Phonon Dispersion and Proton Disorder of Ice VII and VIII

The phonon dispersion of ice VII and that of its proton-ordered analog ice VIII are investigated through a combination of inelastic x-ray scattering (IXS) measurements and first-principles calculations of the oxygen sublattice dynamic structure factor. Particular attention is devoted to hydrogen-disorder in ice VII, addressed theoretically through a statistical ensemble of fictitious ordered supercell configurations. Similar phonon densities of states are found in both phases but are significantly less structured in the case of ice VII. Our data further show that, despite a full proton disorder, the acoustic phonon branches in this phase clearly inherit the periodicity of its body-centered cubic oxygen lattice. The calculations predict, however, the presence of gap openings in the one-atom phonon dispersion. These predictions are supported by revisiting the analysis of previous single-crystal IXS measurements along the longitudinal [111] branch of ice VII.

Study of the X-ray radiation interaction with a multislit collimator for the creation of microbeams in radiation therapy

Microbeam radiation therapy (MRT) is a developing radiotherapy, based on the use of beams only a few tens of micrometres wide, generated by synchrotron X-ray sources. The spatial fractionation of the homogeneous beam into an array of microbeams is possible using a multislit collimator (MSC), i.e. a machined metal block with regular apertures. Dosimetry in MRT is challenging and previous works still show differences between calculated and experimental dose profiles of 10-30%, which are not acceptable for a clinical implementation of treatment. The interaction of the X-rays with the MSC may contribute to the observed discrepancies; the present study therefore investigates the dose contribution due to radiation interaction with the MSC inner walls and radiation leakage of the MSC. Dose distributions inside a water-equivalent phantom were evaluated for different field sizes and three typical spectra used for MRT studies at the European Synchrotron Biomedical beamline ID17. Film dosimetry was utilized to determine the contribution of radiation interaction with the MSC inner walls; Monte Carlo simulations were implemented to calculate the radiation leakage contribution. Both factors turned out to be relevant for the dose deposition, especially for small fields. Photons interacting with the MSC walls may bring up to 16% more dose in the valley regions, between the microbeams. Depending on the chosen spectrum, the radiation leakage close to the phantom surface can contribute up to 50% of the valley dose for a 5 mm × 5 mm field. The current study underlines that a detailed characterization of the MSC must be performed systematically and accurate MRT dosimetry protocols must include the contribution of radiation leakage and radiation interaction with the MSC in order to avoid significant errors in the dose evaluation at the micrometric scale.

High resolution radiochromic film dosimetry: Comparison of a microdensitometer and an optical microscope

PURPOSE

Microbeam radiation therapy is a developing technique that promises superior tumour control and better normal tissue tolerance using spatially fractionated X-ray beams only tens of micrometres wide. Radiochromic film dosimetry at micrometric scale was performed using a microdensitometer, but this instrument presents limitations in accuracy and precision, therefore the use of a microscope is suggested as alternative. The detailed procedures developed to use the two devices are reported allowing a comparison.

METHODS

Films were irradiated with single microbeams and with arrays of 50 µm wide microbeams spaced by a 400 µm pitch, using a polychromatic beam with mean energy of 100 keV. The film dose measurements were performed using two independent instruments: a microdensitometer (MDM) and an optical microscope (OM).

RESULTS

The mean values of the absolute dose measured with the two instruments differ by less than 5% but the OM provides reproducibility with a standard deviation of 1.2% compared to up to 7% for the MDM. The resolution of the OM was determined to be ~ 1 to 2 µm in both planar directions able to resolve pencil beams irradiation, while the MDM reaches at the best 20 µm resolution along scanning direction. The uncertainties related to the data acquisition are 2.5-3% for the OM and 9-15% for the MDM.

CONCLUSION

The comparison between the two devices validates that the OM provides equivalent results to the MDM with better precision, reproducibility and resolution. In addition, the possibility to study dose distributions in two-dimensions over wider areas definitely sanctions the OM as substitute of the MDM.

A high-energy-resolution resonant inelastic X-ray scattering spectrometer at ID20 of the European Synchrotron Radiation Facility

An end-station for resonant inelastic X-ray scattering and (resonant) X-ray emission spectroscopy at beamline ID20 of ESRF - The European Synchrotron is presented. The spectrometer hosts five crystal analysers in Rowland geometry for large solid angle collection and is mounted on a rotatable arm for scattering in both the horizontal and vertical planes. The spectrometer is optimized for high-energy-resolution applications, including partial fluorescence yield or high-energy-resolution fluorescence detected X-ray absorption spectroscopy and the study of elementary electronic excitations in solids. In addition, it can be used for non-resonant inelastic X-ray scattering measurements of valence electron excitations.

