Anisotropic excitonic effects in the energy loss function of hexagonal boron nitride

We demonstrate that the valence energy-loss function of hexagonal boron nitride (hBN) displays a strong anisotropy in shape, excitation energy and dispersion for momentum transfer q parallel or perpendicular to the hBN layers. This is manifested by e.g. an energy shift of 0.7 eV that cannot be captured by single-particle approaches and is a demonstration of a strong anisotropy in the two-body electron-hole interaction. Furthermore, for in-plane directions of q we observe a splitting of the -plasmon in the M direction that is absent in the K direction and this can be traced back to band-structure effects.Comment: 10 pages, 4 figure

Electrode thickness measurement of a Si(Li) detector for the SIXA array

Cathode electrodes of the Si(Li) detector elements of the SIXA X-ray spectrometer array are formed by gold-palladium alloy contact layers. The equivalent thickness of gold in one element was measured by observing the characteristic L-shell X-rays of gold excited by monochromatised synchrotron radiation with photon energies above the L3 absorption edge of gold. The results obtained at 4 different photon energies below the L2 edge yield an average value of 22.4(35) nm which is consistent with the earlier result extracted from detection efficiency measurements. PACS: 29.40.Wk; 85.30.De; 07.85.Nc; 95.55.Ka Keywords: Si(Li) detectors, X-ray spectrometers, X-ray fluorescence, detector calibration, gold electrodes, synchrotron radiationComment: 10 pages, 4 PostScript figures, uses elsart.sty, submitted to Nucl. Instrum. Meth.

Improving the spatial and statistical accuracy in X-ray Raman scattering based direct tomography

An algorithm to simultaneously increase the spatial and statistical accuracy of X-ray Raman scattering (XRS) based tomographic images is presented. Tomography that utilizes XRS spectroscopy signals as a contrast for the images is a new and promising tool for investigating local atomic structure and chemistry in heterogeneous samples. The algorithm enables the spatial resolution to be increased based on a deconvolution of the optical response function of the spectrometer and, most importantly, it allows for the combination of data collected from multiple analyzers and thus enhances the statistical accuracy of the measured images.Peer reviewe

Bimagnon studies in cuprates with Resonant Inelastic X-ray Scattering at the O K edge. I - An assessment on La2CuO4 and a comparison with the excitation at Cu L3 and Cu K edges

We assess the capabilities of magnetic Resonant Inelastic X-ray Scattering (RIXS) at the O $K$ edge in undoped cuprates by taking La_{2}CuO_{4} as a benchmark case, based on a series of RIXS measurements that we present here. By combining the experimental results with basic theory we point out the fingerprints of bimagnon in the O $K$ edge RIXS spectra. These are a dominant peak around 450 meV, the almost complete absence of dispersion both with $\pi$ and $\sigma$ polarization and the almost constant intensity vs. the transferred momentum with $\sigma$ polarization. This behavior is quite different from Cu $L_3$ edge RIXS giving a strongly dispersing bimagnon tending to zero at the center of the Brillouin zone. This is clearly shown by RIXS measurements at the Cu $L_3$ edge that we present. The Cu $L_3$ bimagnon spectra and those at Cu $K$ edge - both from the literature and from our data - however, have the same shape. These similarities and differences are understood in terms of different sampling of the bimagnon continuum. This panorama points out the unique possibilities offered by O $K$ RIXS in the study of magnetic excitations in cuprates near the center of the BZ

Li 1s core exciton in LiH studied by x-ray Raman scattering spectroscopy

The Li 1s core excitation spectra in LiH was studied by means of x-ray Raman scattering (XRS) spectroscopy in a wide range of momentum transfers q. The analysis of the near-edge region of the measured spectra in combination with q-dependent ab initio calculations of XRS spectra based on the Bethe-Salpeter equation (BSE) reveals that the prominent peak at the excitation onset arises from two main contributions, namely a pre-edge peak associated to a p-type core exciton and strong transitions to empty states near the bottom of the conduction band, which is in contrast to previous experimental studies that attributed that feature to a single excitonic peak. The p-like angular symmetry of the core exciton is supported by BSE calculations of the relative contributions to the XRS spectra from monopole and dipole transitions and by the observed decrease of its normalised intensity for increasing momentum transfers. Higher energy spectral features in the measured XRS spectra are well reproduced by BSE, as well as by real-space multiple-scattering calculations.Peer reviewe

