Partial crystallinity in alkyl side chain polymers dictates surface freezing

We have studied the structure of a novel crystalline surface monolayer on top of a disordered melt of the same material [poly(n-alkyl acrylate)s] using grazing incidence x-ray diffraction. The grazing incidence x-ray diffraction, surface tension, and bulk latent heat results show that side chains crystallize except the nine methylene units of the alkyl side chains closest to the polymer backbone. The partial crystallinity along with a thicker surface layer, due to the additional length of the linker group, explains why the difference between the surface order-to-disorder transition temperature and bulk melting temperature increases with a decrease in the length of the alkyl side chain.     

Observation of heterodyne mixing in surface x-ray photon correlation spectroscopy experiments

We report measurements of propagating capillary waves on a liquid water surface at T=5 degrees C with x-ray photon correlation spectroscopy. The experiment has been performed under grazing incidence conditions with an incoming x-ray beam below the critical angle of total external reflection. In the q region investigated the measured intensity-intensity autocorrelation functions of the liquid water surface were found to be heterodyne signals, i.e., a combination of first- and second-order correlation functions g(1)(tau) and g(2)(tau).     

Kohn anomaly in MgB2 by inelastic X-ray scattering

We study phonons in MgB2 using inelastic x-ray scattering (1.6 and 6 meV resolution). We clearly observe the softening and broadening of the crucial E(2g) mode through the Kohn anomaly along GammaM, in excellent agreement with ab initio calculations. Low temperature measurements (just above and below T(c)) show negligible changes for the momentum transfers investigated and no change in the E(2g) mode at A between room temperature and 16 K. We report the presence of a longitudinal mode along GammaA near in energy to the E(2g) mode that is not predicted by theory.     

Grazing incidence scattering of vibrationally excited H2 molecules from metal surfaces

We have studied the scattering of vibrationally excited H₂ molecules from metal surfaces under fast grazing incidence conditions, by means of quasi-classical calculations based on six-dimensional potential energy surfaces. We show that, in spite of the fast parallel motion, the reorientation of the molecule along the trajectory plays a fundamental role on the scattering, being responsible for the nonmonotonic behavior observed as a function of the normal incidence energy, similar to that observed under slow normal incidence conditions. The present study has allowed us to further prove that the interaction between a H₂ molecule and an ordered metal surface under fast grazing incidence conditions is, in general, governed by the normal momentum of the molecule.     

Probing the local order of single phospholipid membranes using grazing incidence x-ray diffraction

We report the first grazing incidence x-ray diffraction measurements of a single phospholipid bilayer at the solid-liquid interface. Our grazing incidence x-ray diffraction and reflectivity measurements reveal that the lateral ordering in a supported DPPE (1, 2-Dipalmitoyl-sn-Glycero-3-Phosphoethanolamine) bilayer is significantly less than that of an equivalent monolayer at the air-liquid interface. Our findings also indicate that the leaflets of the bilayer are uncoupled in contrast to the scattering from free standing phosphatidylcholine bilayers. The methodology presented can be readily implemented to study more complicated biomembranes and their interaction with proteins.     

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.     

Measuring surface stress discontinuities in self-organized systems with X rays

We have performed a grazing incidence x-ray diffraction study of the self-organized N/Cu(001) system. Diffraction satellites associated with self-organization are particularly intense around Bragg conditions of the bulk crystal. Bulk elastic relaxations due to surface stress discontinuities at domain boundaries are responsible for this feature. A quantitative analysis shows that these relaxations, computed by molecular dynamics or continuum elasticity, explain very well the whole diffraction study. A difference in surface stress of 7 N m(-1) between uncovered and N-covered regions of the Cu surface is shown to be the driving force for self-organization.     

Lattice dynamics of molybdenum at high pressure

We have determined the lattice dynamics of molybdenum at high pressure to 37 GPa using high-resolution inelastic x-ray scattering. Over the investigated pressure range, we find a significant decrease in the H-point phonon anomaly. We also present calculations based on density functional theory that accurately predict this pressure dependence. Based on these results, we infer that the likely explanation for the H-point anomaly in molybdenum is strong electron-phonon coupling, which decreases upon compression due to the shift of the Fermi level with respect to the relevant electronic bands.     

