Multiple inelastic electron scattering processes in thin molecular films: a simple model

Electron energy loss spectra of multilayer molecular films contain bands resulting from inelastic collisions at two or more sites within the film. The contributions of twofold loss processes are analysed for selected vibrational spectra of benzene and SF₆ using a simple model that was suggested previously and is extended here. It is shown that this analysis can be used not only to detect dipole scattering contributions to particular vibrational excitations. Combined with an estimate of the twofold loss spectrum based on the pattern of the fundamental bands the model also enables the identification of overtone and combination bands which is important for the detection of resonant processes.     

Microscopic collective dynamics of atoms in the amorphous metallic alloy Ni33Zr67

The structural properties and microscopic collective dynamics of atoms in the amorphous metallic alloy Ni₃₃Zr₆₇ are studied using molecular dynamics simulations with a pair-additive model potential. The calculated equilibrium structural and dynamic characteristics are compared with experimental data on neutron diffraction and inelastic X-ray scattering. Theoretical analysis of the structural relaxation of microscopic density fluctuations for amorphous metallic alloys is performed within the Lee’s recurrent relation approach. The results of theoretical calculations for the intensity of scattering I(k, ω) for the amorphous metallic alloy Ni₃₃Zr₆₇ are in good agreement with the results of computer simulation and experimental inelastic X-ray scattering data. The low-frequency excitations observed in the longitudinal current spectra are related to the vibrational motions of individual atom clusters, which include Ni and Zr atoms.     

Large harmonic softening of the phonon density of states of uranium

Phonon density-of-states curves were obtained from inelastic neutron scattering spectra from the three crystalline phases of uranium at temperatures from 50 to 1213 K. The alpha-phase showed an unusually large thermal softening of phonon frequencies. Analysis of the vibrational power spectrum showed that this phonon softening originates with the softening of a harmonic solid, as opposed to vibrations in anharmonic potentials. It follows that thermal excitations of electronic states are more significant thermodynamically than are the classical volume effects. For the alpha-beta and beta-gamma phase transitions, vibrational and electronic entropies were comparable.     

A second-order vibrational model applied to the scattering of electrons and α-particles from 12C

Transition densities for monopole excitations and for virtual excitations are derived in the second-order vibrational model. Form factors for electron scattering calculated in the Born approximation and angular distributions for α-particle scattering, obtained in coupled channel calculations, are compared with experimental results for the elastic scattering and the inelastic scattering from the 2⁺ level at 4.44 MeV, the 0⁺ level at 7.65 MeV and the 3^? level at 9.64 MeV in ¹²C.     

Comparative analysis of characteristic electron energy loss spectra and inelastic scattering cross-section spectra of Fe

The inelastic electron scattering cross section spectra of Fe have been calculated based on experimental spectra of characteristic reflection electron energy loss as dependences of the product of the inelastic mean free path by the differential inelastic electron scattering cross section on the electron energy loss. It has been shown that the inelastic electron scattering cross-section spectra have certain advantages over the electron energy loss spectra in the analysis of the interaction of electrons with substance. The peaks of energy loss in the spectra of characteristic electron energy loss and inelastic electron scattering cross sections have been determined from the integral and differential spectra. It has been shown that the energy of the bulk plasmon is practically independent of the energy of primary electrons in the characteristic electron energy loss spectra and monotonically increases with increasing energy of primary electrons in the inelastic electron scattering cross-section spectra. The variation in the maximum energy of the inelastic electron scattering cross-section spectra is caused by the redistribution of intensities over the peaks of losses due to various excitations. The inelastic electron scattering cross-section spectra have been analyzed using the decomposition of the spectra into peaks of the energy loss. This method has been used for the quantitative estimation of the contributions from different energy loss processes to the inelastic electron scattering cross-section spectra of Fe and for the determination of the nature of the energy loss peaks.     

Spin excitations in few-electrons AlGaAs/GaAs quantum dots probed by inelastic light scattering

We present recent studies of electronic excitations in nanofabricated AlGaAs/GaAs semiconductor quantum dots (QDs) by resonant inelastic light scattering. The resonant light scattering spectra are dominated by excitations from parity-allowed inter-shell transitions between Fock–Darwin levels. In QDs with very few electrons the resonant spectra are characterized by distinct charge and spin excitations that reveal the strong impact of both exchange and correlation effects. A sharp inter-shell spin excitation of the triplet spin QD state with four electrons is identified.     