Erratum: Lattice Dynamics of EuO: Evidence for Giant Spin-Phonon Coupling [Phys. Rev. Lett. 116, 185501 (2016)]

This corrects the article DOI: 10.1103/PhysRevLett.116.185501.

A large-solid-angle X-ray Raman scattering spectrometer at ID20 of the European Synchrotron Radiation Facility

An end-station for X-ray Raman scattering spectroscopy at beamline ID20 of the European Synchrotron Radiation Facility is described. This end-station is dedicated to the study of shallow core electronic excitations using non-resonant inelastic X-ray scattering. The spectrometer has 72 spherically bent analyzer crystals arranged in six modular groups of 12 analyzer crystals each for a combined maximum flexibility and large solid angle of detection. Each of the six analyzer modules houses one pixelated area detector allowing for X-ray Raman scattering based imaging and efficient separation of the desired signal from the sample and spurious scattering from the often used complicated sample environments. This new end-station provides an unprecedented instrument for X-ray Raman scattering, which is a spectroscopic tool of great interest for the study of low-energy X-ray absorption spectra in materials under in situ conditions, such as in operando batteries and fuel cells, in situ catalytic reactions, and extreme pressure and temperature conditions.

Critical scattering and incommensurate phase transition in antiferroelectric PbZrO3 under pressure

Antiferroelectric lead zirconate is the key ingredient in modern ferroelectric and piezoelectric functional solid solutions. By itself it offers opportunities in new-type non-volatile memory and energy storage applications. A highly useful and scientifically puzzling feature of this material is the competition between the ferro- and antiferroelectric phases due to their energetic proximity, which leads to a challenge in understanding of the critical phenomena driving the formation of the antiferroelectric structure. We show that application of hydrostatic pressure drastically changes the character of critical lattice dynamics and enables the soft-mode-driven incommensurate phase transition sequence in lead zirconate. In addition to the long known cubic and antiferroelectric phases we identify the new non-modulated phase serving as a bridge between the cubic and the incommensurate phases. The pressure effect on ferroelectric and incommensurate critical dynamics shows that lead zirconate is not a single-instability-driven system.

Persistent Paramagnons Deep in the Metallic Phase of Sr_{2-x}La_{x}IrO_{4}

We have studied the magnetic excitations of electron-doped Sr_{2-x}La_{x}IrO_{4} (0≤x≤0.10) using resonant inelastic x-ray scattering at the Ir L_{3} edge. The long-range magnetic order is rapidly lost with increasing x, but two-dimensional short-range order (SRO) and dispersive magnon excitations with nearly undiminished spectral weight persist well into the metallic part of the phase diagram. The magnons in the SRO phase are heavily damped and exhibit anisotropic softening. Their dispersions are well described by a pseudospin-1/2 Heisenberg model with exchange interactions whose spatial range increases with doping. We also find a doping-independent high-energy magnetic continuum, which is not described by this model. The spin-orbit excitons arising from the pseudospin-3/2 manifold of the Ir ions broaden substantially in the SRO phase, but remain largely separated from the low-energy magnons. Pseudospin-1/2 models are therefore a good starting point for the theoretical description of the low-energy magnetic dynamics of doped iridates.

All-in-all-Out Magnetic Order and Propagating Spin Waves in Sm_{2}Ir_{2}O_{7}

Using resonant magnetic x-ray scattering we address the unresolved nature of the magnetic ground state and the low-energy effective Hamiltonian of Sm_{2}Ir_{2}O_{7}, a prototypical pyrochlore iridate with a finite temperature metal-insulator transition. Through a combination of elastic and inelastic measurements, we show that the magnetic ground state is an all-in-all-out (AIAO) antiferromagnet. The magnon dispersion indicates significant electronic correlations and can be well described by a minimal Hamiltonian that includes Heisenberg exchange [J=27.3(6)  meV] and Dzyaloshinskii-Moriya interactions [D=4.9(3)  meV], which provides a consistent description of the magnetic order and excitations. In establishing that Sm_{2}Ir_{2}O_{7} has the requisite inversion symmetry preserving AIAO magnetic ground state, our results support the notion that pyrochlore iridates may host correlated Weyl semimetals.

