Diana Y. Qiu

Studying materials under extreme pressure in diamond anvil cells (DACs) is key to discovering emergent states of matter, yet no method currently allows the direct measurement of the electronic structure in this environment. Solid-state high harmonic generation (sHHG) offers a unique all-optical window into the electronic structure of materials. We demonstrate sHHG spectroscopy inside a DAC by probing 2𝑯-MoS2, up to 30 GPa, revealing a pressure-induced crossover of the lowest direct bandgap from the K-point to the 𝚪-point. This transition manifests as a sharp minimum in harmonic intensity and a 30° rotation of the sHHG polarization anisotropy, despite the absence of a structural phase change. First principles simulations attribute these features to interference between competing excitation pathways at distinct points in the Brillouin zone. Our results establish sHHG as a sensitive probe of electronic transitions at high pressure, enabling access to quantum phenomena that evade detection by conventional techniques. This data repository contains materials and codes for the associated publication in Science Advances titled "Isostructural electronic transition in MoS2 probed by solid-state high harmonic generation spectroscopy" DOI: 10.1126/sciadv.adz5621

High Resolution Image Download MS PowerPoint Slide Understanding exciton-defect interactions is critical for optimizing optoelectronic and quantum information applications in many materials. However, ab initio simulations of material properties with defects are often limited to high defect density. Here, we study effects of exciton-defect interactions on optical absorption and photoluminescence spectra in monolayer MoS 2 using a first-principles T-matrix approach. We demonstrate that exciton-defect bound states can be captured by the disorder-averaged Green’s function with the T-matrix approximation and further analyze their optical properties. Our approach yields photoluminescence spectra in good agreement with experiments and provides a new, computationally efficient framework for simulating optical properties of disordered 2D materials from first-principles.

Moiré heterobilayers exhibiting spatially varying exciton localization that can be precisely controlled through the twist angle have emerged as exciting platforms for studying complex quantum phenomena. Here, we study the exciton landscape in MoSe2/WS2 heterobilayers through synergistic first-principles GW plus Bethe Salpeter equation (GW-BSE) calculations and complementary time- and angle-resolved photoemission spectroscopy (tr-ARPES). We find that the MoSe2/WS2 heterobilayer has a type I band alignment at large twist angles. In contrast, at small twist angles, there exist simultaneous spatially modulated regions of local type I band alignment, hosting bright intralayer excitons, and local type II band alignment, hosting long-lived interlayer excitons, due to lattice reconstruction in different high-symmetry regions. In tr-ARPES this manifests in the observation of long-lived excitons with electron population in only MoSe2 at large twist angles, while in samples with small twist angles, signals from two distinct long-lived exciton states with electron population in both layers are observed. Contrary to earlier studies, we find no excitonic hybridization near the low-energy absorption peaks in MoSe2/WS2, whose splitting can, instead, be explained by the lattice reconstruction. Here, the authors reveal how twisting MoSe2/WS2 layers controls exciton behaviour. Combining theory and ultrafast spectroscopy, they show that lattice reconstruction creates distinct bright and long-lived excitons without hybridization effects.

Advancing both the fundamental understanding and technological application of two-dimensional semiconducting transition metal dichalcogenides (TMDs) hinges on precise control and identification of atomic-scale defects. Although self-flux growth yields exceptionally pure TMD crystals, the nature of residual defects has remained an open question. Here, we use scanning tunneling microscopy (STM) to directly image and identify point defects in both monolayer and bulk self-flux grown WSe 2 . We find that the dominant defects reside on chalcogen sites and are unaffected by exfoliation or oxygen exposure. Combining STM observations with first-principles simulations and bulk impurity analysis, we attribute these defects to substitutional oxygen (O Se ). This finding goes against the prevailing wisdom that vacancies are the most common defects in exfoliated TMDs. By establishing substitutional oxygen as the dominant defect, our work provides a crucial reference point for interpreting structure–property relationships and informs ongoing efforts to further improve material quality and device performance.