Dispersive crystal field excitations and quadrupolar interactions in UPd3

We report inelastic neutron scattering measurements and random phase approximation calculations of the dispersive crystal field excitations of UPd(3). The measured spectra at lower energies agree with those calculated using quadrupolar interaction parameters deduced from bulk and x-ray scattering measurements. The more intense excitations arising from the hexagonal sites were used to obtain exchange parameters which proved to be anisotropic.     

Line shape analysis of high energy X-ray induced Auger- and photoelectron spectra of thin Cu and Ni films

A detailed study of the line shape of high energy Cu X-rays excited Auger- and photoelectron spectra of layered samples is presented. The samples consisted of polycrystalline Cu and Ni overlayers of various thicknesses deposited on a Si substrate. The raw data show that the contribution caused by bulk inelastic scattering in the spectra increases significantly with overlayer thickness. Employing a general deconvolution procedure, the contribution of bulk inelastic scattering was consistently eliminated. The resulting spectra are in excellent agreement. The contribution caused by surface excitations has a very moderate effect at these high energies, while intrinsic losses were estimated to make up ∼30–50% of the total intensity. The true energy distribution at the source as seen by the spectrometer was established by elimination of the features due to surface excitations and the contribution from intrinsic excitations of the delocalized electronic states was determined.     

Inelastic light scattering by electron excitations with large wave vectors in a 2D magnetoplasma

Microscopic mechanisms of inelastic light scattering in an interacting electron plasma in semiconductor heterostructures are considered. In the dipole limit, the cross section consists of two main contributions: the first is related to a disorder-induced mechanism and the second arises from the Coulomb interaction. The spectra of disorder-induced light scattering are described in terms of correlation functions of a random potential. The spectrum induced by the Coulomb interaction arises from two-quasiparticle excitations. The mechanisms which are studied in this paper result in the appearance of large wave vector excitations in the spectra of resonant light scattering. These results can be used to model the experimentally observed appearance of the roton density of states in light scattering spectra in the integer quantum Hall regime of a two-dimensional system. Furthermore, we show that the lineshape of spectra strongly depends on the character of disorder and, in particular, on the spatial positions of impurities with respect to a quantum well. Zh. éksp. Teor. Fiz. 112, 1041–1054 (September 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Spectrum of vibrational frequencies of crystalline tungsten at temperatures of 293 and 2400 K

The slow neutron inelastic scattering spectra for a refractory (T _melt = 3680 K) Group VI transition metal of the Periodic Table, namely, tungsten, were measured for the first time in the range from room temperature to 2400 K. Measurements of the neutron scattering spectra of tungsten were performed on a DIN-2PI time-of-flight spectrometer installed at the IBR-2 reactor (Dubna, Russia). The sample was heated in a TS3000 K high-temperature thermostat. The spectrum of vibrational frequencies of the crystal lattice of tungsten at temperatures of 293 and 2400 K was determined from the measured neutron scattering spectra by the iterative method. A softening of the frequency spectrum of tungsten was observed with increasing temperature. This was explained by the increasing role of vibrational anharmonicity effects at high temperatures. The experimental results were compared with model calculations of the frequency spectrum of tungsten.     

Momentum dependence of charge excitations in the electron-doped superconductor Nd1.85 Ce0.15 CuO4: a resonant inelastic x-ray scattering study

We report a resonant inelastic x-ray scattering (RIXS) study of charge excitations in the electron-doped high-T(c) superconductor Nd1.85 Ce0.15 CuO4. The intraband and interband excitations across the Fermi energy are separated for the first time by tuning the experimental conditions properly to measure charge excitations at low energy. A dispersion relation with q-dependent width emerges clearly in the intraband excitation, while the intensity of the interband excitation is concentrated around 2 eV near the zone center. The experimental results are consistent with theoretical calculation of the RIXS spectra based on the Hubbard model.     

Slow neutron scattering and intermolecular excitations in liquid hydrogen

A simple analytical model for inelastic neutron scattering in liquid hydrogen with the effects of delayed diffusion and intermolecular vibrations with a single frequency parameter is proposed. Calculated results show good agreements with experimental results of total and differential scattering cross sections as well as the energy spectra of neutrons in liquid hydrogen moderators, and thus indicate the importance of the consideration of intermolecular excitations in the dynamics of liquid hydrogen.     

