Organic-inorganic metal halide perovskites (HaPs) are intensively studied for their light-harvesting properties. Owing to the interplay between strong electron-electron interaction and spin-orbit coupling (SOC), their quantitative theoretical description is still a challenge as evidenced by the wide variety of results available in literature. Here, various methodologies for computing their electronic structure are evaluated, also accounting for SOC. More specific, the GW approach as well as variants of the hybrid functionals PBE0 and HSE are at the center of our investigations. For both functionals, we explore methods to determine the mixing parameter $\alpha$, and for HSE, we investigate the impact of the screening-parameter $\omega$. An extensive investigation of PbI2, a precursor of many HaPs, leads to the conclusion that hybrid functionals with $\alpha$ tuned by the density-based mixing method are most suitable for obtaining band gaps comparable to $G_0W_0$ results. Moreover, this methodology is transferable to CsPbI3, and the same behaviour is expected for the entire family of lead-iodine perovskites.

In this work, we report results of the quasiparticle self-consistent GW method within the framework of the linearized augmented plane-wave method. The impact of self-consistency on the electronic structure is investiand GaAs. Possible reasons for discrepancies between different studies and implementations are discussed. For LiF, MgO, and CaO, we assess the charge-density redistribution upon self-consistency. For a representative set of materials, we investigate and confirm the absence of (any) starting-point dependence. The off-diagonal terms in the self-energy matrix are found to considerably impact the electronic structure. For better reproducibility and quality assessment, we describe the implementation of QSGW in the all-electron full-potential code EXCITING.

Among the transparent conducting oxides, the perovskite barium stannate is most promising for various electronic applications due to its outstanding carrier mobility achieved at room temperature. Most important characteristics however, i.e., its band gap, effective masses, and absorption edge remain controversial. Here, we provide a fully consistent picture by combining state-of-the-art ab initio methodology with forefront electron energy-loss spectroscopy (EELS) and optical absorption measurements. Onand off-axis valence EELS spectra, featuring signals originating from band gap transitions, are acquired on defect-free sample regions of a BaSnO3 single crystal. These highenergy-resolution measurements are able to capture also very weak excitations below the optical gap, attributed to indirect transitions. By temperature-dependent optical absorption measurements, we assess band-gap renormalization effects induced by electron-phonon coupling. Overall, we find for the effective electronic mass, the direct and the indirect gap, the optical gap as well as the absorption onsets and spectra excellent agreement between both experimental techniques and the theoretical many-body results, supporting also the picture of a phonon-mediated mechanism where indirect transitions are activated by phonon-induced symmetry lowering. This work demonstrates a fruitful connection between different high-level theoretical and experimental methods for exploring the characteristics of advanced materials.

We present a combined theoretical and experimental study of the electronic structure of stoichiometric Bi4Te3, a natural superlattice of alternating Bi2Te3 quintuple layers and Bi bilayers. In contrast to the related semiconducting compounds Bi2Te3 and Bi1Te1, density functional theory predicts Bi4Te3 is a semimetal. In this work, we compute the quasiparticle electronic structure of Bi4Te3 in the framework of the GW approximation within many-body perturbation theory. The quasiparticle corrections are found to modify the dispersion of the valence and conduction bands in the vicinity of the Fermi energy, leading to the opening of a small indirect band gap. Based on the analysis of the eigenstates, Bi4Te3 is classified as a dual topological insulator with bulk topological invariants Z(2) (1; 111) and magnetic mirror Chern number n(M) = 1. The bulk GW results are used to build a Wannier-function-based tight-binding Hamiltonian that is further applied to study the electronic properties of the (111) surface. The comparison with our angle-resolved photoemission measurements shows excellent agreement between the computed and measured surface states and indicates the dual topological nature of Bi4Te3.