Self-trapped excitons (STEs), which have one or both carriers spatially trapped by a lattice distortion, are associated with broadband emission and a large Stokes shift that is desirable for many applications. The fundamental physical processes that lead to their formation are difficult to observe, mainly due to the ultrafast time scales involved and the low oscillator strength of STE transitions. Here, we employ ultrafast transient absorption spectroscopy with sub-20 fs temporal resolution in the ultraviolet to study the STE formation process in a pair of lead-free double perovskites, Cs 2 AgInCl 6 and Cs 2 (Ag 0.6 Na 0.4 )InCl 6 . Using first-principles calculations, we assign a broad photoinduced absorption band in Cs 2 AgInCl 6 to an intraband transition in the valence band that tracks the initial 70 fs hot-hole cooling step. Furthermore, exciton–phonon coupling calculations unravel the phonon modes that couple strongly with excitons in the lowest absorption peak to cause self-trapping. The transient absorption data shows the buildup of a stimulated emission band from the STE on a 200 fs time scale and long-lived coherent oscillations corresponding to the phonons of the lattice modified by the STE formation process.

Many-body interactions are essential for understanding non-linear optics and ultrafast spectroscopy of materials. Recent first principles approaches based on nonequilibrium Green’s function formalisms, such as the time-dependent adiabatic GW (TD-aGW) approach, can predict nonequilibrium dynamics of excited states including electron-hole interactions. However, the high-dimensionality of the electron-hole kernel poses significant computational challenges. Here, we develop a data-driven low-rank approximation for the electron-hole kernel, leveraging localized excitonic effects in the Hilbert space of crystalline systems to achieve significant data compression through singular value decomposition (SVD). We show that the subspace of non-zero singular values remains small even as the k-grid grows, ensuring computational tractability with extremely dense k-grids. This low-rank property enables at least 95% data compression and an order-of-magnitude speedup of TD-aGW calculations. Our approach avoids intensive training processes and eliminates time-accumulated errors, seen in previous approaches, providing a general framework for high-throughput, nonequilibrium simulation of light-driven dynamics in materials.

Studying materials under extreme pressure in diamond anvil cells (DACs) is key to discovering new states of matter, yet no method currently allows the direct measurement of the electronic structure in this environment. Solid-state high harmonic generation (sHHG) offers a new all-optical window into the electronic structure of materials. We demonstrate sHHG spectroscopy inside a DAC by probing $2H$-MoS$_2$, up to 30 GPa, revealing a pressure-induced crossover of the lowest direct bandgap from the $\textbf{K}$-point to the $Γ$-point. This transition manifests as a sharp minimum in harmonic intensity and a 30° rotation of the sHHG polarization anisotropy, despite the absence of a structural phase change. First-principles simulations attribute these features to interference between competing excitation pathways at distinct points in the Brillouin zone. Our results establish sHHG as a sensitive probe of electronic transitions at high pressure, enabling access to quantum phenomena that evade detection by conventional techniques.

Understanding exciton-defect interactions is critical for optimizing optoelectronic and quantum information applications in many materials. However, ab initio simulations of material properties with defects are often limited to high defect density. Here, we study effects of exciton-defect interactions on optical absorption and photoluminescence spectra in monolayer MoS2 using a first-principles T-matrix approach. We demonstrate that exciton-defect bound states can be captured by the disorder-averaged Green's function with the T-matrix approximation and further analyze their optical properties. Our approach yields photoluminescence spectra in good agreement with experiments and provides a new, computationally efficient framework for simulating optical properties of disordered 2D materials from first-principles.

Understanding exciton thermalization is critical for optimizing optoelectronic and photocatalytic processes in many materials. However, it is hard to access the dynamics of such processes experimentally, especially on systems such as monolayer transition metal dichalcogenides, where various low-energy excitations pathways can compete for exciton thermalization. Here, we study exciton dynamics due to exciton-phonon scattering in monolayer ${\mathrm{MoS}}_{2}$ from a first-principles, interacting Green's function approach, to obtain the relaxation and thermalization of low-energy excitons following different initial excitations at different temperatures. We find that the thermalization occurs on a picosecond time scale at 300 K but can increase by an order of magnitude at 100 K. The long total thermalization time, owing to the nature of its excitonic band structure, is dominated by slow spin-flip scattering processes in monolayer ${\mathrm{MoS}}_{2}$. In contrast, thermalization of excitons in individual spin-aligned and spin-anti-aligned channels can be achieved within a few hundred fs when exciting higher-energy excitons. We further simulate the intensity spectrum of time-resolved angle-resolved photoemission spectroscopy experiments and anticipate that such calculations may serve as a map to correlate spectroscopic signatures with microscopic exciton dynamics.