Phonon dispersion and lifetimes in MgB2

We measure phonon dispersion and linewidth in a single crystal of MgB2 along the Gamma-A, Gamma-M, and A-L directions using inelastic x-ray scattering. We use density functional theory to compute the effect of both electron-phonon coupling and anharmonicity on the linewidth, obtaining excellent agreement with experiment. Anomalous broadening of the E(2g) phonon mode is found all along Gamma-A. The dominant contribution to the linewidth is always the electron-phonon coupling.     

Segregation in thin films

The longitudinal and transverse components of the image force acting on a moving charge near a metal surface are calculated. In particular, it is found that the transverse force is finite at the surface in contrast to the classical image force. The transverse force leads to deviations in trajectory in grazing incidence reflection inelastic electron scattering experiments, which are estimated. A consequence is the existence of a limiting maximum probability for excitation of surface plasmons.     

Total-reflection inelastic X-ray scattering from a 10-nm thick La0.6Sr0.4CoO3 thin film

To study equilibrium changes in composition, valence, and electronic structure near the surface and into the bulk, we demonstrate the use of a new approach, total-reflection inelastic x-ray scattering, as a sub-keV spectroscopy capable of depth profiling chemical changes in thin films with nanometer resolution. By comparing data acquired under total x-ray reflection and penetrating conditions, we are able to separate the O K-edge spectra from a 10 nm La0.6Sr0.4CoO3 thin film from that of the underlying SrTiO3 substrate. With a smaller wavelength probe than comparable soft x-ray absorption measurements, we also describe the ability to easily access dipole-forbidden final states, using the dramatic evolution of the La N4,5 edge with momentum transfer as an example.     

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.     

Brillouin and Umklapp scattering in polybutadiene: comparison of neutron and x-ray scattering

We report a comparison of high resolution inelastic x-ray Brillouin scattering to coherent inelastic neutron scattering for amorphous deuterated polybutadiene, done for one temperature in the glass phase and another one in the melt. The x-ray scattering proves to be by far the better technique for such a polymer within its present resolution bounds. The neutron scattering allows one to extend these measurements to a much better resolution, showing an additional quasielastic signal in the melt. The results suggest x-ray measurements at higher momentum transfer, to see whether they are complementary to neutrons.     

Probing simultaneously the volume and surface structure of nanospheres adsorbed at a solid-liquid interface by GISANS

We demonstrate in this paper the possibilities offered by Grazing Incidence Small Angle Neutron Scattering (GISANS) for the study of solid/liquid interfaces. We present experimental results obtained by Specular Neutron Reflectivity (SNR) and GISANS on a model system made of silica nanospheres adsorbed on a silicon wafer by electrostatic interactions both at solid/air interface and solid/liquid interfaces. At the solid/liquid interface, we demonstrate that grazing incidence scattering enables to discriminate the surface and the bulk scattering. The surface structure factor derived from GISANS shows that the nanospheres are organized as a repulsive liquid system, with a surface fraction occupation consistent with values obtained by SNR. This original setup highlights a direct correlation between the structure of the silica nanospheres in solution and their organization on the surface: due to the strong electrostatic repulsions between spheres, their organization at the surface is close to the projection in 2D of the 3D organization of the nanospheres in solution.     

High-frequency subsurface and bulk dynamics of liquid indium

We have performed bulk and surface-sensitive inelastic x-ray scattering experiments on liquid indium with 3 meV energy resolution. The experimental data are well reproduced within a generalized hydrodynamic model including structural and microscopic relaxation processes. We find a longitudinal viscosity of 22 mPa s in the near-surface region compared to 7.4 mPa s in the bulk. The origin of the increase is associated with a slowing down of the collective dynamics in a subsurface region of 4.6 nm.     