The sixth sentence of the fifth paragraph under Results contained two redundant ‘>’ and originally incorrectly read ‘the standard procedure of approximating the quasiparticle wave functions by the KS wave functions GW (diag)> breaks down at that point, and we are forced to calculate the quasiparticle wave function explicitly by diagonalization of the quasiparticle equation GW (full)>’. The correct version reads ‘the standard procedure of approximating the quasiparticle wave functions by the KS wave functions GW (diag) breaks down at that point, and we are forced to calculate the quasiparticle wave function explicitly by diagonalization of the quasiparticle equation GW (full)’. The seventh sentence of the fifth paragraph under Results incorrectly omitted the figure number and ‘GW’ and originally read ‘We also see from Fig. that, for the most part, the two approaches give nearly identical results’. The correct version reads ‘We also see from Fig. 3 that, for the most part, the two GW approaches give nearly identical results’.

The GW method is a standard method to calculate the electronic band structure from first principles. It has been applied to a large variety of semiconductors and insulators but less often to metallic systems, in particular, with respect to a self-consistent employment of the method. In this work, we take a look at all-electron quasiparticle self-consistent GW (QSGW) calculations for simple metals (alkali and alkaline earth metals) based on the full-potential linearized augmented-plane-wave approach and compare the results to single-shot (i.e., non-selfconsistent) G0W0 calculations, density-functional theory (DFT) calculations in the local-density approximation, and experimental measurements. We show that, while DFT overestimates the bandwidth of most of the materials, the GW quasiparticle renormalization corrects the bandwidths in the right direction, but a full self-consistent calculation is needed to consistently achieve good agreement with photoemission data. The results mainly confirm the common belief that simple metals can be regarded as nearly free electron gases with weak electronic correlation. The finding is particularly important in light of a recent debate in which this seemingly established view has been contested.

This work combines two powerful self-energy techniques: the well-known GW method and a self-energy recently developed by us that describes renormalization effects caused by the scattering of electrons with magnons and Stoner excitations. This GT self-energy, which is fully k -dependent and contains infinitely many spin-flip ladder diagrams, was shown to have a profound impact on the electronic band structure of Fe, Co, and Ni. In the present work, we refine the method by combining GT with the GW self-energy. The resulting GWT spectral functions exhibit strong lifetime effects and emergent dispersion anomalies. They are in an overall better agreement with experimental spectra than those obtained with GW or GT alone, even showing partial improvements over local-spin-density approximation dynamical mean-field theory. The performed analysis provides a basis for applying the GWT technique to a wider class of magnetic materials.

We provide a combined theoretical and experimental study of the electronic structure and the optical absorption edge of the orthorhombic perovskite $\mathrm{La}\mathrm{In}{\mathrm{O}}_{3}$, employing density functional theory and many-body perturbation theory. We find the lowest-energy excitation at 0.2 eV below the fundamental gap (5 eV), reflecting a sizable electron-hole attraction. Since the transition from the valence band maximum ($\mathrm{\ensuremath{\Gamma}}$ point) is, however, dipole forbidden, the onset is characterized by weak excitations from transitions around it. The first intense excitation appears about 0.32 eV above. Interestingly, this value coincides with an experimental value obtained by ellipsometry (4.80 eV) which is higher than the onset from optical absorption spectroscopy (4.35 eV). The latter discrepancy is attributed to the fact that the weak transitions that define the optical gap are not well enough resolved by the ellipsometry measurement. Through temperature-dependent measurements of the optical gap, we assess renormalization effects by electron-phonon coupling, enhancing the quantitative comparison between theoretical and experimental results.

The emerging interest in van der Waals heterostructures as new materials for opto-electronics and photonics poses questions about their stability and structure-property relations. In the framework of density-functional and many-body perturbation theory, we investigate the structural, electronic, and optical properties of periodic heterostructures formed by graphene and hexagonal boron nitride (h-BN). To understand how the constituents affect each other depending on the layer stacking, we examine 12 commensurate arrangements. We find that interaction with graphene improves the stability of bulk h-BN also in those configurations that are predicted to be energetically metastable. In return, the interaction with h-BN can open a band gap of a few hundred meV in graphene. Its actual size can be tuned by the arrangement of the layers. In the semiconducting configurations, the character and spatial distribution of optical excitations are affected by the specific stacking, that determines the electronic states involved in the transitions. Remarkably, six out of the 12 explored heterostructures remain semi-metallic.