Lattice Dynamics of EuO: Evidence for Giant Spin-Phonon Coupling

Comprehensive studies of lattice dynamics in the ferromagnetic semiconductor EuO have been performed by a combination of inelastic x-ray scattering, nuclear inelastic scattering, and ab initio calculations. A remarkably large broadening of the transverse acoustic phonons was discovered at temperatures above and below the Curie temperature T_{C}=69  K. This result indicates a surprisingly strong momentum-dependent spin-phonon coupling induced by the spin dynamics in EuO.

A miniature closed-circle flow cell for high photon flux X-ray scattering experiments

A closed-circle miniature flow cell for high X-ray photon flux experiments on radiation-sensitive liquid samples is presented. The compact cell is made from highly inert material and the flow is induced by a rotating magnetic stir bar, which acts as a centrifugal pump inside the cell. The cell is ideal for radiation-sensitive yet precious or hazardous liquid samples, such as concentrated acids or bases. As a demonstration of the cell's capabilities, X-ray Raman scattering spectroscopy data on the oxygen K-edge of liquid water under ambient conditions are presented.

Lattice dynamics in the paraelectric phase of PbHfO₃ studied by inelastic x-ray scattering

We report the results of an inelastic x-ray scattering study of the lattice dynamics in the paraelectric phase of the antiferroelectric lead hafnate PbHfO3. The study reveals an avoided crossing between the transverse acoustic and transverse optic phonon modes propagating along the [1 1 0] direction with [1  -1 0] polarization. The static susceptibility with respect to the generally incommensurate modulations is shown to increase on cooling for the entire Γ-M direction. We consider different approaches to the data analysis that correspond to different models for the temperature evolution of the dynamic susceptibility function. A number of similarities and differences between the lattice dynamics of PbHfO3 and PbZrO3 are described.

Planning, performing and analyzing X-ray Raman scattering experiments

A compilation of procedures for planning and performing X-ray Raman scattering (XRS) experiments and analyzing data obtained from them is presented. In particular, it is demonstrated how to predict the overall shape of the spectra, estimate detection limits for dilute samples, and how to normalize the recorded spectra to absolute units. In addition, methods for processing data from multiple-crystal XRS spectrometers with imaging capability are presented, including a super-resolution method that can be used for direct tomography using XRS spectra as the contrast. An open-source software package with these procedures implemented is also made available.

CaIrO3: a spin-orbit Mott insulator beyond the j(eff) ground state

In CaIrO3, electronic correlation, spin-orbit coupling, and tetragonal crystal field splitting are predicted to be of comparable strength. However, the nature of its ground state is still an object of debate, with contradictory experimental and theoretical results. We probe the ground state of CaIrO3 and assess the effective tetragonal crystal field splitting and spin-orbit coupling at play in this system by means of resonant inelastic x-ray scattering. We conclude that insulating CaIrO3 is not a j(eff) = 1/2 iridate and discuss the consequences of our finding to the interpretation of previous experiments. In particular, we clarify how the Mott insulating state in iridates can be readily extended beyond the j(eff) = 1/2 ground state.

Role of disorder in the thermodynamics and atomic dynamics of glasses

We measured the density of vibrational states (DOS) and the specific heat of various glassy and crystalline polymorphs of SiO2. The typical (ambient) glass shows a well-known excess of specific heat relative to the typical crystal (α-quartz). This, however, holds when comparing a lower-density glass to a higher-density crystal. For glassy and crystalline polymorphs with matched densities, the DOS of the glass appears as the smoothed counterpart of the DOS of the corresponding crystal; it reveals the same number of the excess states relative to the Debye model, the same number of all states in the low-energy region, and it provides the same specific heat. This shows that glasses have higher specific heat than crystals not due to disorder, but because the typical glass has lower density than the typical crystal.

Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate

The need for both high electrical conductivity and low thermal conductivity creates a design conflict for thermoelectric systems, leading to the consideration of materials with complicated crystal structures. Rattling of ions in cages results in low thermal conductivity, but understanding the mechanism through studies of the phonon dispersion using momentum-resolved spectroscopy is made difficult by the complexity of the unit cells. We have performed inelastic X-ray and neutron scattering experiments that are in remarkable agreement with our first-principles density-functional calculations of the phonon dispersion for thermoelectric Na(0.8)CoO2, which has a large-period superstructure. We have directly observed an Einstein-like rattling mode at low energy, involving large anharmonic displacements of the sodium ions inside multi-vacancy clusters. These rattling modes suppress the thermal conductivity by a factor of six compared with vacancy-free NaCoO2. Our results will guide the design of the next generation of materials for applications in solid-state refrigerators and power recovery.