Simulation of the lattice dynamics of an Al-Cu-Fe icosahedral quasicrystal

The total and partial vibrational spectra of aluminum, copper, and iron atoms in an Al-Cu-Fe icosahedral quasicrystal are calculated by the recursive method. The calculations are based on the 1/1 crystal approximant. The interaction of atoms in the Al-Cu-Fe quasicrystal is described within the EAM model. The calculated spectra are in satisfactory agreement with the experimental data on neutron inelastic scattering.     

Low energy electronic excitations in the layered cuprates studied by copper L3 resonant inelastic x-ray scattering

We have measured the resonant inelastic x-ray scattering (RIXS) spectra at the Cu L3 edge in a variety of cuprates. Exploiting a considerably improved energy resolution (0.8 eV) we recorded significant dependencies on the sample composition and orientation, on the scattering geometry, and on the incident photon polarization. The RIXS final states correspond to two families of electronic excitations, having local (dd excitations) and nonlocal (charge-transfer) character. The dd energy splitting can be estimated with a simple crystal field model. The RIXS at the L3 edge demonstrates here a great potential, thanks to the resonance strength and to the large 2p spin-orbit splitting.     

Probing collective modes of correlated states of few electrons in semiconductor quantum dots

Low-lying collective excitations above highly correlated ground states of few interacting electrons confined in GaAs semiconductor quantum dots are probed by resonant inelastic light scattering. We highlight that separate studies of the changes in the spin and charge degrees of freedom offer unique access to the fundamental interactions. The case of quantum dots with four electrons is found to be determined by a competition between triplet and singlet ground states that is uncovered in the rich light scattering spectra of spin excitations. These light scattering results are described within a configuration-interaction framework that captures the role of electron correlation with quantitative accuracy. Recent light scattering results that reveal the impact of anisotropic confining potentials in laterally coupled quantum dots are also reviewed. In these studies, inelastic light scattering methods emerge as powerful probes of collective phenomena and spin configurations in quantum dots with few electrons.     

Neutron Scattering and Vibrational Spectra of Molecular Crystals

Infrared and Raman spectra of a number of molecular crystals have been measured for studying molecular vibrations and the intermolecular and intramolecular force fields. The infrared absorption bands arise from interaction of the electric wave with the oscillating dipole moment of the crystal. Raman scattering covers inelastic photon scattering processes and accordingly Raman lines arise from the oscillating polarizability of the crystal. Thus, the vibrational modes observed in infrared absorption or Raman scattering spectra are k = 0 modes, for which translationally equivalent molecules vibrate in phase.     

Density of vibrational states in amorphous solid media: Measurement by single-molecule spectroscopy

The energy spectrum of low-frequency vibrational modes in amorphous polyisobutylene doped with chromophore tetra-tert-butylterrylene molecules has been measured. The technique for measuring the spectra of vibrational modes in solid amorphous media by single-molecule spectroscopy is described. The measured spectrum is compared with the data on the density of vibrational states (bosonic peak) in pure polyisobutylene, which were obtained by inelastic neutron scattering. It is shown that incorporation of small amounts of tetra-tert-butylterrylene into polyisobutylene did not significantly change the low-temperature vibrational dynamics of the matrix.     

Production of pseudoscalar bosons upon stimulated Raman scattering in dielectric media

Conditions for the emergence of pseudoscalar vibrational excitations in the spectra of spontaneous and stimulated Raman scattering in molecular liquids and crystals are determined. The spectra of spontaneous and stimulated Raman scattering by pseudoscalar bosons are measured experimentally for several liquids and crystals.     

Theory of resonant inelastic X-ray scattering spectrum for Ni impurities in cuprates

We perform the numerically exact diagonalization calculation for small Cu-O clusters with a Ni impurity site, representing the Ni-substituted cuprate, to examine the single-particle excitation spectra as well as the resonant inelastic X-ray scattering (RIXS) spectra. We clarify relations between low-energy electronic structures near the Ni site and excitations seen in the RIXS spectra.     

Magnetic excitation spectra in BaFe2As2: a two-particle approach within a combination of the density functional theory and the dynamical mean-field theory method

We study the magnetic excitation spectra in the paramagnetic state of BaFe(2)As(2) from the ab initio perspective. Both the one-particle and the magnetic two-particle excitation spectra are determined within the combination of the density functional theory and the dynamical mean-field theory method. This method reproduces all the experimentally observed features in inelastic neutron scattering and relates them to both the one-particle excitations and the collective modes. The magnetic excitation dispersion is well accounted for by our theoretical calculation in the paramagnetic state without any broken symmetry; hence, nematic order is not needed to explain the inelastic neutron scattering experimental data.