Connection between inelastic x-ray scattering and complementary x-ray spectroscopies: probing excitons at Al K and L1 edges of α\alpha-Al2_2O3_3

We present an ab initio study of core excitations at the aluminum K and L1 edges in ${\alpha}$-Al2O3 within an all-electron many-body perturbation theory (MBPT) framework. Calculated XAS reveals excellent agreement with experiments, highlighting the dipole-forbidden nature of the pre-peak, which in experiments is enabled by sp mixing due to atomic vibrations. Non-resonant inelastic X-ray scattering (NRIXS) is employed to go beyond the dipole approximation and probe transition channels with s, p, and d character, enhancing multipole transitions that contribute to the pre-peak. The RIXS spectra at K and L1 edges are remarkably similar, opening the way to soft X-ray RIXS experiments to probe semi-core s states. The RIXS calculations reveal two distinct regimes based on the behavior with incoming photon energy ($\omega_1$). For $\omega_1$ in resonance with the XAS threshold, we observe Raman-like behavior, where the RIXS spectra show significant dependence on $\omega_1$ , reflecting the coupling between absorption and emission processes. For higher $\omega_1$ , above the XAS threshold, the study reveals fluorescence features that appear at constant emission energy, and can be explained via X-ray emission spectroscopy (XES)

Excitons in van der Waals materials : From monolayer to bulk hexagonal boron nitride

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 analyze in detail the excitation spectrum of hexagonal boron nitride (hBN) that we have obtained from the 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.Peer reviewe

Epoxy resin doped with Coumarin 6: Example of accessible luminescent collectors

We report on the preparation of luminescent collectors based on epoxy resins containing Coumarin 6 as fluorescent dye. Fluorescent epoxy slabs were obtained by carefully mixing from 60 to 150 ppm of the fluorophore with bisphenol A diglycidyl ether and 4,4′-methylenebis(2-methylcyclohexylamine) as curing agent. Spectroscopic (FT-IR, solid-state NMR, Raman) investigations and calorimetric analysis evidence the success of the preparation procedure in terms of slab homogeneity, fluorophore dispersibility and its role in promoting the crosslinking extent. The concentrating ability and the derived optical efficiencies of the epoxy-based collectors are determined with a properly designed set-up and result greater (∼10%) than that of poly(methyl methacrylate) concentrators with the same fluorophore and geometry. Optical efficiencies as high as 7.4% are obtained and enable the potential use of epoxy resins as bulk thermosetting materials for solar collectors

Exciton band structure of molybdenum disulfide: from monolayer to bulk

Exciton band structures analysis provides a powerful tool to identify the exciton character of materials, from bulk to isolated systems, and goes beyond the mere analysis of the optical spectra. In this work, we focus on the exciton properties of molybdenum sisulfide (MoS 2 ) by solving the ab initio many-body Bethe–Salpeter equation, as a function of momentum, to obtain the excitation spectra of both monolayer and bulk MoS 2 . We analyse the spectrum and the exciton dispersion on the basis of a model excitonic Hamiltonian capable of providing an efficient description of the excitations in the bulk crystal, starting from the knowledge of the excitons of a single layer. In this way, we obtain a general characterization of both bright and darks excitons in terms of the interplay between the electronic band dispersion (i.e. interlayer hopping) and the electron–hole exchange interaction. We identify for both the 2D and the 3D limiting cases the character of the lowest-energy excitons in MoS 2 , we explain the effects and relative weights of both band dispersion and electron–hole exchange interaction and finally we interpret the differences observed when changing the dimensionality of the system

Multiple satellites in materials with complex plasmon spectra: From graphite to graphene

International audienceThe photoemission spectrum of graphite is still debated. To help resolve this issue, we present photoemission measurements at high photon energy and analyze the results using a Green's function approach that takes into account the full complexity of the loss spectrum. Our measured data show multiple satellite replicas. We demonstrate that these satellites are of intrinsic origin, enhanced by extrinsic losses. The dominating satellite is due to the π+σ plasmon of graphite, whereas the π plasmon creates a tail on the high-binding energy side of the quasiparticle peak. The interplay between the two plasmons leads to energy shifts, broadening, and additional peaks in the satellite spectrum. We also predict the spectral changes in the transition from graphite towards graphene

Exciton energy-momentum map of hexagonal boron nitride

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.Peer reviewe

Characterization of a 6×6-mm2 75-μm cell MPPC suitable for the Cherenkov Telescope Array project

This paper presents the latest characterization results of a novel Low Cross-Talk (LCT) large-area (6×6-mm2) Multi-Pixel Photon Counter (MPPC) detector manufactured by Hamamatsu, belonging to the recent LCT5 family and achieving a fill-factor enhancement and cross-talk reduction. In addition, the newly adopted resin coating is demonstrated to yield improved photon detection capabilities in the 290–350 nm spectral range, making the new LCT MPPC particularly suitable for emerging applications like Cherenkov Telescopes. For a 3×3-mm2 version of the new MPPC under test, a comparative analysis of the large pixel pitch (75-µm) detector versus the smaller pixel pitch (50-µm) detector is also undertaken. Furthermore, measurements of the 6×6-mm2 MPPC response versus the angle of incidence are provided for the characterized device