Water dynamics as affected by interaction with biomolecules and change of thermodynamic state: a neutron scattering study

The dynamics of water as subtly perturbed by both the interaction with biomolecules and the variation of temperature and pressure has been investigated via neutron scattering spectroscopy. A measurement of inelastic neutron scattering devoted to the study of the coherent THz dynamics of water in a water-rich mixture with DNA (hydration level of 1 g DNA/15 g D(2)O) at room temperature is reported. The DNA hydration water coherent dynamics is characterised by the presence of collective modes, whose dispersion relations are similar to those observed in bulk water. These dispersion relations are well described by the interaction model developed in the case of bulk water, and the existence of a fast sound is experimentally demonstrated. The behaviour of the collective water dynamics was complemented by studying the single-particle dynamics of bulk water along the isotherm T = 298 K in the pressure range 0.1-350 MPa by means of incoherent scattering. This experiment is an attempt to simulate the change of the water molecular arrangement due to the interaction with DNA, by increasing the pressure as the presence of the biomolecule produces an increase in the density. An anomaly is found in the behaviour of the relaxation time derived from the quasi-elastic scattering signal, which can be related to the hypothetical second critical point in water. This anomaly and the transition from slow to fast sound take place in the same Q range, thus suggesting that the two phenomena could be related at some microscopic level.     

4f delocalization in Gd: inelastic x-ray scattering at ultrahigh pressure

We present resonant inelastic x-ray scattering and x-ray emission spectroscopy results on Gd metal to 113 GPa which suggest Kondo-like aspects in the delocalization of 4f electrons. Analysis of the resonant inelastic x-ray scattering data reveals a prolonged and continuous delocalization with volume throughout the entire pressure range, so that the volume-collapse transition at 59 GPa is only part of the phenomenon. Moreover, the Lgamma1 x-ray emission spectroscopy spectra indicate no apparent change in the bare 4f moment across the collapse, suggesting that Kondo screening is responsible for the expected Pauli-like behavior in magnetic susceptibility.     

Probing the transition in bulk Ce under pressure: a direct investigation by resonant inelastic X-ray scattering

We report on the most complete investigation to date of the -electron properties at the transition in elemental Ce by resonant inelastic x-ray scattering (RIXS). The Ce 2p3d-RIXS spectra were measured directly in the bulk material as a function of pressure through the transition. The spectra were simulated within the Anderson impurity model. The occupation number n(f) and f double occupancy were derived from the calculations in both gamma and alpha phases in the ground state. We find that the electronic structure changes result mainly from band formation of 4f electrons which concurs with reduced electron correlation and increased Kondo screening at high pressure.     

Momentum-resolved charge excitations in a prototype one-dimensional mott insulator

We report momentum-resolved charge excitations in a one-dimensional (1D) Mott insulator studied using high resolution inelastic x-ray scattering over the entire Brillouin zone for the first time. Excitations at the insulating gap edge are found to be highly dispersive (momentum dependent) compared to excitations observed in two-dimensional Mott insulators. The observed dispersion in 1D cuprates ( SrCuO2 and Sr2CuO3) is consistent with charge excitations involving holons which is unique to spin-1/2 quantum chain systems. These results point to the potential utility of momentum-resolved inelastic x-ray scattering in providing valuable information about electronic structure of strongly correlated insulators.     

Electron spectroscopy of iron disilicide

We have reported on the results of a complex investigation of iron disilicide FeSi₂ using characteristic electron energy loss spectroscopy, inelastic electron scattering cross section spectroscopy, and X-ray photoelectron spectroscopy. It has been shown that the main peak in the spectra of inelastic electron scattering for FeSi₂ is a superposition of two unresolved peaks, viz., surface and bulk plasmons. An analysis of the fine structure of the spectra of inelastic electron scattering cross section by their decomposition into Lorentzlike Tougaard peaks has made it possible to quantitatively estimate the contributions of individual energy loss processes to the resulting spectrum and determine their origin and energy.