The interaction between excitons and phonons is fundamental to the nature and fate of photoexcitatons in solids, with important implications for spectroscopy and transport measurements, and for applications in optoelectronics and clean energy. In this dissertation, we present recent advances in computing exciton-phonon interactions from first principles. We implement a recently proposed linear-response reciprocal space-based framework which involves contracting electron-phonon matrix elements computed from density functional perturbation theory (DFPT) with exciton expansion coefficients obtained after building and diagonalizing the ab initio Bethe-Salpeter Hamiltonian, which is built on density functional theory and the GW approximation. We apply this formalism in unique ways to study phenomena related to exciton-phonon interactions, namely the nature of exciton diffusion in acene crystals, the dynamical screening of excitons in halide perovskites due to lattice vibrations, and the asymmetric lineshapes in MoS2 stemming from off-diagonal exciton-phonon coupling.

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## Scholarly Works (126 results)

Electronic structure theory enables fundamental understanding of material properties. Using first-principles based methods, several different classes of complex materials are studied including perovskite oxide heterostructures, halide perovskites, alkali metals under pressure, and high-throughput screening of multiferroic materials.

• In Chapter 2 we discuss the basis of electronic structure methods used in this thesis and introduce the calculation of topological properties from first-principles.

• In Chapter 3 the properties of lithium, an ostensibly simple metal, under pressure are introduced and our explorations into the emergence of nontrivial topological features at high pressures are shown.

• Halide perovskites have emerged as a promising material class for solar energy con- version due to their optoelectronic properties. We explore three different halide per- ovskite materials and, in collaboration with experiment, develop an understanding of how their properties change through chemical substitutions and structural manipula- tions. This includes the optoelectronic properties of Sn–alloyed Cs2AgBiBr6 (Chapter 4) and Cs8Au4XCl23 (X = In; Bi) (Chapter 6); and the increase in conductivity of (EA)2CuBr4 through pressure (Chapter 5).

• Similarly, perovskite oxides are a highly tunable class of materials which display a wide range of interesting phenomena. Here we exploit the fact that experimentalists can control the synthesis of perovskite oxide heterostructures at the precision of single atomic layers. Given this level of control, we use first-principles calculations to de- sign monolayers and bilayers that would tune the work function of bulk SrRuO3 for thermionic applications in Chapter 7

• In Chapter 8, we discuss our work developing a high throughput workflow based on symmetry and first-principles calculations to screen tens of thousands of materials for candidates which are both magnetic and ferroelectric.

• Finally, Chapter 9 presents an overview of the work.

The accurate prediction of electronic excitation energies in molecules is an area of intense research of significant fundamental interest and is critical for many applications. Today, most excited state calculations use time-dependent density functional theory (TDDFT) in conjunction with an approximate exchange-correlation functional. In this dissertation, I have examined and critically assessed an alternative method for predicting charged and low-lying neutral excitations with similar computational cost: the \textit{ab initio} Bethe-Salpeter equation (BSE) approach. Rigorously based on many-body Green's function theory but incorporating information from density functional theory, the predictive power of the BSE approach remained at the beginning of this work unexplored for the neutral and charged electronic excitations of organic molecules. Here, the results and implications of several systematic benchmarks are laid out in detail.

Here, we use and develop first-principles methods based on density functional theory (DFT) and beyond to understand and predict charge transport phenomena in the novel class of nanostructured devices: molecular junctions. Molecular junctions, individual molecules contacted to two metallic leads, which can be systematically altered by modifying the chemistry of each component, serve as test beds for the study of transport at the nanoscale. To date, various experimental methods have been designed to reliably assemble and mea- sure transport properties of molecular junctions. Furthermore, theoretical methods built on DFT designed to yield quantitative agreement with these experiments for certain classes of molecular junctions have been developed. In order to gain insight into a broader range of molecular junctions and environmental effects associated with the surrounding solution, this dissertation will employ, explore and extend first-principles DFT calculations coupled with approximate self-energy corrections known to yield quantitative agreement with experiments for certain classes of molecular junctions.

To start we examine molecular junctions in which the molecule is strongly hybridized with the leads: a challenging limit for the existing methodology. Using a physically motivated tight-binding model, we find that the experimental trends observed for such molecules can be explained by the presence of a so-called “gateway” state associated with the chemical bond that bridges the molecule and the lead. We discuss the ingredients of a self-energy corrected DFT based approach to quantitatively predict conductance in the presence of these hybridization effects.

We also develop and apply an approach to account for the surrounding environment on the conductance, which has been predominantly ignored in past transport calculations due to computational complexity. Many experiments are performed in a solution of non-conducting molecules; far from benign, this solution is known to impact the measured conductance by as much as a factor of two. Here, we show that the dominant effect of the solution stems from nearby molecules binding to the lead surface surrounding the junction and altering the local electrostatics. This effect operates in much the same way adsorbates alter the work function of a surface. We develop a framework which implicitly includes the surrounding molecules through an electrostatic-based lattice model with parameters from DFT calculations, reducing the computational complexity of this problem while retaining predictive power. Our approach for computing environmental effects on charge transport in such junctions will pave the way for a better understanding of the physics of nanoscale devices, which are known to be highly sensitive to their surroundings.