Dynamics and Thermodynamics beyond the critical point

Sudden changes in the dynamical properties of a supercritical fluid model have been found as a function of pressure and temperature (T/T(c) = 2-5 and P/P(c) = 10-10(3)), striving with the notion of a single phase beyond the critical point established by thermodynamics. The sound propagation in the Terahertz frequency region reveals a sharp dynamic crossover between the gas like and the liquid like regimes along several isotherms, which involves, at sufficiently low densities, the interplay between purely acoustic waves and heat waves. Such a crossover allows one to determine a dynamic line in the phase diagram which exhibits a very tight correlation with a number of thermodynamic observables, showing that the supercritical state is remarkably more complex than thought so far.

Lattice dynamics and superconductivity in cerium at high pressure

We have measured phonon dispersion relations of the high-pressure phase cerium-oC4 (α' phase with the α-uranium crystal structure) at 6.5 GPa by using inelastic x-ray scattering. Pronounced phonon anomalies are observed, which are remarkably similar to those of α-U. First-principles electronic structure calculations reproduce the anomalies and allow us to identify strong electron-phonon coupling as their origin. At the low-pressure end of its stability range, Ce-oC4 is on the verge of a lattice-dynamical instability and possibly a charge density wave. The superconducting transition temperatures of the fcc, oC4, and mC4 phases of Ce have been calculated, and the superconductivity observed experimentally by Wittig and Probst is attributed to the oC4 phase.

Understanding the complex phase diagram of uranium: the role of electron-phonon coupling

We report an experimental determination of the dispersion of the soft phonon mode along [100] in uranium as a function of pressure. The energies of these phonons increase rapidly, with conventional behavior found by 20 GPa, as predicted by recent theory. New calculations demonstrate the strong pressure (and momentum) dependence of the electron-phonon coupling, whereas the Fermi-surface nesting is surprisingly independent of pressure. This allows a full understanding of the complex phase diagram of uranium and the interplay between the charge-density wave and superconductivity.

Plasmons in sodium under pressure: increasing departure from nearly free-electron behavior

We have measured plasmon energies in Na under high pressure up to 43 GPa using inelastic x-ray scattering (IXS). The momentum-resolved results show clear deviations, growing with increasing pressure, from the predictions for a nearly free-electron metal. Plasmon energy calculations based on first-principles electronic band structures and a quasiclassical plasmon model allow us to identify a pressure-induced increase in the electron-ion interaction and associated changes in the electronic band structure as the origin of these deviations, rather than effects of exchange and correlation. Additional IXS results obtained for K and Rb are addressed briefly.

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.

Influence of deuteration on lithium acetate dihydrate studied by inelastic X-ray scattering, density functional theory, thermal expansion, elastic and thermodynamic measurements

The influence of deuteration on the properties of lithium acetate dihydrate has been investigated by thermal expansion measurements, ultrasound spectroscopy and calorimetry. Inelastic X-ray scattering has been employed to investigate if the low temperature structural phase transition can be detected by a change in the vibrational spectrum. Density functional theory, DFT, calculations have been employed to complement the experimental investigations. The thermal expansion coefficients and the specific heat of the deuterated compound differ significantly from the protonated form. The differences in the elastic stiffness coefficients are just above the detection limit of the technique employed here. Temperature dependent inelastic X-ray spectroscopic measurements show no significant change of the vibrational spectrum when crossing the transition temperature. The DFT calculations show that the methyl group dynamics are best described in the framework of coupled rotators of opposing methyl groups. One of the coupled rotational modes corresponds to a hindered rotator with a barrier of 15 meV, while the other is a free rotator.

Symmetry of platelet defects in diamond: new insights with synchrotron light

Mapping of reciprocal space for Ia-type diamond single crystals with synchrotron radiation has uncovered a variety of diffuse scattering features, some of them have not been observed before. The main component of diffuse scattering in the form of diffuse rods corresponds to a set of platelets which join together blocks of diamond structure. The platelets are ordered structural entities with lattice periodicity 8(1/2)a(0) x (1/2)(1/2)a(0) x 0.55a(0), where a(0) is a unit-cell dimension of diamond. Intensity distribution along the rods has been measured and used for recognition of symmetry elements of the platelet structure. These findings, together with previously reported transmission electron microscopy (TEM) observations, provide strong constraints for atomistic modelling of the platelet structure.