K -shell diagram and hypersatellite spectra of 4d transition elements

The K-shell diagram (K alpha(1,2) and K beta(1,3)) and hypersatellite (HS) (K-h alpha(1,2)) spectra of Y, Zr, Mo, and Pd have been measured with high energy-resolution using photoexcitation by 90 keV synchrotron radiation. Comparison of the measured and ab initio calculated HS spectra demonstrates the importance of quantum electrodynamical (QED) effects for the HS spectra. Phenomenological fits of the measured spectra by Voigt functions yield accurate values for the shift of the HS from the diagram lines, the splitting of the HS lines, and their intensity ratio. Good agreement with theory was found for all quantities except for the intensity ratio, which is dominated by the intermediacy of the coupling of the angular momenta. The observed deviations imply that our current understanding of the variation of the coupling scheme from LS to jj across the periodic table may require some revision.The K-shell diagram (K alpha(1,2) and K beta(1,3)) and hypersatellite (HS) (K-h alpha(1,2)) spectra of Y, Zr, Mo, and Pd have been measured with high energy-resolution using photoexcitation by 90 keV synchrotron radiation. Comparison of the measured and ab initio calculated HS spectra demonstrates the importance of quantum electrodynamical (QED) effects for the HS spectra. Phenomenological fits of the measured spectra by Voigt functions yield accurate values for the shift of the HS from the diagram lines, the splitting of the HS lines, and their intensity ratio. Good agreement with theory was found for all quantities except for the intensity ratio, which is dominated by the intermediacy of the coupling of the angular momenta. The observed deviations imply that our current understanding of the variation of the coupling scheme from LS to jj across the periodic table may require some revision.The K-shell diagram (K alpha(1,2) and K beta(1,3)) and hypersatellite (HS) (K-h alpha(1,2)) spectra of Y, Zr, Mo, and Pd have been measured with high energy-resolution using photoexcitation by 90 keV synchrotron radiation. Comparison of the measured and ab initio calculated HS spectra demonstrates the importance of quantum electrodynamical (QED) effects for the HS spectra. Phenomenological fits of the measured spectra by Voigt functions yield accurate values for the shift of the HS from the diagram lines, the splitting of the HS lines, and their intensity ratio. Good agreement with theory was found for all quantities except for the intensity ratio, which is dominated by the intermediacy of the coupling of the angular momenta. The observed deviations imply that our current understanding of the variation of the coupling scheme from LS to jj across the periodic table may require some revision.Peer reviewe

Assessing temperature effects on multipole contributions and angular dependence in core-level spectroscopies

This study aims at assessing the thermal nuclei motion effects on the multipole transition channels involved in two core-level spectroscopies, x-ray absorption spectroscopy (XAS) and x-ray Raman scattering (XRS). Temperature effects on the 1s -> s monopole, 1s -> p dipole, and 1s -> d quadrupole transitions are investigated using two reference systems for which we present original experimental data: alpha-Al2O3 at the Al K edge probed by XRS at room temperature and rutile TiO2 at the Ti K pre-edge probed by XAS at temperatures ranging from 6 to 700 K. Through the latter, this work enlightens the part of the pre-edge peak enhancement due to temperature in the K pre-edge region of 3d transition metal, which is known to be routinely used to determine the concentration, valence or symmetry of the probed element in a given sample. Nuclear thermal fluctuations are taken into account using a method based on density functional theory that consists in averaging spectra over atomic configurations, generated within the harmonic approximation and obeying quantum statistics at finite temperature. Since only a finite number of such configurations are used, the numerically averaged spectra generally lose the symmetry of the equilibrium crystal positions. In this paper, we demonstrate that the physical average has to be symmetric and propose a method to restore the physical angular dependence of the spectra. The approach is successfully applied to investigate the angular dependent XAS spectra in rutile as a function of temperature. The two systems under study allow to draw general conclusions regarding the effect of nuclear quantum fluctuations on the different transition channels available to both core-level spectroscopies.Peer reviewe

Direct observation of the momentum distribution and renormalization factor in lithium

We have measured the momentum distribution and renormalization factor Z(kF) in liquid and solid lithium by high-resolution Compton scattering. High-resolution data over a wide momentum range exhibit a clear feature of the renormalization and a sharp drop of momentum densities at the Fermi momentum k(F). These results are compared with those computed by quantum Monte Carlo simulation performed both on a disordered crystal and a liquid exhibiting very good agreement. Asymptotic behavior of the experimental and theoretical momentum distributions are examined to estimate Z(kF). The experimentally obtained Z(kF) = 0.43(-0.01)(+0.11) for liquid Li and 0.54(-0.02)(+0.11) for solid Li are in good agreement with theoretical results of 0.54 +/- 0.01 and 0.64 +/- 0.01, respectively.Peer reviewe

Comparing C-60 and C-70 as acceptor in organic solar cells : Influence of the electronic structure and aggregation size on the photovoltaic characteristics

The difference in aggregation size of the C-60 and C-70 fullerenes affect the photovoltaic performance of devices assembled in the so-called bilayer architecture with poly [2,7-(9,9- dioctyl- dibenzosilole)- alt-4,7- bis(thiophen-2-yl)benzo- 2,1,3- thiadiazole] (PSiF-DBT) as the electron donor material. Despite the better performance of the C-70 devices, which is related to the high absorption coefficient in the visible range and the superior charge transport properties, the short-circuit current variation upon annealing treatment at 100 degrees C is approximately twice bigger when the C-60 is the acceptor. We attribute this effect to the tendency of C-60 in form smaller aggregate domains relatively to the C-70. The increased roughness on the polymeric surface after annealing results in an enhanced donor/acceptor contact area and assists the fullerene diffusion deeper inside the polymeric layer. This effect leads to a better mixing between donor and acceptor species and create a interpenetrating layer close to the so-called bulk heterojunction. Since C-60 forms smaller aggregates, this mechanism is more pronounced for this molecule. Therefore, a significant variation in the performance of the C-60 devices is observed after this kind of treatment. Density Functional Theory calculations of the potential energy of interaction between two fullerene molecules and X-Ray measurements gives evidences to support this idea. In addition, combining spectrally resolved external quantum efficiency measurements with optical modeling our results also indicate the occurrence of the bilayer interfacial mixing for PSiF-DBT/C-60.Peer reviewe