Received 6 March 2019DOI:https://doi.org/10.1103/PhysRevB.99.099905©2019 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBand gapElectronic structureFirst-principles calculationsSemiconductorsTechniquesDensity functional theoryDensity functional theory developmentElectron-correlation calculationsGW methodHigh-throughput calculationsRandom phase approximationk dot p methodCondensed Matter, Materials & Applied Physics

By combining all-electron density-functional theory with many-body perturbation theory, a prototypical inorganic/organic hybrid system, composed of pyridine molecules that are chemisorbed on the nonpolar ZnO(1010) surface is investigated. The G0W0 approximation is employed to describe its one-particle excitations in terms of the quasiparticle band structure, and the Bethe–Salpeter equation is solved to obtain the absorption spectrum. The different character of the constituents leads to very diverse self-energy corrections of individual Kohn–Sham states, and thus the G0W0 band structure is distinctively different from its DFT counterpart, that is, many-body effects cannot be regarded as a rigid shift of the conduction bands. The nature of the optical excitations at the interface over a wide energy range is explored and it is shown that various kinds of electron-hole pairs are formed, comprising hybrid excitons and (hybrid) charge-transfer excitations. The absorption onset is characterized by a strongly bound bright ZnO-dominated hybrid exciton. For the selected examples of either exciton type, the individual contributions from the valence and conduction bands are analyzed and the binding strength and extension of the electron-hole wavefunctions are discussed.

By combining all-electron density-functional theory with many-body perturbation theory, a prototypical inorganic/organic hybrid system, composed of pyridine molecules that are chemisorbed on the nonpolar ZnO( 10 1 ¯ 0 ) surface is investigated. The G 0 W 0 approximation is employed to describe its one-particle excitations in terms of the quasiparticle band structure, and the Bethe–Salpeter equation is solved to obtain the absorption spectrum. The different character of the constituents leads to very diverse self-energy corrections of individual Kohn–Sham states, and thus the G 0 W 0 band structure is distinctively different from its DFT counterpart, that is, many-body effects cannot be regarded as a rigid shift of the conduction bands. The nature of the optical excitations at the interface over a wide energy range is explored and it is shown that various kinds of electron-hole pairs are formed, comprising hybrid excitons and (hybrid) charge-transfer excitations. The absorption onset is characterized by a strongly bound bright ZnO-dominated hybrid exciton. For the selected examples of either exciton type, the individual contributions from the valence and conduction bands are analyzed and the binding strength and extension of the electron-hole wavefunctions are discussed.

Controlling the electrical conductivity of organic semiconductors is a key asset for organic electronics, nowadays realized mostly by molecular dopants. Two doping mechanisms have been reported - charge-transfer complex (CTC) and ion pair (IPA) formation. However, their occurrence depending on molecular structure, energy levels, and structure of thin films remains elusive. Here, we study p-type doping of the planar organic semiconductor dibenzotetrathiafulvalene (DBTTF) in combination with the electron acceptors tetracyanonaphthoquinodimethane (TCNNQ) and hexafluorotetracyanonaphthoquinodimethane (F6TCNNQ) as planar dopants. The conductivity of DBTTF films increases by more than two orders of magnitude upon doping with F6TCNNQ and only slightly with TCNNQ. The highest conductivity is reached at about 10 mol % dopant concentration as a result of two counteracting effects: (1) increasing carrier concentration and (2) reduced carrier mobility due to the growing density of structural defects. We identified two different CTCs for DBTTF:TCNNQ blends and both types of charge-transfer interactions (CTC and IPA) in films of DBTTF doped with F6TCNNQ from absorption measurements. No signature of the charge-transfer interaction is found for DBTTF and TCNNQ in solution, whereas IPA formation only is observed for DBTTF and F6TCNNQ Many-body perturbation theory calculations of the electronic and optical properties of one-dimensional stacks complement the experimental data and help in understanding the behavior of CTCs. The degree of charge transfer turns out to be higher for the DBTTF:F6TCNNQ complexes than for DBTTF:TCNNQ as derived from the C N stretching mode softening in infrared absorption. We discuss the different fundamental semiconductor-dopant interactions in solution as compared to the solid state with the aid of the state-of-matter-dependent energy levels of the materials. The presence of both charge-transfer mechanisms in the material combinations investigated here gives us access to their doping efficiency, which is higher for IPA than for CTC formation. Avoiding the CTC formation by structural imperfections seems to be a way to increase the doping efficiency for crystalline materials. The determination of energy levels both in solution and in thin films is beneficial for understanding charge-transfer behavior.

