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Fil: Paredes Mellone, Oscar Ariel. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Cordoba. Instituto de Fisica Enrique Gaviola. Universidad Nacional de Cordoba. Instituto de Fisica Enrique Gaviola; Argentina. Universidad Nacional de Cordoba. Facultad de Matematica, Astronomia y Fisica; Argentina

We present a general picture of the exciton properties of layered materials in terms of the excitations of their single-layer building blocks. To this end, we derive a model excitonic hamiltonian by drawing an analogy with molecular crystals, which are other prototypical van der Waals materials. We employ this simplified model to analyse in detail the excitation spectrum of hexagonal boron nitride (hBN) that we have obtained from the {\it ab initio} solution of the many-body Bethe-Salpeter equation as a function of momentum. In this way we identify the character of the lowest-energy excitons in hBN, discuss the effects of the interlayer hopping and the electron-hole exchange interaction on the exciton dispersion, and illustrate the relation between exciton and plasmon excitations in layered materials., Accepted for Phys. Rev. B

In this paper we report an experimental and computational study of liquid acetonitrile (H3C-C[triple bond, length as m-dash]N) by resonant inelastic X-ray scattering (RIXS) at the N K-edge. The experimental spectra exhibit clear signatures of the electronic structure of the valence states at the N site and incident-beam-polarization dependence is observed as well. Moreover, we find fine structure in the quasielastic line that is assigned to finite scattering duration and nuclear relaxation. We present a simple and light-to-evaluate model for the RIXS maps and analyze the experimental data using this model combined with ab initio molecular dynamics simulations. In addition to polarization-dependence and scattering-duration effects, we pinpoint the effects of different types of chemical bonding to the RIXS spectrum and conclude that the H2C-C[double bond, length as m-dash]NH isomer, suggested in the literature, does not exist in detectable quantities. We study solution effects on the scattering spectra with simulations in liquid and in vacuum. The presented model for RIXS proved to be light enough to allow phase-space-sampling and still accurate enough for identification of transition lines in physical chemistry research by RIXS. In this paper we report an experimental and computational study of liquid acetonitrile (H3C-C equivalent to N) by resonant inelastic X-ray scattering (RIXS) at the N K-edge. The experimental spectra exhibit clear signatures of the electronic structure of the valence states at the N site and incident-beam-polarization dependence is observed as well. Moreover, we find fine structure in the quasielastic line that is assigned to finite scattering duration and nuclear relaxation. We present a simple and light-to-evaluate model for the RIXS maps and analyze the experimental data using this model combined with ab initio molecular dynamics simulations. In addition to polarization-dependence and scattering-duration effects, we pinpoint the effects of different types of chemical bonding to the RIXS spectrum and conclude that the H2C-C equivalent to NH isomer, suggested in the literature, does not exist in detectable quantities. We study solution effects on the scattering spectra with simulations in liquid and in vacuum. The presented model for RIXS proved to be light enough to allow phase-space-sampling and still accurate enough for identification of transition lines in physical chemistry research by RIXS.

We present a comprehensive study of the electronic, magnetic, and optical properties of ${\mathrm{CuGa}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{S}}_{2}$, as a promising candidate for intermediate-band (IB) solar cells. We use hybrid exchange-correlation functional within the density functional theory framework, and show that Fe doping induces unoccupied states 1.6--1.9 eV above the valence band. The IBs significantly enhance the optical absorption in lower energy part of the spectrum. We find that at moderate $n$-type co-doping concentration, the added charge occupies part of the IB in the gap, but large concentrations lower the energy of the occupied IB toward the valence band. Moreover, we show that Fe impurities tend to cluster within the compound and they choose antiferromagnetic ordering. The findings can have a significant effect in understanding this material and help to synthesize more efficient IB solar cells.

Hydration of sulfuric acid plays a key role in new-particle formation in the atmosphere. It has been recently proposed that proton dynamics is crucial in the stabilization of these clusters. One key question is how water molecules mediate proton transfer from sulfuric acid, and hence how the deprotonation state of the acid molecule behaves as a function concentration. We address the proton transfer in aqueous sulfuric acid with O K edge and S L edge core-excitation spectra recorded using inelastic X-ray scattering and with ab initio molecular dynamics simulations in the concentration range of 0-18.0 M. Throughout this range, we quantify the acid-water interaction with atomic resolution. Our simulations show that the number of donated hydrogen bonds per Owater increases from 1.9 to 2.5 when concentration increases from 0 to 18.0 M, in agreement with a rapid disappearance of the pre-edge feature in the O K edge spectrum. The simulations also suggest that for 1.5 M sulfuric acid SO4(2-) is most abundant and that its concentration falls monotonously with increasing concentration. Moreover, the fraction of HSO4(-) peaks at ∼12 M.

Understanding and controlling the way excitons propagate in solids is a key for tailoring materials with improved optoelectronic properties. A fundamental step in this direction is the determination of the exciton energy-momentum dispersion. Here, thanks to the solution of the parameter-free Bethe- Salpeter equation (BSE), we draw and explain the exciton energy-momentum map of hexagonal boron nitride (h-BN) in the first three Brillouin zones. We show that h-BN displays strong excitonic effects not only in the optical spectra at vanishing momentum $\mathbf{q}$, as previously reported, but also at large $\mathbf{q}$. We validate our theoretical predictions by assessing the calculated exciton map by means of an inelastic x-ray scattering (IXS) experiment. Moreover, we solve the discrepancies between previous experimental data and calculations, proving then that the BSE is highly accurate through the whole momentum range. Therefore, these results put forward the combination BSE and IXS as the tool of choice for addressing the exciton dynamics in complex materials. Understanding and controlling the way excitons propagate in solids is a key for tailoring materials with improved optoelectronic properties. A fundamental step in this direction is the determination of the exciton energy-momentum dispersion. Here, thanks to the solution of the parameter-free Bethe-Salpeter equation (BSE), we draw and explain the exciton energy-momentum map of hexagonal boron nitride (h-BN) in the first three Brillouin zones. We show that h-BN displays strong excitonic effects not only in the optical spectra at vanishing momentum q, as previously reported, but also at large q. We validate our theoretical predictions by assessing the calculated exciton map by means of an inelastic x-ray scattering (IXS) experiment. Moreover, we solve the discrepancies between previous experimental data and calculations, proving then that the BSE is highly accurate through the whole momentum range. Therefore, these results put forward the combination BSE and IXS as the tool of choice for addressing the exciton dynamics in complex materials.

We present a comprehensive simulation study on the solid-liquid phase transition of the ionic liquid 1,3-dimethylimidazolium chloride in terms of the changes in the atomic structure and their effect on the Compton profile. The structures were obtained by using ab initio molecular dynamics simulations. Chosen radial distribution functions of the liquid structure are presented and found generally to be in good agreement with previous ab initio molecular dynamics and neutron scattering studies. The main contributions to the predicted difference Compton profile are found to arise from intermolecular changes in the phase transition. This prediction can be used for interpreting future experiments.

We report a Compton scattering study on the molecular-level structural changes of aqueous poly(N-isopropylacrylamide) (PNIPAM) across the conformational phase transition. PNIPAM is a thermoresponsive polymer that changes its conformation in water from the hydrophilic coil state to the collapsed hydrophobic globule state at 32 °C. Combined with density functional theory calculations, the Compton scattering experiments detect two type of changes in the phase transition. The amount of hydrogen bonds is found to reduce, and an elongation of the internal covalent bond lengths is observed. The elongation of the bonds indicates that not only the hydrogen bonding changes but there are other processes, most likely related to hydrophobic interaction, that should be taken into account in the phase transition.