of calculating band gaps of periodic systems -- and what an appropriate theoretical formulation would be to describe periodic systems. We explored how EDFT could calculate band gaps by estimating the thermodynamic limit with increasingly wide finite versions of the one-dimensional Kronig-Penney (KP) periodic model. We use Octopus, an ab initio, open-source, real-space DFT code, as in our previous work [R. J. Leano et al., Electron. Struct. 6, 035003 (2024)] in which we found with "particle in a box" models that EDFT can provide a reasonable effective mass correction for the homogeneous electron gas. Now, we use a periodic reference that is gapped. We find that the finite systems' Kohn-Sham gap approaches the same periodic limit for each of three ways of terminating the finite system, though the appropriate states corresponding to the valence band maximum and conduction band minimum have to be carefully identified in each case. Finally, our EDFT results, using a simple ensemblized LDA approximation, have a reasonable nonzero correction to the bandgap in the periodic limit. The results indicate that EDFT is promising for periodic systems, to motivate further work on developing a suitable formalism.

cond-mat.mtrl-sci↗

arXiv2025

Computational and Experimental Investigation of Chiral and Achiral 2D Organic Lead Bromide Perovskites: Octahedral Distortions and Electronic and Optical Properties

We present a computational investigation, in conjunction with synthesis and experimental characterization, into the structural, electronic, and optical properties of layered 2D organic lead bromide perovskites. We contrast materials based on the chiral (R/S)-4-fluoro-$\alpha$-methylbenzylammonium (R/S-FMBA), which have been shown to lead to bright room-temperature circularly polarized luminescence, with the similar achiral 4-fluorobenzylammonium (FBA). Using density functional theory (DFT) with van der Waals (vdW) corrections, we study relaxed structures (compared with X-ray diffraction, XRD) and optical absorption spectra (compared with experiments), as well as bandstructure and orbital character of transitions. We develop and provide a Python code to calculate octahedral distortions and compare DFT and XRD results, finding that vdW corrections are important for accuracy and that DFT overestimates octahedral tilt angles. (FMBA)$_2$PbBr$_4$ shows among the largest tilt angle differences (often termed $\Delta \beta$) reported, $14^\circ$, indicating strong inversion symmetry-breaking which enables its chiral emission. The lowest-energy optical transitions involve the perovskite only and are polarized within the layer. This work furthers understanding of structure-property relations with applications to optoelectronics and spintronics.

cond-mat.mtrl-sci↗

arXiv2025

How to choose efficiently the size of the Bethe-Salpeter Equation Hamiltonian for accurate exciton calculations on supercells

The Bethe-Salpeter Equation (BSE) is the workhorse method to study excitons in materials. The BSE Hamiltonian size, which depends on how many valence-to-conduction band transitions are considered, needs to be chosen to be sufficiently large to converge excitons' energies and wavefunctions but should be minimized to make calculations tractable, as BSE calculations are expensive and scale with the number of atoms as $\mathcal{O}(N_{\rm{atoms}}^6)$. In particular, in the case of supercell (SC) calculations composed of $N_{\rm{rep}}$ replicas of a primitive cell (PC), a natural choice to build this BSE Hamiltonian is to include all transitions derived from PC calculations by zone folding. However, this leads to a very large BSE Hamiltonian, as the number of matrix elements in it is $(N_k N_c N_v)^2$, where $N_k$ is the number of $k$-points and $N_{c(v)}$ is the number of conduction (valence) states used. When creating a SC, the number of $k$-points decreases by a factor $N_{\rm{rep}}$ but both the number of conduction and valence states increase by the same factor, therefore the number of matrix elements in the BSE Hamiltonian increases by a factor $N_{\rm{rep}}^2$, making exactly corresponding calculations prohibitive. Here, we provide a workflow to decide how many transitions are necessary to achieve comparable results, based on only PC results. With our method, we show that to converge the first exciton binding energy of a LiF SC composed of 64 PCs, to an energy tolerance of 0.15 eV, we only need 12\% of the valence-to-conduction matrix elements that result from zone folding with a minimal set of bands. As an example, we use the number of bands from our method to obtain the absorption spectrum of LiF with a V$_k$-like defect. The procedure in our work helps in evaluating excitonic properties in large SC calculations.

