Electronic Structure of (Organic-)Inorganic Metal Halide Perovskites: The Dilemma of Choosing the Right Functional

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 approach as well as variants of the hybrid functionals PBE0 and HSE are at the center of the investigations. For both functionals, methods to determine the mixing parameter α are explored, and for HSE, the impact of the screening-parameter ω is investigated. An extensive investigation of PbI2, a precursor of many HaPs, leads to the conclusion that hybrid functionals with α tuned by the density-based mixing method are most suitable for obtaining band gaps comparable toPeer Reviewe

Bulk and surface electronic structure of Bi4_4Te3_3 from GWGW calculations and photoemission experiments

We present a combined theoretical and experimental study of the electronic structure of stoichiometric Bi$_4$Te$_3$, a natural superlattice of alternating Bi$_2$Te$_3$ quintuple layers and Bi bilayers. In contrast to the related semiconducting compounds Bi$_2$Te$_3$ and Bi$_1$Te$_1$, density functional theory predicts Bi$_4$Te$_3$ to be a semimetal. In this work, we compute the quasiparticle electronic structure of Bi$_4$Te$_3$ 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, Bi$_4$Te$_3$ is classified as a dual topological insulator with bulk topological invariants $\mathbb{Z}_2$ (1;111) and magnetic mirror Chern number $n_M=1$. The bulk $GW$ results are used to build a Wannier-functions 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 Bi$_4$Te$_3$

Excitations in cubic BaSnO3_{3}: a consistent picture revealed by combining theory and experiment

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. On- and off-axis valence EELS spectra, featuring signals originating from band gap transitions, are acquired on defect-free sample regions of a BaSnO$_{3}$ single crystal. These high-energy-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