Extreme Ultraviolet Lithography (EUVL) has been introduced to meet the need for smaller feature sizes. EUVL imaging chemistry is revolutionary because of ionizing radiation is involved. Novel physical and chemical processes has made material development more akin to shooting in the dark. In this work, the author delineates the material challenge of EUV photo-sensitization at the molecular level, presents investigations in elucidating sensitization mechanisms and explores new possibilities. The author argues that chemical activation, which connects EUV photon absorption and generation of reactive species, is one of the main driver of resist performance. Three electron spectroscopy techniques are introduced and characterized to interrogate the electron cascade subsequent to photo-ionization. These experiments mainly answer questions about how reactive radical cations and slow electrons are generated. For example, the size of this cascade was found to be between 1 to 4 nanometers in various materials. Once the radical cations are cooled and the slow electrons have attached to electron acceptors, existing computational chemistry techniques are used to understand the subsequent generation of active species. Specifically, photo acid generators (PAG) and a prototypical organo-tin oxo cluster are studied in detail. This study revealed that the said organo-tin oxo cluster reacts to both ionization and electron attachment with similar chemical outcomes, thus partially explaining its superior performance. Manipulating dose by modifying interface dipole is also investigated. Finally, consolidating what he has learned along the way, the author discusses possibilities in EUV material engineering.

In this thesis we use first-principles density functional theory (DFT) and related methods to predict and understand the properties of two categories of materials with particular promise for technological applications: topological semimetals, and antiferromagnets whose magnetic order can be controlled by electrical current.

Topological semimetals (TSMs), a subset of topological materials which are the particular focus of the first part of this thesis, have robust band crossings in reciprocal space protected by crystalline symmetries and characterized by mathematical invariants. They exhibit a variety of exotic phenomena such as ultrahigh mobility of electrons, giant magnetoresistance, and chiral anomalies. Moreover, analogously to the better known topological insulators (TIs), the ``bulk-boundary correspondence", related to the change in topological invariant in going from material to vacuum, implies the existence of electronic states localized at the compound surface which can differ significantly from the semimetallic bulk states in TSMs. The development of group-theoretical methods to identify TSMs and TIs have revealed that TSMs are far more ubiquitous than initially hypothesized; to date over $10000$ candidates have been identified. While this might seem to imply that the goal of harnessing properties of TSMs for practical purposes is a solved problem, most candidate materials have one or more features which make experimental manipulation and detection of the topological properties difficult if not impossible. If the symmetry-protected band crossings occur at energies far from the Fermi level, or if they are obscured by other trivial bands at the same energy, the topological signatures will be obscured. Thus, the identification of specific materials, structural motifs, and possible tuning parameters through which one can realize ``functional" TSMs is highly desirable.\\indent The first section of this thesis describes a set of studies focused on the interplay of symmetry, orbital character and magnetism in yielding electronic structures with controllable TSM features near the Fermi level and free from interfering trivial bands. First, we use a combination of DFT and tight-binding to examine the electronic structure of a previously synthesized compound, $\mathrm{TiRhAs}$. We find that $\mathrm{TiRhAs}$ hosts a topological nodal line protected by a mirror plane nearly exactly at the Fermi level, with no other energetically degenerate trivial bands. Next, in combination with experimental ARPES data which confirmed our DFT findings, we investigate the transition metal dichalcogenide (TMD) $\mathrm{NiTe_2}$, and find that is a Dirac semimetal with a bulk tilted Dirac cone and topological surface states. While previous isostructural $\mathrm{MX_2}$ ($\mathrm{M}=\mathrm{Pd}$, $\mathrm{Pt}$: $\mathrm{X}=\mathrm{Te}$, $\mathrm{Se}$) compounds have been shown to host similar ``ladders" of topologically protected bulk and surface states, the features of interest occur at large binding energies which render their topological properties irrelevant to transport. We show that the increased hybridization between $\mathrm{Ni}$ $\mathrm{d}$ and $\mathrm{Te}$ $\mathrm{p}$ states as compared to the other $\mathrm{MX_2}$ compounds is responsible for tuning the Dirac cone very close to the Fermi level; thus substitution of the transition metal element is an effective method for designing functional TSMs within this class of TMDs. Finally, we examine the possibility of realizing TSM features in compounds isostructural to the multiferroic hexagonal manganites. This was motivated by the numerous order parameters in multiferroic compounds which can be controlled by external fields; thus, a multiferroic compound with TSM features in a particular phase would provide an opportunity to switch from nontrivial to trivial topology by tuning of the ferroic order parameters. We find through our DFT calculations that by enforcing a metastable ferromagnetic order in the nonpolar centrosymmetric phase, hexagonal $\mathrm{YCrO_3}$ and $\mathrm{YVO_3}$ become topological nodal line semimetals, in contrast to the insulating band structures that occur with the ground state antiferromagnetic order of the transition metal ions.\ \indent The second section of this thesis focuses on first-principles characterization of ``functional materials" in which the feature to leverage for functionality is magnetic, rather than topological, order. We focus on antiferromagnetic (AFM) materials. There has been a recent surge of interest in using AFMs rather than their traditional ferromagnetic (FM) counterparts for spintronic devices whose magnetic order can be manipulated by an electrical current. The vanishing bulk magnetization of AFMs makes them particularly robust to magnetic field perturbations, and the limiting rate of spin dynamics (i.e. the rate at which spins can rotate) in AFMs is order $\sim \mathrm{THz}$ as opposed to $\sim \mathrm{GHz}$ for FMs.\ \indent Our studies focus on one example, the iron-intercalated TMD, $\mathrm{Fe_{1/3}NbS_2}$. The triangular lattice of $\mathrm{Fe}$ ions has an antiferromagnetic (AFM) order which can be manipulated with electrical pulses of very low current density. While numerous experimental characterizations have been performed on this compound, ambiguities regarding the magnetic ground state, spin exchange constants, and the specifics of the current-induced magnetization dynamics, remain. In one study, we calculate the nearest-neighbor Heisenberg exchange constants in $\mathrm{Fe_{1/3}NbS_2}$ and find that competition between strong nearest-neighbor interplanar and intraplanar $\mathrm{Fe}$ exchange constants is responsible for an observed half-magnetization plateau. In the second part of our first-principles characterization of $\mathrm{Fe_{1/3}NbS_2}$, we explore the working hypothesis that the current-induced manipulation of AFM order, which is detected by changes in electrical resistance, is due to a repopulation of three energetically equivalent AFM domains on the triangular lattice. Based on calculated conductivity tensors within a constant relaxation time approximation with experimentally proposed AFM magnetic orders, we verify that the transport parallel to the $\mathrm{Fe}$ layers is anisotropic, a necessary condition for the domain repopulation hypothesis. Finally, by comparing our ab-initio transport with experimental changes in resistance for specific pulse directions, we infer the likely current-domain response for $\mathrm{Fe_{1/3}NbS_2}$, that is, which domains are favored for a given current direction.