cond-mat.mtrl-sci↗

arXiv2025

Revisiting ab-initio excited state forces from many-body Green's function formalism: approximations and benchmark

Ab initio techniques for studying the optical and vibrational properties of materials are well-established, but only a few recent studies have focused on the interaction between excitons and atomic vibrations. In this paper, we revisit the excited state forces method, based on GW/BSE and DFPT calculations and provide a practical implementation and straightforward workflow. We fixed issues from Ismail-Beigi and Louie's implementation in \textcolor{blue}{Phys. Rev. Lett. 90, 076401 (2003)} and use an approximation for GW-level electron-phonon coefficients that improves our calculations accuracy. We explore its technical aspects, including convergence and the quality of approximations used for CO molecule, LiF and monolayer MoS$_2$. We successfully apply this method to investigate diverse kinds of self-trapped excitons in LiF, including polaronic excitons, discuss excited state relaxation strategies and project excited state forces in phonon displacement to explore exciton-phonon interactions. Our results provide the tools to study exciton-phonon related phenomena in molecules in materials, including coherent phonon generation, such as resonant Raman, self-trapped excitons and excitonic insulators.

cond-mat.mtrl-sci↗

arXiv2024

Resolving Discrepancies in Calculations of Mechanical Properties of CH\textsubscript{3}NH\textsubscript{3}PbI\textsubscript{3} Perovskites

The mechanical properties of hybrid perovskite materials are important for device flexibility, resistance to fracture, epitaxial growth, surface energetics of quantum dots, and induction or relief of stress in thin films due to thermal expansion and phase changes. These issues are particularly salient for solar cells in space applications. Nonetheless, few studies are available on the mechanical properties of the paradigmatic hybrid perovskite CH$_3$NH$_3$PbI$_3$ (MAPI). Experimental results are only available for the room-temperature tetragonal phase, and there are significant discrepancies among them. Results from density functional theory (DFT) are available for all three phases but have even larger discrepancies from each other and from experiments. To clarify the disorder in the literature, we have studied the elastic properties of all three phases in detail with DFT calculations. We have examined the effect of different aspects in calculation methodology such as use of energy or stress, the structure, exchange-correlation functionals, Van der Waals corrections, pseudopotentials, $k$-point sampling, and formulas for anisotropic elasticity and polycrystalline averages. Our results provide accurate reference values and an appropriate general methodology for elastic properties of metal halide perovskites.

cond-mat.mtrl-sci↗

arXiv2024

Embedding theory contributions to average atom models for warm dense matter

Accurate modeling in the warm dense matter regime is a persistent challenge with the most detailed models such as quantum molecular dynamics and path integral Monte Carlo being immensely computationally expensive. Density functional theory (DFT)-based average atom models (AAM) offer significant speed-ups in calculation times while still retaining fair accuracy in evaluating equations of state, mean ionizations, and more. Despite their success, AAMs struggle to precisely account for electronic interactions -- in particular, they do not account for effects on the kinetic energy arising from overlaps in neighboring atom densities. We aim to enhance these models by including such interactions via the non-additive kinetic potential $v^{\rm nadd}$ as in DFT embedding theories. $v^{\rm nadd}$ can be computed using Thomas-Fermi, von Weizs\"acker, or more sophisticated kinetic energy functionals. The proposed model introduces $v^{\rm nadd}$ as a novel interaction term in existing ion-correlation models, which include interactions beyond the central atom. We have applied this model to hydrogen at 5 eV and densities ranging 0.008 to 0.8 g/cm$^3$, and investigated the effects of $v^{\rm nadd}$ on electron densities, Kohn-Sham energy level shifts, mean ionization, and total energies.

cond-mat.mtrl-sci↗

arXiv2024

A computational materials science paradigm for a Course-based Undergraduate Research Experience (CURE)

Course-based Undergraduate Research Experiences (CUREs) bring the excitement of research into the classroom to improve learning and the sense of belonging in the field. They can reach more students, earlier in their studies, than typical undergraduate research. Key aspects are: students learn and use research methods, give input into the project, generate new research data, and analyze it to draw conclusions that are not known beforehand. CUREs are common in other fields but have been rare in materials science and engineering. I propose a paradigm for computational material science CUREs, enabled by web-based simulation tools from nanoHUB.org that require minimal computational skills. After preparatory exercises, students each calculate part of a set of closely related materials, following a defined protocol to contribute to a novel class dataset which they analyze, and also calculate an additional property of their choice. This approach has been used successfully in several class projects.