Water wires, quasi-one-dimensional chains composed of hydrogen-bonded (H-bonded) water molecules, play a fundamental role in numerous chemical, physical, and physiological processes. Yet direct experimental detection of water wires has been elusive so far. Based on advanced many-body theory that includes electron-hole interactions, we report that optical absorption spectroscopy can serve as a sensitive probe of water wires and their ordering. In both liquid and solid water, the main peak of the spectrum is discovered to be a charge-transfer exciton. In water, the charge-transfer exciton is strongly coupled to the H-bonding environment where the exciton is excited between H-bonded water molecules with a large spectral intensity. In regular ice, the spectral weight of the charge-transfer exciton is enhanced by a collective excitation occurring on proton-ordered water wires, whose spectral intensity scales with the ordering length of water wire. The spectral intensity and excitonic interaction strength reaches its maximum in ice XI, where the long-range ordering length yields the most pronounced spectral signal. Our findings suggest that water wires, which widely exist in important physiological and biological systems and other phases of ice, can be directly probed by this approach.

Excitons -- elementary excitations formed by bound electron-hole pairs -- govern the optical properties and excited-state dynamics of materials. In two-dimensions (2D), excitons are theoretically predicted to have a linear energy-momentum relation with a non-analytic discontinuity in the long wavelength limit, mimicking the dispersion of a photon. This results in an exciton that behaves like a massless particle, despite the fact that it is a composite boson composed of massive constituents. However, experimental observation of massless excitons has remained elusive. In this work, we unambiguously experimentally observe the predicted linear exciton dispersion in freestanding monolayer hexagonal boron nitride (hBN) using momentum-resolved electron energy-loss spectroscopy. The experimental result is in excellent agreement with our theoretical prediction based on ab initio many-body perturbation theory. Additionally, we identify the lowest dipole-allowed transition in monolayer hBN to be at 6.6 eV, illuminating a long-standing debate about the band gap of monolayer hBN. These findings provide critical insights into 2D excitonic physics and open new avenues for exciton-mediated superconductivity, Bose-Einstein condensation, and high-efficiency optoelectronic applications.

Many-body electron-hole interactions are essential for understanding non-linear optical processes and ultrafast spectroscopy of materials. Recent first principles approaches based on nonequilibrium Green's function formalisms, such as the time-dependent adiabatic GW (TD-aGW) approach, can predict the nonequilibrium dynamics of excited states including electron-hole interactions. However, the high dimensionality of the electron-hole kernel poses significant computational challenges for scalability. Here, we develop a data-driven low-rank approximation for the electron-hole kernel, leveraging localized excitonic effects in the Hilbert space of crystalline systems. Through singular value decomposition (SVD) analysis, we show that the subspace of non-zero singular values, containing the key information of the electron-hole kernel, retains a small size even as the k-grid grows, ensuring computational feasibility with extremely dense k-grids for converged calculations. Utilizing this low-rank property, we achieve at least 95% compression of the kernel and an order-of-magnitude speedup of TD-aGW calculations. Our method, rooted in physical interpretability, outperforms existing machine learning approaches by avoiding intensive training processes and eliminating time-accumulated errors, providing a general framework for high-throughput, nonequilibrium simulation of light-driven dynamics in materials.

Understanding exciton thermalization is critical for optimizing optoelectronic and photocatalytic processes in many materials. However, it is hard to access the dynamics of such processes experimentally, especially on systems such as monolayer transition metal dichalcogenides, where various low-energy excitations pathways can compete for exciton thermalization. Here, we study exciton dynamics due to exciton-phonon scattering in monolayer MoS2 from a first-principles, interacting Green's function approach, to obtain the relaxation and thermalization of low-energy excitons following different initial excitations at different temperatures. We find that the thermalization occurs on a picosecond timescale at 300 K but can increase by an order of magnitude at 100 K. The long total thermalization time, owing to the nature of its excitonic band structure, is dominated by slow spin-flip scattering processes in monolayer MoS2. In contrast, thermalization of excitons in individual spin-aligned and spin-anti-aligned channels can be achieved within a few hundred fs when exciting higher-energy excitons. We further simulate the intensity spectrum of time-resolved angle-resolved photoemission spectroscopy (TR-ARPES) experiments and anticipate that such calculations may serve as a map to correlate spectroscopic signatures with microscopic exciton dynamics.