With first-principles calculations based on density functional theory, we can predict with good accuracy the electronic ground state properties of a fixed arrangement of nuclei in a molecule or crystal. However, the potential of this formalism and approach is not fully utilized; most calculations are performed on experimentally determined structures and stoichiometric substitutions of those systems.

This in part stems from the difficulty of systematically generating 3D geometries that are chemically valid under the complex interactions existing in materials. Designing materials is a bottleneck for computational materials exploration; there is a need for systematic design tools that can keep up with our calculation capacity. Identifying a higher level language to articulate designs at the atomic scale rather than simply points in 3D space can aid in developing these tools.

Constituent atoms of materials tend to arrange in recognizable patterns with defined symmetry such as coordination polyhedra in transition metal oxides or subgroups of organic molecules; we call these structural motifs. In this thesis, we advance a variety of systematic strategies for understanding complex materials from structural motifs on the atomic scale with an eye towards future design.

In collaboration with experiment, we introduce the harmonic honeycomb iridates with frustrated, spin-anisotropic magnetism. At the atomic level, the harmonic honeycomb iridates have identical local geometry where each iridium atom octahedrally coordinated by oxygen hosts a $J_{eff}=1/2$ spin state that experiences interactions in orthogonal spin directions from three neighboring iridium atoms. A homologous series of harmonic honeycomb can be constructed by changing the connectivity of their basic structural units.

Also in collaboration with experiment, we investigate the metal-organic chalcogenide assembly [AgSePh]$_\infty$ that hosts 2D physics in a bulk 3D crystal. In this material, inorganic AgSe layers are scaffolded by organic phenyl ligands preventing the inorganic layers from strongly interacting. While bulk Ag$_2$Se is an indirect band gap semiconductor, [AgSePh]$_\infty$ has a direct band gap and photoluminesces blue. We propose that these hybrid systems present a promising alternative approach to exploring and controlling low-dimensional physics due to their ease of synthesis and robustness to the ambient environment, contrasting sharply with the difficulty of isolating and maintaining traditional low-dimensional materials such as graphene and MoS$_2$.

Automated density functional theory via high throughput approaches are a promising means of identifying new materials with a given property. We automate a search for ferroelectric materials by integrating density functional theory calculations, crystal structure databases, symmetry tools, workflow software, and a custom analysis toolkit. Structural distortions that occur in the structural motifs of ferroelectrics give rise to a switchable spontaneous polarization. In ferroelectrics lattice, spin, and electronic degrees of freedom couple leading to exotic physical phenomena and making them technologically useful (e.g. non-volatile RAM).

We also propose a new neural network architecture that encodes the symmetries of 3D Euclidean space for learning the structural motifs of atomic systems. We describe how these networks can be used to speed up important components of the computational materials discovery pipeline and generate hypothetical stable atomic structures.

Finally, we conclude with a discussion of the materials design tools deep learning may enable and how these tools could be guided by the intuition of materials scientists.