physics.ed-ph↗

arXiv2024

Implementation of time-dependent Hartree-Fock in real space

Time-dependent Hartree-Fock (TDHF) is one of the fundamental post-Hartree-Fock (HF) methods to describe excited states. In its Tamm-Dancoff form, equivalent to Configuration Interaction Singles, it is still widely used and particularly applicable to big molecules where more accurate methods may be unfeasibly expensive. However, it is rarely implemented in real space, mostly because of the expensive nature of the exact-exchange potential in real space. Compared to widely used Gaussian-type orbitals (GTO) basis sets, real space often offers easier implementation of equations and more systematic convergence of Rydberg states, as well as favorable scaling, effective domain parallelization, flexible boundary conditions, and ability to treat model systems. We implemented TDHF in the Octopus real-space code as a step toward linear-response hybrid time-dependent density-functional theory (TDDFT), other post-HF methods, and ensemble density-functional theory methods involving exact exchange. Calculation of HF's non-local exact exchange is very expensive in real space. We overcome this limitation with Octopus' implementation of Adaptively Compressed Exchange (ACE), and find the appropriate mixing scheme and starting point to complete the ground-state calculation in a practical amount of time, and thus enable TDHF. We compared our results to those from GTOs on a set of small molecules and confirmed close agreement of results, though with larger deviations than in the case of semi-local TDDFT. We find that convergence of TDHF demands a finer real-space grid than semi-local TDDFT. We also present the subtleties in benchmarking a real-space calculation against GTOs, relating to Rydberg and vacuum states.

physics.chem-ph↗

arXiv2024

Low-energy nine-layer rhombohedral stacking of transition metal dichalcogenides

Transition-metal dichalcogenides (TMDs) show unique physical, optical, and electronic properties. The known phases of TMDs are 2H and 3R in bulk form, 1T and associated reconstructions, and 1H in monolayer form. This paper reports a hypothetical phase, 9R, that may exist in TMDs (Mo, W)(S, Se, Te)$_2$, meeting both dynamical stability and elastic stability criteria. 9R phase has the same space group as 3R, $i.e.$ rhombohedral $R3m$ without inversion symmetry, and has 9 layers in a conventional unit cell. We find that 9R has an energy within 1 meV per formula unit of 3R and can be energetically favored by a particular strain condition. We further calculate the electronic, elastic, piezoelectric, Raman, and second-harmonic generation signatures of 9R TMDs and compare them with the corresponding 2H and 3R phases. 9R has similar properties to 3R but shows distinctive Raman peaks in the low-frequency regime, improved piezoelectric properties, and unique band splitting arising from layer coupling at the conduction band minimum. These distinct properties make 9R an attractive candidate for applications in piezotronics and valleytronics.

cond-mat.mtrl-sci↗

arXiv2024

Computation of the expectation value of the spin operator $\hat{S}^2$ for the Spin-Flip Bethe-Salpeter Equation

Spin-flip methods applied to excited-state approaches like the Bethe-Salpeter Equation allow access to the excitation energies of open-shell systems, such as molecules and defects in solids. The eigenstates of these solutions, however, are generally not eigenstates of the spin operator $\hat{S}^2$. Even for simple cases where the excitation vector is expected to be, for example, a triplet state, the value of $\langle \hat{S}^2 \rangle$ may be found to differ from 2.00; this difference is called ``spin contamination.'' The expectation values $\langle \hat{S}^2 \rangle$ must be computed for each excitation vector, to assist with the characterization of the particular excitation and to determine the amount of spin contamination of the state. Our aim is to provide for the first time in the spin-flip methods literature a comprehensive resource on the derivation of the formulas for $\langle \hat{S}^2 \rangle$ as well as its computational implementation. After a brief discussion of the theory of the Spin-Flip Bethe-Salpeter Equation and some examples further illustrating the need for calculating $\langle \hat{S}^2 \rangle$, we present the derivation for the general equation for computing $\langle \hat{S}^2 \rangle$ with the eigenvectors from an SF-BSE calculation, how it is implemented in a Python script, and timing information on how this calculation scales with the size of the SF-BSE Hamiltonian.