Water wires, quasi-one-dimensional chains composed of hydrogen-bonded (H-bonded) water molecules, play a fundamental role in numerous chemical, physical, and physiological processes. Yet direct experimental detection of water wires has been elusive so far. Based on advanced $ab$ $initio$ many-body theory that includes electron-hole interactions, we report that optical absorption spectroscopy can serve as a sensitive probe of water wires and their ordering. In both liquid and solid water, the main peak of the spectrum is discovered to be a charge transfer exciton. In water, the charge transfer exciton is strongly coupled to the H-bonding environment where the exciton is excited between H-bonded water molecules with a large spectral intensity. In regular ice, the spectral weight of the charge transfer exciton is enhanced by a collective excitation occurring on proton-ordered water wires, whose spectral intensity scales with the ordering length of water wire. The spectral intensity and excitonic interaction strength reaches its maximum in ice XI, where the long-range ordering length yields the most pronounced spectral signal. Our findings suggest that water wires, which widely exist in important physiological and biological systems and other phases of ice, can be directly probed by this approach.

Representation learning for the electronic structure problem is a major challenge of machine learning in computational condensed matter and materials physics. Within quantum mechanical first principles approaches, density functional theory (DFT) is the preeminent tool for understanding electronic structure, and the high-dimensional DFT wavefunctions serve as building blocks for downstream calculations of correlated many-body excitations and related physical observables. Here, we use variational autoencoders (VAE) for the unsupervised learning of DFT wavefunctions and show that these wavefunctions lie in a low-dimensional manifold within latent space. Our model autonomously determines the optimal representation of the electronic structure, avoiding limitations due to manual feature engineering. To demonstrate the utility of the latent space representation of the DFT wavefunction, we use it for the supervised training of neural networks (NN) for downstream prediction of quasiparticle bandstructures within the GW formalism. The GW prediction achieves a low error of 0.11 eV for a combined test set of two-dimensional metals and semiconductors, suggesting that the latent space representation captures key physical information from the original data. Finally, we explore the generative ability and interpretability of the VAE representation.

Among existing band structure predictions of the topological monolayer 1$T$`-WTe${}_{2}$, only a semi-empirical hybrid functional has been able to reproduce the gapped electronic structure observed by angle-resolved photoemission spectroscopy (ARPES). Here, the authors demonstrate that the electronic band structure can be obtained by updating the quasiparticle wave functions within many-body perturbation theory (MBPT). Moreover, Bethe-Salpeter equation (BSE) on top of the gapped band structure leads to negative exciton frequencies, suggesting a possible exciton instability consistent with experimental evidence.

Abstract Many-body effects play an important role in enhancing and modifying optical absorption and other excited-state properties of solids in the perturbative regime, but their role in high harmonic generation (HHG) and other nonlinear response beyond the perturbative regime is not well-understood. We develop here an ab initio many-body method to study nonperturbative HHG based on the real-time propagation of the non-equilibrium Green’s function with the GW self energy. We calculate the HHG of monolayer MoS 2 and obtain good agreement with experiment, including the reproduction of characteristic patterns of monotonic and nonmonotonic harmonic yield in the parallel and perpendicular responses, respectively. Here, we show that many-body effects are especially important to accurately reproduce the spectral features in the perpendicular response, which reflect a complex interplay of electron-hole interactions (or exciton effects) in tandem with the many-body renormalization and Berry curvature of the independent quasiparticle bandstructure.

High Resolution Image Download MS PowerPoint Slide Despite the weak, van der Waals interlayer coupling, photoinduced charge transfer vertically across atomically thin interfaces can occur within surprisingly fast, sub-50 fs time scales. An early theoretical understanding of charge transfer is based on a noninteracting picture, neglecting excitonic effects that dominate optical properties of such materials. We employ an ab initio many-body perturbation theory approach, which explicitly accounts for the excitons and phonons in the heterostructure. Our large-scale first-principles calculations directly probe the role of exciton–phonon coupling in the charge dynamics of the WS 2 /MoS 2 heterobilayer. We find that the exciton–phonon interaction induced relaxation time of photoexcited excitons at the K valley of MoS 2 and WS 2 is 67 and 15 fs at 300 K, respectively, which sets a lower bound to the intralayer-to-interlayer exciton transfer time and is consistent with experiment reports. We further show that electron–hole correlations facilitate novel transfer pathways that are otherwise inaccessible to noninteracting electrons and holes.

