InN thin film lattice dynamics by grazing incidence inelastic x-ray scattering

Achieving comprehensive information on thin film lattice dynamics so far has eluded well established spectroscopic techniques. We demonstrate here the novel application of grazing incidence inelastic x-ray scattering combined with ab initio calculations to determine the complete elastic stiffness tensor, the acoustic and low-energy optic phonon dispersion relations of thin wurtzite indium nitride films. Indium nitride is an especially relevant example, due to the technological interest for optoelectronic and solar cell applications in combination with other group III nitrides.     

Phonon dispersion curves in wurtzite-structure GaN determined by inelastic x-ray scattering

We have investigated the lattice dynamics of a wurtzite GaN single crystal by inelastic x-ray scattering. Several dispersion branches and phonons at high-symmetry points have been measured, including the two zone-center Raman- and infrared-inactive silent modes. The experiments have been complemented by ab initio calculations. They are in very good agreement with our measurements, not only for phonon energies, but also for scattering intensities, thus validating the correctness of the eigenvectors. Other phenomenological and ab initio theories exhibit significant differences.     

Elasticity of cobalt at high pressure studied by inelastic x-ray scattering

The five independent elastic moduli of single-crystalline hcp cobalt were determined by inelastic x-ray scattering to 39 GPa and compared to ultrasonic measurements and first principles calculations. In general the agreement is good, in particular, for the evolution of the longitudinal sound velocity in the a-c plane. This confirms the calculations, suggesting that a similar evolution is valid for hcp iron, the main constituent of the Earth's inner core, up to the highest investigated pressure. Our results represent an important benchmark to further refine ab initio calculations.     

Phonon eigenvectors in Si determined by inelastic neutron scattering

We have determined the eigenvectors of longitudinal phonons with wavevectors in the direction in Si at 12 K from inelastic neutron scattering intensities. The eigenvectors obtained from different model and quantum-mechanical calculations are at variance. Comparison of experimental and theoretical results shows that of the various theoretical predictions the ones from the bond-charge model are in best agreement with experiment. Internal-strain constants from these models and from experiment are compared.     

Formation of a new dynamical mode in -uranium observed by inelastic x-ray and neutron scattering

Phonon dispersion curves were obtained from inelastic x-ray and neutron scattering measurements on alpha-uranium single crystals at temperatures from 298 to 573 K. Both measurements showed a softening and an abrupt loss of intensity in the longitudinal optic branch along [00zeta] above 450 K. Above the same temperature a new dynamical mode of comparable intensity emerges along the [01zeta] zone boundary with energy near the top of the phonon spectrum. The new mode forms without a structural transition but coincides with an anomaly in the mechanical deformation behavior. We argue that the mode is an intrinsically localized vibration and formed as a result of a strong electron-phonon interaction.     

Suppression of coherent inelastic Mössbauer scattering of synchrotron radiation at grazing angles of incidence

Coherent inelastic Mössbauer scattering (CIMS) of synchrotron radiation (SR) by an isotopic interface (a flat interface between two regions of matter differing in the concentration of the Mössbauer isotope) at grazing angles of incidence of the SR beam is investigated theoretically. The qualitative features of CIMS as compared with conventional optics at small grazing angles of the SR beam are determined. Specifically, it is shown that for CIMS by an isotopic interface, instead of the phenomenon of total external (internal) reflection well known in optics, total suppression of reflection of the CIMS beam occurs at grazing angles of the SR beam less than a critical angle, and a maximum of the transmission and reflection at distinguished CIMS frequencies appears near the critical angle.     

On high-frequency scattering by a strip at nearly grazing incidence

The problem of diffraction by an absolutely soft segment is considered in the high-frequency approximation. The asymptotic field decomposition is obtained, which makes it possible to trace the transition from classical asymptotics valid for grazing incidence to geometrical optics asymptotics, which describes scattering at a finite (not small) angle.     

Off-diagonal response by means of inelastic x-ray scattering

Off-diagonal elements ?^-1 (q, q + g_h, ω) of the inverse dielectric matrix are shown to be measurable by means of inelastic scattering of X-ray photons from two different modes of the wave field, which is generated within the analyzer of a Laue-case X-ray interferometer at the interference position.     

Ferromagnetic in-plane spin fluctuations in NaxCoO2 observed by neutron inelastic scattering

We present neutron scattering spectra taken from a single crystal of Na0.75CoO2, the precursor to a novel cobalt-oxide superconductor. The data contain a prominent inelastic signal at low energies ( approximately 10 meV), which is localized in wave vector about the origin of two-dimensional reciprocal space. The signal is highly dispersive, and decreases in intensity with increasing temperature. We interpret these observations as direct evidence for the existence of ferromagnetic spin fluctuations within the cobalt-oxygen layers.     

Modelling of the electromagnetic scattering by sea surfaces at grazing incidence. Application to HF surface wave radars

The object of the study is to model the electromagnetic scattering by time-evolving sea surfaces illuminated by high frequency surface wave radars. A common way to simulate this case is to consider the scattering at grazing incidence. Recent studies have focused on so-called exact methods. As such methods are very time and memory consuming, it becomes important to implement techniques to reduce the calculation costs. Another important issue is to better understand the interactions between the electromagnetic waves and moving sea surfaces. We present in this article our simulator main characteristics and a comparison between simulated data and measurements.     

Modeling of neutral-atom scattering at the surface of a crystal for the case of grazing incidence

The features of the angular distributions of accelerated neutral atoms at grazing angles of incidence on the Al(001) surface are studied using the mathematical modeling method. The interaction of accelerated atoms with crystal-lattice atoms and the electronic properties and atomic structure of the Al(001) surface are calculated using the electron-density-functional method. The angular distributions of scattered atoms are modeled by taking into account their interaction with several atomic layers in the crystal lattice and atomic displacements during thermal oscillations. The influence of crystal surface-layer relaxation on rainbow scattering, i.e., the difference between the distances of planes on the surface and in the volume, is established. The possibilities of using the effect of rainbow scattering to study the structural features of a crystal surface are discussed.     

Resonant coherent excitation in atom-surface scattering at grazing incidence

Within a semiclassical approximation the excited-state density matrix is calculated for a resonant 1s→2p transition in a hydrogen-like projectile. The resonant coherent excitation process is induced by the periodic surface potential seen by the particle during its surface passage at grazing incidence. We find an anisotropic magnetic sublevel population which yields a high degree of polarization in a subsequent photon emission. The dependence of the Stokes parameters on both the relative orientation between the beam velocityv and the crystal axes and the surface potential decaying along the surface-normal are studied.