Effect of dynamical screening in the Bethe-Salpeter framework: Excitons in crystalline naphthalene

Solving the Bethe-Salpeter equation (BSE) for the optical polarization functions is a first principles means to model optical properties of materials including excitonic effects. One almost ubiquitously used approximation neglects the frequency dependence of the screened electron-hole interaction. This is commonly justified by the large difference in magnitude of electronic plasma frequency and exciton binding energy. We incorporated dynamical effects into the screening of the electron-hole interaction in the BSE using two different approximations as well as exact diagonalization of the exciton Hamiltonian. We compare these approaches for a naphthalene organic crystal, for which the difference between exciton binding energy and plasma frequency is only about a factor of ten. Our results show that in this case, corrections due to dynamical screening are about 15\,\% of the exciton binding energy. We analyze the effect of screening dynamics on optical absorption across the visible spectral range and use our data to establish an \emph{effective} screening model as a computationally efficient approach to approximate dynamical effects in complex materials in the future.Comment: 11 pages main text, 5 figures main text, 9 pages supplemental, 6 figures supplementa

Ab initio calculation of carrier mobility in semiconductors including ionized-impurity scattering

The past decade has seen the emergence of ab initio computational methods for calculating phonon-limited carrier mobilities in semiconductors with predictive accuracy. More realistic calculations ought to take into account additional scattering mechanisms such as, for example, impurity and grain-boundary scattering. In this work, we investigate the effect of ionized-impurity scattering on the carrier mobility. We model the impurity potential by a collection of randomly distributed Coulomb scattering centers, and we include this relaxation channel into the ab initio Boltzmann transport equation, as implemented in the EPW code. We demonstrate this methodology by considering silicon, silicon carbide, and gallium phosphide, for which detailed experimental data are available. Our calculations agree reasonably well with experiments over a broad range of temperatures and impurity concentrations. For each compound investigated here, we compare the relative importance of electron-phonon scattering and ionized-impurity scattering, and we critically assess the reliability of Matthiessen's rule. We also show that an accurate description of dielectric screening and carrier effective masses cam improve quantitative agreement with experiments

Monolayer 1T-NbSe2 as a 2D-correlated magnetic insulator

Monolayer group V transition metal dichalcogenides in their 1T phase have recently emerged as a platform to investigate rich phases of matter, such as spin liquid and ferromagnetism, resulting from strong electron correla- tions. Newly emerging 1T-NbSe2 has inspired theoretical investigations predicting collective phenomena such as charge transfer gap and ferromagnetism in two dimensions; however, the experimental evidence is still lacking. Here, by controlling the molecular beam epitaxy growth parameters, we demonstrate the successful growth of high-quality single-phase 1T-NbSe2. By combining scanning tunneling microscopy/spectroscopy and ab initio calculations, we show that this system is a charge transfer insulator with the upper Hubbard band located above the valence band maximum. To demonstrate the electron correlation resulted magnetic property, we create a vertical 1T/2H NbSe2 heterostructure, and we find unambiguous evidence of exchange interactions between the localized magnetic moments in 1T phase and the metallic/superconducting phase exemplified by Kondo reso- nances and Yu-Shiba-Rusinov–like bound states.Center for Dynamics and Control of Materials: an NSF MRSEC under cooperative agreement no. DMR-1720595. J.L. and F.G. were supported by the Robert A. Welch Foundation under award number F-1990-20190330. Other supports were from NSF grant nos. DMR-1808751 and DMR-1949701, the Welch Foundation F-1672, F-1814, and the National Natural Science Foundation of China (grant nos. 11774268 and 11974012).Center for Dynamics and Control of Material

Electron-phonon physics from first principles using the EPW code

EPW is an open-source software for $\textit{ab initio}$ calculations of electron-phonon interactions and related materials properties. The code combines density functional perturbation theory and maximally-localized Wannier functions to efficiently compute electron-phonon coupling matrix elements on ultra-fine Brillouin zone grids. This data is employed for predictive calculations of temperature-dependent properties and phonon-assisted quantum processes in bulk solids and low-dimensional materials. Here, we report on significant new developments in the code that occurred during the period 2016-2022, namely: a transport module for the calculation of charge carrier mobility and conductivity under electric and magnetic fields within the $\textit{ab initio}$ Boltzmann transport equation; a superconductivity module for the calculation of critical temperature and gap structure in phonon-mediated superconductors within the $\textit{ab initio}$ anisotropic multi-band Eliashberg theory; an optics module for calculations of phonon-assisted indirect transitions; a module for the calculation of small and large polarons without supercells using the $\textit{ab initio}$ polaron equations; and a module for calculating electron-phonon couplings, band structure renormalization, and temperature-dependent optical spectra using the special displacement method. For each capability, we outline the methodology and implementation, and provide example calculations. We describe recent code refactoring to prepare EPW for exascale architectures, we discuss efficient parallelization strategies, and report on extreme parallel scaling tests.Comment: 61 pages, 9 figure