Multiple polytypes of MoTe 2 with distinct structures and intriguing electronic properties can be accessed by various physical and chemical approaches. Here, we report electrochemical lithium (Li) intercalation into 1T′-MoTe 2 nanoflakes, leading to the discovery of two previously unreported lithiated phases. Distinguished by their structural differences from the pristine 1T′ phase, these distinct phases were characterized using in situ polarization Raman spectroscopy and in situ single-crystal X-ray diffraction. The lithiated phases exhibit increasing resistivity with decreasing temperature, and their carrier densities are two to 4 orders of magnitude smaller than the metallic 1T′ phase, as probed through in situ Hall measurements. The discovery of these gapped phases in initially metallic 1T′-MoTe 2 underscores electrochemical intercalation as a potent tool for tuning the phase stability and electron density in two-dimensional (2D) materials.

In two-dimensional chiral metal-halide perovskites, chiral organic spacers endow structural and optical chirality to the metal-halide sublattice, enabling exquisite control of light, charge, and electron spin. The chiroptical properties of metal-halide perovskites have been measured by transmissive circular dichroism spectroscopy, which necessitates thin-film samples. Here, by developing a reflection-based approach, we characterize the intrinsic, circular polarization-dependent complex refractive index for a prototypical two-dimensional chiral lead-bromide perovskite and report large circular dichroism for single crystals. Comparison with ab initio theory reveals the large circular dichroism arises from the inorganic sublattice rather than the chiral ligand and is an excitonic phenomenon driven by electron-hole exchange interactions, which breaks the degeneracy of transitions between Rashba-Dresselhaus-split bands, resulting in a Cotton effect. Our study suggests that previous data for spin-coated films largely underestimate the optical chirality and provides quantitative insights into the intrinsic optical properties of chiral perovskites for chiroptical and spintronic applications.

Nonradiative exciton relaxation processes are critical for energy transduction and transport in optoelectronic materials, but how these processes are connected to the underlying crystal structure and the associated electron, exciton, and phonon band structures, as well as the interactions of all these particles, is challenging to understand. Here, we present a first-principles study of exciton-phonon relaxation pathways in pentacene, a paradigmatic molecular crystal and optoelectronic semiconductor. We compute the momentum- and band-resolved exciton-phonon interactions, and use them to analyze key scattering channels. We find that both exciton intraband scattering and interband scattering to parity-forbidden dark states occur on the same ∼100 fs timescale as a direct consequence of the longitudinal-transverse splitting of the bright exciton band. Consequently, exciton-phonon scattering exists as a dominant nonradiative relaxation channel in pentacene. We further show how the propagation of an exciton wave packet is connected with crystal anisotropy, which gives rise to the longitudinal-transverse exciton splitting and concomitant anisotropic exciton and phonon dispersions. Our results provide a framework for understanding the role of exciton-phonon interactions in exciton nonradiative lifetimes in molecular crystals and beyond.

Inducing novel quantum phases and topologies in materials using intense light fields is a key objective of modern condensed matter physics, but nonetheless faces significant experimental challenges. Alternately, theory predicts that in the dense limit, excitons - collective excitations composed of Coulomb-bound electron-hole pairs - could also drive exotic quantum phenomena. However, the direct observation of these phenomena requires the resolution of electronic structure in momentum space in the presence of excitons, which became possible only recently. Here, using time- and angle-resolved photoemission spectroscopy of an atomically thin semiconductor in the presence of a high-density of resonantly and coherently photoexcited excitons, we observe the Bardeen-Cooper-Schrieffer (BCS) excitonic state - analogous to the Cooper pairs of superconductivity. We see the valence band transform from a conventional paraboloid into a Mexican-hat like Bogoliubov dispersion - a hallmark of the excitonic insulator phase; and we observe the recently predicted giant exciton-driven Floquet effects. Our work realizes the promise that intense bosonic fields, other than photons, can also drive novel quantum phenomena and phases in materials.