cond-mat.mtrl-sci↗

arXiv2024

Approaching Periodic Systems in Ensemble Density Functional Theory via Finite One-Dimensional Models

Ensemble Density Functional Theory (EDFT) is a generalization of ground-state Density Functional Theory (GS DFT), which is based on an exact formal theory of finite collections of a system's ground and excited states. EDFT in various forms has been shown to improve the accuracy of calculated energy level differences in isolated model systems, atoms, and molecules, but it is not yet clear how EDFT could be used to calculate band gaps for periodic systems. We extend the application of EDFT toward periodic systems by estimating the thermodynamic limit with increasingly large finite one-dimensional ``particle in a box'' systems, which approach the uniform electron gas (UEG). Using ensemble-generalized Hartree and Local Spin Density Approximation (LSDA) exchange-correlation functionals, we find that corrections go to zero in the infinite limit, as expected for a metallic system. However, there is a correction to the effective mass, with results comparable to other calculations on 1D, 2D, and 3D UEGs, which indicates promise for non-trivial results from EDFT on periodic systems.

cond-mat.mtrl-sci↗

arXiv2023

Energy Gap from Step Structure of the Analytically Inverted Non-Additive Kinetic Potential

The bandgap constitutes a challenging problem in density functional theory (DFT) methodologies. It is known that the energy gap values calculated by common DFT approaches are underestimated. The bandgap was also found to be related to the derivative discontinuity (DD) of the exchange-correlation potential in the Kohn-Sham formulation of DFT. Several reports have shown that DD appears as a step on the potential curve. The step structure is a mandatory structure for aligning the KS energy levels in the ionization potentials in a dissociated molecule in both fragments and is a function of electron localisation. Reproducing the step in the DFT framework gives the charge transfer process and the correct energy gap and describes the source of dissociation. This step phenomenon has not yet been studied in the non-additive kinetic potential $v^{\text{NAD}}[\rho_A,\rho_B](\textbf{r})$, a key quantity used in embedding theories. While $v^{\text{NAD}}[\rho_A,\rho_B](\textbf{r})$ is known to be difficult to approximate, in this work, we explain how an accurate energy gap can be produced from the analytically inverted $v^{\text{NAD}}[\rho_A,\rho_B](\textbf{r})$, even if we use the input densities calculated by the local and semi-local functionals. We used the precisely calculated $v^{\text{NAD}}[\rho_A,\rho_B](\textbf{r})$ reported in our previous publication [Phys. Rev. A 106, 042812 (2022)] to produce the energy gap for some model systems and report in this work the promising accuracy of our results through the comparison with the results obtained from one of the most accurate calculations, OEP theory with the KLI local approximation.

physics.chem-ph↗

arXiv2023

A substitutional quantum defect in WS$_2$ discovered by high-throughput computational screening and fabricated by site-selective STM manipulation

Point defects in two-dimensional materials are of key interest for quantum information science. However, the space of possible defects is immense, making the identification of high-performance quantum defects extremely challenging. Here, we perform high-throughput (HT) first-principles computational screening to search for promising quantum defects within WS$_2$, which present localized levels in the band gap that can lead to bright optical transitions in the visible or telecom regime. Our computed database spans more than 700 charged defects formed through substitution on the tungsten or sulfur site. We found that sulfur substitutions enable the most promising quantum defects. We computationally identify the neutral cobalt substitution to sulfur (Co$_{\rm S}^{0}$) as very promising and fabricate it with scanning tunneling microscopy (STM). The Co$_{\rm S}^{0}$ electronic structure measured by STM agrees with first principles and showcases an attractive new quantum defect. Our work shows how HT computational screening and novel defect synthesis routes can be combined to design new quantum defects.

cond-mat.mtrl-sci↗

arXiv2023

Pausing ultrafast melting by timed multiple femtosecond-laser pulses

An intense femtosecond-laser excitation of a solid induces highly nonthermal conditions. In materials like silicon, laser-induced bond-softening leads to a highly incoherent ionic motion and eventually nonthermal melting. But is this outcome an inevitable consequence, or can it be controlled? Here, we performed ab initio molecular dynamics simulations of crystalline silicon after timed multiple femtosecond-laser pulse excitations with fluence above the nonthermal melting threshold. Our results demonstrate an excitation mechanism that pauses nonthermal melting and creates a metastable state instead, with an electronic structure similar to the ground state. This mechanism can be generalized to other materials, potentially enabling structural and/or electronic transitions to metastable phases in the high-excitation regime. In addition, our approach could be used to switch off nonthermal contributions in experiments, allowing reliable electron-phonon coupling constants to be obtained more easily.

