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TheabinitioBethe-Salpeter equation (BSE) approach, an established method for the study of excitons in materials, is typically solved in a limit where only static screening from electrons is captured. Here, we generalize this framework to include dynamical screening from phonons at lowest order in the electron-phonon interaction. We apply this generalized BSE approach to a series of inorganic lead halide perovskites,CsPbX3, withX = Cl, Br, and I. We find that inclusion of screening from phonons significantly reduces the computed exciton binding energies of these systems. By deriving a simple expression for phonon screening effects, we reveal general trends for their importance in semiconductors and insulators, based on a hydrogenic exciton model. We demonstrate that the magnitude of the phonon screening correction in isotropic materials can be reliably predicted using four material specific parameters: the reduced effective mass, static and optical dielectric constants, and frequency of the most strongly coupled longitudinal-optical phonon mode. This framework helps to elucidate the importance of phonon screening and its relation to excitonic properties in a broad class of semiconductors.

Accurate prediction of fundamental band gaps of crystalline solid state systems entirely within density functional theory is a long standing challenge. Here, we present a simple and inexpensive method that achieves this by means of non-empirical optimal tuning of the parameters of a screened range-separated hybrid functional. The tuning involves the enforcement of an ansatz that generalizes the ionization potential theorem to the removal of an electron in an occupied state described by a localized Wannier function in a modestly sized supercell calculation. The method is benchmarked against experiment for a set of systems ranging from narrow band gap semiconductors to large band gap insulators, spanning a range of fundamental band gaps from 0.2 to 14.2 eV and is found to yield quantitative accuracy across the board, with a mean absolute error of $\sim$0.1 eV and a maximal error of $\sim$0.2 eV., Comment: 10 pages, 2 figures

Antimony sulphide ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ has emerged as a promising material for a variety of energy applications ranging from solar cells to thermoelectrics and solid-state batteries. The most distinctive feature of ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ is its crystal structure, which consists of parallel 1-nm-wide ribbons held together by weak van der Waals forces. This structure clearly suggests that it should be possible to isolate individual ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ ribbons using micromechanical or liquid-phase exfoliation techniques. However, it is not clear yet how to identify the ribbons postexfoliation using standard optical probes. Using state-of-the-art first-principles calculations based on many-body perturbation theory, here we show that individual ribbons of ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ carry optical signatures clearly distinct from those of bulk ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$. In particular, we find a large blueshift of the optical absorption edge (from 1.38 to 2.30 eV) resulting from the interplay between a reduced screening and the formation of bound excitons. In addition, we observe a transition from an indirect band gap to a direct gap, suggesting an enhanced photoluminescence in the green. These unique fingerprints will enable extending the research on van der Waals materials to the case of one-dimensional chalchogenides.

We study the quasiparticle band gap of the hybrid organic-inorganic lead halide perovskite ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$, using many-body perturbation theory based on the $GW$ approximation. We perform a systematic analysis of the band gap sensitivity to relativistic spin-orbit effects, to the description of semicore Pb-$5d$ and I-$4d$ electrons, and to the starting Kohn-Sham eigenvalues. We find that the inclusion of semicore states increases the calculated band gap by 0.2 eV, and self-consistency on the quasiparticle eigenvalues using a scissor correction increases the band gap by 0.5 eV with respect to the ${G}_{0}{W}_{0}$ result. These findings allow us to resolve an inconsistency between previously reported $GW$ calculations for ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$. Our most accurate band gap is 1.72 eV, and is in good agreement with the measured optical gap after considering a small excitonic shift as determined in experiments.

A model for the electronic properties of two dimensional nano-tori has been constructed. To this end, the Schrodinger equation has been approximately solved for the entire system, and the average number of electrons has been studied. The electron spread proved to have important implications in these structure's field emission properties, further studied using the one dimensional Wentzel — Kramers — Brillouin (WKB) approximation.

Department of Optics, Palack´y University, 17. listopadu 50, 77200 Olomouc, Czech Republic(Dated: February 1, 2008)We show theoretically and experimentally that single copy distillation of squeezing from continu-ous variable non-Gaussian states is possible using linear optics and conditional homodyne detection.A specific non-Gaussian noise source, corresponding to a random linear displacement, is investigated.Conditioning the signal on a tap measurement, we observe probabilistic recovery of squeezing.

We propose and experimentally demonstrate quantum purification of coherent light states, using only basic linear optics. It is based on the independent processing of several identically prepared coherent states.

The electronic properties of two-dimensional toroidal surfaces of nanometer size have been investigated by approximately solving the time-independent Schrodinger equation. The effects of the quantum confinement on the electron population of these structures were shown by studying the average electron density on the surface of the torus. The unique electron spread that resulted from such computations encouraged further study on the field emission properties of this two-dimensional manifold. The Wentzel–Kramers–Brillouin approximate approach was used for a preliminary study of the field emission properties of the nanotori for various geometrical parameters.

Fueled by a strong demand in various practical areas, the development of electron sources has reached new heights in recent years, with respect to both intensity and quality of the produced electron beams. In this context, field-emission (FE) cathodes play a crucial role for many special applications. To obtain strong and stable FE currents with relatively small anode voltages, sharper conductive structures have been introduced that may focus the electric field on small areas of their tips up to huge values of the order of 10 V/nm. Carbon nanotubes (CNTs) are well known examples of such structures [1]. Tapered CNTs and even conical graphene nanoneedles have been considered and studied as potential FE sources [2,3]. However, ultra-sharp features present in a quantum structure may determine specific configurations for the allowed electronic energy levels in the whole structure, mostly in devices with coherent transport of electrons. For example, ultra-sharp terminations of tapered CNTs may actually restrict the allowed states for the conduction electrons. As a consequence, the electron population on such sharp features may be lower than in other, more bulky, parts of the structure. Thus, the field enhancement that normally occurs on high-curvature parts of conductive surfaces may be hindered or even compensated by the lack of conduction electrons due to local tight spatial confinement [2].