Peculiar perovskites: Unraveling the unique optical response of hybrid organic-inorganic perovskites from first principles

Hybrid organic-inorganic perovskite (HOP) materials and their layered analogs (LHOP) have been extensively applied to optoelectronic applications including solar cells, light-emitting diodes, optical detectors, and spintronics due to their unique atomic, electronic, and optical properties. HOPs and LHOPs are hosts to fascinating microscopic interactions that influence their macroscopic optical and electronic properties. Researchers still debate how interactions between optically excited charge carriers, free-carriers, lattice vibrations, and atomic geometry influence the measured optical response of HOPs. First principles simulations provide a window to examine how these atomic-scale interactions contribute piece-by-piece to the measurable optical properties of materials. In this PhD thesis, I apply and develop first principles optical calculations based on density functional theory and many-body perturbation theory to determine how the optical response and excitonic properties of HOPs and LHOPs are influenced by the presence of free-carriers, polar lattice vibrations, and layer stoichiometry. In order to describe polar lattice screening, I extend the lattice screened Coulomb interaction in the Fan-Migdal self energy to the electron-hole interaction by the Shindo approximation and subsequently construct model screening functions from the generalized and simple Frohlich models for the electron-phonon vertex. Additionally, I apply optical response calculations to predict how the choice of organic spacer layers in LHOPs can be used to design novel triplet light-emitting materials with emission wavelengths spanning the visible range

Peculiar perovskites: Unraveling the unique optical response of hybrid organic-inorganic perovskites from first principles

Hybrid organic-inorganic perovskite (HOP) materials and their layered analogs (LHOP) have been extensively applied to optoelectronic applications including solar cells, light-emitting diodes, optical detectors, and spintronics due to their unique atomic, electronic, and optical properties. HOPs and LHOPs are hosts to fascinating microscopic interactions that influence their macroscopic optical and electronic properties. Researchers still debate how interactions between optically excited charge carriers, free-carriers, lattice vibrations, and atomic geometry influence the measured optical response of HOPs. First principles simulations provide a window to examine how these atomic-scale interactions contribute piece-by-piece to the measurable optical properties of materials. In this PhD thesis, I apply and develop first principles optical calculations based on density functional theory and many-body perturbation theory to determine how the optical response and excitonic properties of HOPs and LHOPs are influenced by the presence of free-carriers, polar lattice vibrations, and layer stoichiometry. In order to describe polar lattice screening, I extend the lattice screened Coulomb interaction in the Fan-Migdal self energy to the electron-hole interaction by the Shindo approximation and subsequently construct model screening functions from the generalized and simple Frohlich models for the electron-phonon vertex. Additionally, I apply optical response calculations to predict how the choice of organic spacer layers in LHOPs can be used to design novel triplet light-emitting materials with emission wavelengths spanning the visible range

Phonon-Limited Mobility and Electron-Phonon Coupling in Lead-Free Halide Double Perovskites

International audienceLead-free halide double perovskites have attracted considerable attention as complements to lead-based halide perovskites in a range of optoelectronic applications. Experiments on Cs(2)AgBiBr(6) indicate carrier mobilities in the range of 0.3-11 cm(2)/(V s) at room temperature, considerably lower than in lead-based perovskites. The origin of low mobilities is currently unclear, calling for an atomic-scale investigation. We report state-of-the-art ab initio calculations of the phonon-limited mobility of charge carriers in lead-free halide double perovskites Cs(2)AgBiX(6) (X = Br, Cl). For Cs(2)AgBiBr(6), we obtain room-temperature electron and hole mobilities of 17 and 14 cm(2)/(V s), respectively, in line with experiments. We demonstrate that the cause for the lower mobility of this compound, compared to CH(3)NH(3)PbI(3), resides in the heavier carrier effective masses. A mode-resolved analysis of scattering rates reveals the predominance of Fröhlich electron-phonon scattering, similar to lead-based perovskites. Our results indicate that, to increase the mobility of lead-free perovskites, it is necessary to reduce the effective masses, for example by cation engineering

Bottleneck-Free Hot Hole Cooling in CH3NH3PbI3 Revealed by Femtosecond XUV Absorption

Femtosecond carrier cooling in the organohalide perovskite semiconductor CH3NH3PbI3 is measured using extreme ultraviolet (XUV) and optical transient absorption spectroscopy. XUV absorption between 44 eV and 58 eV measures transitions from the I 4d core to the valence and conduction bands and gives distinct signals for hole and electron dynamics. The core-to-valence-band signal directly maps the photoexcited hole distribution and provides a quantitative measurement of the hole temperature. The combination of XUV and optical probes reveals that upon excitation at 400 nm, the initial hole distribution is 3.5 times hotter than the electron distribution. At an initial carrier density of 1.4×1020 cm-3 both carriers are subject to a hot phonon bottleneck, but at 4.2×1019 cm-3 the holes cool to less than 1000 K within 400 fs. This result places significant constraints on the use of organohalide perovskites in hot-carrier photovoltaics.<br /