By means of full-potential all-electron density-functional theory and many-body perturbation theory, we compute the band alignment at a prototypical hybrid inorganic/organic interface. The electronic properties of a model system built of poly(para-phenylene) and rs-ZnO are studied in two different geometries, employing several approaches of increasing sophistication. To this extent, we explore models for predicting the level alignment, which are based on the knowledge of the electronic structure of the individual constituents and are commonly used for semiconductor interfaces. For their evaluation in the context of hybrid materials, we perform an ab initio study of the entire system, including a quasiparticle description of the electronic structure within the G0W0 approximation. Based on this, we quantify the impact of structure, charge redistribution, orbital hybridization, and molecular polarization on the band offsets and the alignment type. We highlight not only known limitations of predicting the level alignment at a hybrid inorganic/organic interface by simple models but also demonstrate how structural details of the interface components impact the results.

Hybrid systems, combining inorganic and organic components, present an ex-citing pathway towards tailoring materials with specific desired properties. By a first-principles approach based on many-body Green function techniques, in article number 1800108, Olga Turkina, Claudia Draxl, and co-workers explore the opto-electronic spectra of pyridine/ZnO(1010). In this prototypical hybrid system, various kinds of novel exciton types are formed. The image shows the hole probability distribution of a hybrid charge-transfer exciton.

We present a novel approach to electron-lattice interaction beyond the linear-coupling regime. Based on the solution of a Holstein-Peierls-type model, we derive explicit analytical expressions for the eigenvalue spectrum of the Hamiltonian, resulting in a narrowing of bands as a function of temperature. Our approach enables the intuitive interpretation in terms of quasiparticles, i.e. polaron bands and dressed-phonon frequencies. Being nonperturbative, the formalism also applies in the strong-coupling case. We apply it to the organic crystal naphthalene, with the coupling strengths obtained by ab initio calculations.

By combining all-electron density-functional theory with many-body perturbation theory, we investigate a prototypical inorganic/organic hybrid system, composed of pyridine molecules that are chemisorbed on the non-polar ZnO($10\overline{1}0$) surface. We employ the $G_0W_0$ approximation to describe its one-particle excitations in terms of the quasi-particle band structure, and solve the Bethe-Salpeter equation for obtaining the absorption spectrum. The different character of the constituents leads to very diverse self-energy corrections of individual Kohn-Sham states, and thus the $G_0W_0$ band structure is distinctively different from its DFT counterpart, i.e., many-body effects cannot be regarded as a rigid shift of the conduction bands. We explore the nature of the optical excitations at the interface over a wide energy range and show that various kinds of electron-hole pairs are formed, comprising hybrid excitons and (hybrid) charge-transfer excitations. The absorption onset is characterized by a strongly bound bright ZnO-dominated hybrid exciton. For selected examples of either exciton type, we analyze the individual contributions from the valence and conduction bands and discuss the binding strength and extension of the electron-hole wavefunctions.

With the example of hexagonal boron nitride, we demonstrate how the character of electron-hole (e-h) pairs in van der Waals bound low-dimensional systems is driven by layer stacking. Four types of excitons appear, with either a two- or three-dimensional spatial extension. Electron and hole distributions are either overlapping or exhibit a charge-transfer nature. We discuss under which structural and symmetry conditions they appear and they are either dark or bright. This analysis provides the key elements to identify, predict, and possibly tailor the character of e-h pairs in van der Waals materials.

We present a combined experimental and theoretical study to solve the unit-cell and molecular arrangement of the tetracene thin film (TF) phase. TF phases, also known as substrate induced phases (SIPs), are polymorphs that exist at interfaces and decisively impact the functionality of organic thin films, e.g., in a transistor channel, but also change the optical spectra due to the different molecular packing. As SIPs only exist in textured ultrathin films, their structure determination remains challenging compared to bulk materials. Here, we use grazing incidence X-ray diffraction and atomistic simulations to extract the TF unit-cell parameters of tetracene together with the atomic positions within the unit-cell.