Despite the weak, van-der-Waals interlayer coupling, photoinduced charge transfer vertically across atomically thin interfaces can occur within surprisingly fast, sub-50fs timescales. Early theoretical understanding of the charge transfer is based on a noninteracting picture, neglecting excitonic effects that dominate the optical properties of such materials. Here, we employ an ab initio many-body perturbation theory approach which explicitly accounts for the excitons and phonons in the heterostructure. Our large-scale first-principles calculations directly probe the role of exciton-phonon coupling in the charge dynamics of the WS$_2$/MoS$_2$ heterobilayer. We find that the exciton-phonon interaction induced relaxation time of photo-excited excitons at the $K$ valley of MoS$_2$ and WS$_2$ is 67 fs and 15 fs at 300 K, respectively, which sets a lower bound to the intralayer-to-interlayer exciton transfer time and is consistent with experiment reports. We further show that electron-hole correlations facilitate novel transfer pathways which are otherwise inaccessible to non-interacting electrons and holes.

. The versatile optical methodology for probing TMD/metal interfaces can shed light on the intricate charge transfer characteristics within various 2D heterostructures, facilitating the development of more efficient and scalable nano-electronic and optoelectronic technologies.

Floquet engineering, in which a temporal periodic drive breaks the continuous temporal symmetry and dynamically engineers the electronic structure, has attracted enormous attention in condensed matter physics. However, only a handful of studies have experimentally demonstrated Floquet effects driven by optical fields [5,6]. In this talk, we will discuss the experimental observation of the Floquet-Bloch states induced by the excitons in 2D semiconductors.

Excitons in solids often reside in a narrow region of reciprocal space. Yet, it is standard for first-principles exciton calculations to sample the entire Brillouin zone uniformly, at great computational cost and resulting in underconverged exciton binding energies. Here, the authors demonstrate that a straightforward-to-implement nonuniform sampling approach yields highly converged exciton binding energies, which can significantly differ from previous best theoretical estimates, at a fraction of the computational expense of uniform sampling approaches. This will enable the prediction of exciton binding energies of increasingly complex materials.

Charged dopants in 2D transition metal dichalcogenides (TMDs) have been associated with the formation of hydrogenic bound states, defect-bound trions, and gate-controlled magnetism. Charge-transfer at the TMD–substrate interface and the proximity to other charged defects can be used to regulate the occupation of the dopant’s energy levels. In this study, we examine vanadium-doped WSe 2 monolayers on quasi-freestanding epitaxial graphene, by high-resolution scanning probe microscopy and ab initio calculations. Vanadium atoms substitute W atoms and adopt a negative charge state through charge donation from the graphene substrate. V W –1 dopants exhibit a series of occupied p -type defect states, accompanied by an intriguing electronic fine-structure that we attribute to hydrogenic states bound to the charged impurity. We systematically studied the hybridization in V dimers with different separations. For large dimer separations, the 2 e – charge state prevails, and the magnetic moment is quenched. However, the Coulomb blockade in the nearest-neighbor dimer configuration stabilizes a 1 e – charge state. The nearest-neighbor V-dimer exhibits an open-shell character for the frontier defect orbital, giving rise to a paramagnetic ground state. Our findings provide microscopic insights into the charge stabilization and many-body effects of single dopants and dopant pairs in a TMD host material.

Data-driven modeling is useful for reconstructing nonlinear dynamical systems when the underlying process is unknown or too expensive to compute. Having reliable uncertainty assessment of the forecast enables tools to be deployed to predict new scenarios unobserved before. In this work, we first extend parallel partial Gaussian processes for predicting the vector-valued transition function that links the observations between the current and next time points, and quantify the uncertainty of predictions by posterior sampling. Second, we show the equivalence between the dynamic mode decomposition and the maximum likelihood estimator of the linear mapping matrix in the linear state space model. The connection provides a data generating model of dynamic mode decomposition and thus, uncertainty of predictions can be obtained. Furthermore, we draw close connections between different data-driven models for approximating nonlinear dynamics, through a unified view of data generating models. We study two numerical examples, where the inputs of the dynamics are assumed to be known in the first example and the inputs are unknown in the second example. The examples indicate that uncertainty of forecast can be properly quantified, whereas model or input misspecification can degrade the accuracy of uncertainty quantification.