cond-mat.mtrl-sci↗

arXiv2023

Development and Application of a ReaxFF Reactive Force Field for Ni-Doped MoS$_2$

The properties of $\mathrm{MoS_2}$ can be tuned or optimized through doping. In particular, Ni doping has been shown to improve the performance of $\mathrm{MoS_2}$ for various applications, including catalysis and tribology. To enable investigation of Ni-doped $\mathrm{MoS_2}$ with reactive molecular dynamics simulations, we developed a new ReaxFF force field to describe this material. The force field parameters were optimized to match a large set of density-functional theory (DFT) calculations of 2H-$\mathrm{MoS_2}$ doped with Ni, at four different sites (Mo-substituted, S-substituted, octahedral intercalation, and tetrahedral intercalation), under uniaxial, biaxial, triaxial, and shear strain. The force field was evaluated by comparing ReaxFF- and DFT-relaxed structural parameters, the tetrahedral/octahedral energy difference in doped 2H, energies of doped 1H and 1T monolayers, and doped 2H structures with vacancies. We demonstrated the application of the force field with reactive simulations of sputtering deposition and annealing of Ni-doped $\mathrm{MoS_2}$ films. Results show that the developed force field can successfully model the phase transition of Ni-doped $\mathrm{MoS_2}$ from amorphous to crystalline. The newly developed force field can be used in subsequent investigations to study the properties and behavior of Ni-doped $\mathrm{MoS_2}$ using reactive molecular dynamics simulations.

cond-mat.mtrl-sci↗

arXiv2023

Quantifying Hidden Symmetry in the Tetragonal CH$_3$NH$_3$PbI$_3$ Perovskite

The assignment of an exact space group to the tetragonal CH$_3$NH$_3$PbI$_3$ perovskite structure is experimentally challenging and controversial in the literature. Average orientation of the methylammonium ion that gives symmetry to the experimental measurement is not captured in a static density functional theory calculation, although the quasi-I4cm and quasi-I4/mcm structures are commonly used in calculations. In this work we have developed a methodology to quantify the hidden symmetry of these structures using group theory, to enable use of symmetries in understanding spectroscopy and other properties. We study the approximate symmetry of vibrational modes, including analysis of degenerate representations, as well as the dielectric, elastic, electro-optic, Born effective charge, and Raman tensors and the dynamical matrix. Comparing to each subgroup of the full tetragonal D$_{4h}$, our results show that the quasi-I4cm is best described by the expected corresponding point group C$_{4v}$, whereas the quasi-I4/mcm (despite corresponding to point group D$_{4h}$) is best described by the lower symmetry C$_{2v}$. Our methodology can be useful generally for analysis of other soft hybrid materials or any approximately symmetric material.

cond-mat.mtrl-sci↗

arXiv2022

Surface Effects on Anisotropic Photoluminescence in One-Dimensional Organic Metal Halide Hybrids

One-dimensional (1D) organic metal halide hybrids exhibit strongly anisotropic optical properties, highly efficient light emission, and large Stokes shift, holding promises for novel photodetection and lighting applications. However, the fundamental mechanisms governing their unique optical properties and in particular the impacts of surface effects are not understood. Here, we investigate 1D C4N2H14PbBr4 by polarization-dependent time-averaged and time-resolved photoluminescence (TRPL) spectroscopy, as a function of photoexcitation energy. Surprisingly, we find that the emission under photoexcitation polarized parallel to the 1D metal halide chains can be either stronger or weaker than that under perpendicular polarization, depending on the excitation energy. We attribute the excitation-energy-dependent anisotropic emission to fast surface recombination, supported by first-principles calculations of optical absorption in this material. The fast surface recombination is directly confirmed by TRPL measurements, when the excitation is polarized parallel to the chains. Our comprehensive studies provide a more complete picture for a deeper understanding of the optical anisotropy in 1D organic metal halide hybrids.

cond-mat.mtrl-sci↗