Over 25 years ago, Marzari and Vanderbilt introduced maximally localized Wannier functions (MLWFs), the most compact real-space representation of electronic wavefunctions in solids. Here, the authors put forward a generalization of this scheme for excitons, correlated electron-hole pairs that dictate the optical properties of materials. Much as MLWFs have transformed our understanding of electrons in solids, from chemical bonding to polarization to topology, these maximally localized exciton Wannier functions should deepen our understanding of photophysical and excited-state phenomena of materials.

We introduce a maximally-localized Wannier function representation of Bloch excitons, two-particle correlated electron-hole excitations, in crystalline solids, where the excitons are maximally-localized with respect to an average electron-hole coordinate in real space. As a proof-of-concept, we illustrate this representation in the case of low-energy spin-singlet and triplet excitons in LiF, computed using the ab initio Bethe-Salpeter equation approach. We visualize the resulting maximally-localized exciton Wannier functions (MLXWFs) in real space, detail the convergence of the exciton Wannier spreads, and demonstrate how Wannier-Fourier interpolation can be leveraged to obtain exciton energies and states at arbitrary exciton crystal momenta in the Brillouin zone. We further introduce an approach to treat the long-range dipolar coupling between singlet MLXWFs and discuss it in depth. The MLXWF representation sheds light on the fundamental nature of excitons and paves the way towards Wannier-based post-processing of excitonic properties, enabling the construction of ab initio exciton tight-binding models, efficient interpolation of the exciton-phonon vertex, the computation of Berry curvature associated with exciton bands, and beyond.

Time-resolved, angle-resolved photoemission spectroscopy (TR-ARPES) is a one-particle spectroscopic technique that can probe excitons (two-particle excitations) in momentum space. We present an ab initio, time-domain GW approach to TR-ARPES and apply it to monolayer MoS 2 . We show that photoexcited excitons may be measured and quantified as satellite bands and lead to the renormalization of the quasiparticle bands. These features are explained in terms of an exciton-Floquet phenomenon induced by an exciton time–dependent bosonic field, which are orders of magnitude stronger than those of laser field–induced Floquet bands in low-dimensional semiconductors. Our findings imply a way to engineer Floquet matter through the coherent oscillation of excitons and open the new door for mechanisms for band structure engineering.

Topological surface states (TSS) in the prototypical topological insulator (TI) Bi_{2}Se_{3} are frequently characterized using optical probes, but electron-hole interactions and their effect on surface localization and optical response of the TSS remain unexplored. Here, we use ab initio calculations to understand excitonic effects in the bulk and surface of Bi_{2}Se_{3}. We identify multiple series of chiral excitons that exhibit both bulk and TSS character, due to exchange-driven mixing. Our results address fundamental questions about the degree to which electron-hole interactions can relax the topological protection of surface states and dipole selection rules for circularly polarized light in TIs by elucidating the complex intermixture of bulk and surface states excited in optical measurements and their coupling to light.

Data-driven modeling is useful for reconstructing nonlinear dynamical systems when the underlying process is unknown or too expensive to compute. Having reliable uncertainty assessment of the forecast enables tools to be deployed to predict new scenarios unobserved before. In this work, we first extend parallel partial Gaussian processes for predicting the vector-valued transition function that links the observations between the current and next time points, and quantify the uncertainty of predictions by posterior sampling. Second, we show the equivalence between the dynamic mode decomposition and the maximum likelihood estimator of the linear mapping matrix in the linear state space model. The connection provides a {probabilistic generative} model of dynamic mode decomposition and thus, uncertainty of predictions can be obtained. Furthermore, we draw close connections between different data-driven models for approximating nonlinear dynamics, through a unified view of generative models. We study two numerical examples, where the inputs of the dynamics are assumed to be known in the first example and the inputs are unknown in the second example. The examples indicate that uncertainty of forecast can be properly quantified, whereas model or input misspecification can degrade the accuracy of uncertainty quantification.

The intrinsic weak and highly nonlocal dielectric screening of two-dimensional materials is well-known to lead to high sensitivity of their optoelectronic properties to environment. Less studied theoretically is the role of free carriers in those properties. Here, we use ab initio GW and Bethe-Salpeter equation calculations, with a rigorous treatment of dynamical screening and local-field effects, to study the doping dependence of the quasiparticle and optical properties of a monolayer transition-metal dichalcogenide, 2H MoTe 2 . We predict a quasiparticle band gap renormalization of several hundreds of meV for experimentally attainable carrier densities and a similarly sizable decrease in the exciton binding energy. This results in an almost constant excitation energy for the lowest-energy exciton resonance with an increasing doping density. Using a newly developed and generally applicable plasmon-pole model and a self-consistent solution of the Bethe-Salpeter equation, we reveal the importance of accurately capturing both dynamical and local-field effects to understand detailed photoluminescence measurements.

Non-radiative exciton relaxation processes are critical for energy transduction efficiencies in optoelectronic materials, but how these processes are connected to the underlying crystal structure and its associated electron, exciton, and phonon band structures is poorly understood. Here, we present a first-principles approach to explore exciton relaxation pathways in pentacene, a paradigmatic molecular crystal and optoelectronic semiconductor. We compute the momentum- and band-resolved exciton-phonon interactions, and use them to analyse key scattering channels. We find that exciton intraband transitions on femtosecond timescales leading to dark-state occupation is a dominant nonradiative relaxation channel in pentacene. We further show how the nature of real-time propagation of the exciton wavepacket is connected with the longitudinal-transverse exciton splitting, stemming from crystal anisotropy, and concomitant anisotropic exciton and phonon dispersions. Our results provide a framework for understanding time-resolved exciton propagation and energy transfer in molecular crystals and beyond.

Layered hybrid organic–inorganic lead halide perovskites have intriguing optoelectronic properties, but some of the most interesting perovskite systems, such as defective, disordered, or mixed perovskites, require multiple unit cells to describe and are not accessible within state-of-the-art ab initio theoretical approaches for computing excited states. The principal bottleneck is the calculation of the dielectric matrix, which scales formally as O ( N 4 ). We develop here a fully ab initio approximation for the dielectric matrix, known as IPSA-2C, in which we separate the polarizability of the organic/inorganic layers into minimal building blocks, thus circumventing the undesirable power-law scaling. The IPSA-2C method reproduces the quasi-particle band structures and absorption spectra for a series of Ruddlesden–Popper perovskites to high accuracy, by including critical nonlocal effects neglected in simpler models, and sheds light on the complicated interplay of screening between the organic and inorganic sublattices.

Exciton dynamics dictates the evolution of photoexcited carriers in photovoltaic and optoelectronic devices. However, interpreting their experimental signatures is a challenging theoretical problem due to the presence of both electron–phonon and many-electron interactions. We develop and apply here a first-principles approach to exciton dynamics resulting from exciton–phonon coupling in monolayer MoS 2 and reveal the highly selective nature of exciton–phonon coupling due to the internal spin structure of excitons, which leads to a surprisingly long lifetime of the lowest-energy bright A exciton. Moreover, we show that optical absorption processes rigorously require a second-order perturbation theory approach, with photon and phonon treated on an equal footing, as proposed by Toyozawa and Hopfield. Such a treatment, thus far neglected in first-principles studies, gives rise to off-diagonal exciton–phonon self-energy, which is critical for the description of dephasing mechanisms and yields exciton line widths in excellent agreement with experiment.

Monolayer transition metal dichalcogenide (TMDC) semiconductors exhibit strong excitonic optical resonances, which serve as a microscopic, noninvasive probe into their fundamental properties. Like the hydrogen atom, such excitons can exhibit an entire Rydberg series of resonances. Excitons have been extensively studied in most TMDCs (MoS 2, MoSe 2, WS 2, and WSe 2 ), but detailed exploration of excitonic phenomena has been lacking in the important TMDC material molybdenum ditelluride (MoTe 2 ). Here, we report an experimental investigation of excitonic luminescence properties of monolayer MoTe 2 to understand the excitonic Rydberg series, up to 3s. We report a significant modification of emission energies with temperature (4 to 300 K), thereby quantifying the exciton–phonon coupling. Furthermore, we observe a strongly gate-tunable exciton–trion interplay for all the Rydberg states governed mainly by free-carrier screening, Pauli blocking, and band gap renormalization in agreement with the results of first-principles GW plus Bethe–Salpeter equation approach calculations. Our results help bring monolayer MoTe 2 closer to its potential applications in near-infrared optoelectronics and photonic devices.