3D imaging of the Fermi surface by thermal diffuse scattering

We use thermal diffuse scattering of x rays to visualize the lens-shaped portions of the Fermi surface in metallic zinc. Our interpretation of the nature of the observed scattered intensity anomalies is supported by the incorporation of inelastic x-ray scattering measurements as well as ab initio calculations of the electronic structure and lattice dynamics. Our work demonstrates that thermal diffuse scattering complements well-established techniques and is a powerful tool in its own right for studying the shape of the Fermi surface through the associated electron-phonon coupling.

High frequency dynamics in liquids and supercritical fluids: A comparative inelastic x-ray scattering study

The microscopic dynamics of four prototype systems (water, ammonia, nitrogen, and neon) across the critical temperature has been investigated by means of high-resolution inelastic x-ray scattering. The experimental line shape has been described using a model based on the memory function formalism. Two main relaxations, the thermal and the structural one, were observed in all the investigated systems. We found that the microscopic mechanism driving the structural relaxation clearly changes, being mainly governed by intermolecular bond rearrangements below the critical temperature and by binary collisions above it. Moreover, we observed that the relative weight of the thermal relaxation systematically increases on approaching the critical temperature, thus allowing for the observation of a transition from an adiabatic to an isothermal regime of sound propagation. Finally, we found the presence of an additional instantaneous relaxation, likely related to the coupling between collective vibrational modes and intramolecular degrees of freedom.

Bond stretching phonon softening and kinks in the angle-resolved photoemission spectra of optimally doped Bi2Sr1.6La0.4Cu2O6+delta superconductors

We report the first measurement of the Cu-O bond stretching phonon dispersion in optimally doped Bi2Sr1.6La0.4Cu2O6+delta using inelastic x-ray scattering. We found a softening of this phonon at q=( approximately 0.25,0,0) from 76 to 60 meV, similar to the one reported in other cuprates. A comparison with angle-resolved photoemission data on the same sample revealed an excellent agreement in terms of energy and momentum between the angle-resolved photoemission nodal kink and the soft part of the bond stretching phonon. Indeed, we find that the momentum space where a 63+/-5 meV kink is observed can be connected with a vector q=(xi,0,0) with xi > or =0.22, corresponding exactly to the soft part of the bond stretching phonon.

Lattice dynamics of incommensurate composite Rb-IV and a realization of the monatomic linear chain model

Longitudinal-acoustic (LA) phonons have been studied by inelastic x-ray scattering in the high-pressure incommensurate host-guest system Rb-IV in the pressure range of 16.3 to 18.4 GPa. Two LA-like phonon branches are observed along the direction of the incommensurate wave vector, which are attributed to separate lattice vibrations in the host and guest subsystems. The derived sound velocities for the host and the guest, v(h) and v(g), respectively, are similar in magnitude [v(h)=v(g)=3840(100) m/s at 18 GPa], but our results indicate rather different pressure dependences of dv(h)/dP=140(60) m/s GPa(-1) and dv(g)/dP=280(80) m/s GPa(-1). The observations for the one-dimensional Rb guest chains are reproduced quantitatively on the basis of the monatomic linear chain model and the measured compressibility of the chains.

High-frequency dynamics of liquid and supercritical water

The dynamic structure factor S(Q,omega) of water has been determined by high-resolution inelastic x-ray scattering (IXS) in a momentum (Q) and energy (E) transfer range extending from 2 to 4 nm{-1} and from +/-40meV . IXS spectra have been recorded along an isobaric path (400bar) in a temperature (T) interval ranging from ambient up to supercritical (T>647K) conditions. The experimental data have been described in the frame of the generalized hydrodynamic theory, utilizing a model based on the memory function approach. This model allows identifying the active relaxation processes which affect the time decay of density fluctuations, as well as a direct determination of the Q , T , and density (rho) dependencies of the involved transport parameters. The experimental spectra are well described by considering three different relaxation processes: the thermal, the structural, and the instantaneous one. On approaching supercritical conditions, we observe that the microscopic mechanism responsible for the structural relaxation is no longer related to the making and breaking of intermolecular bonds, but to binary intermolecular collisions.

Vibrational properties of hexagonal boron nitride: inelastic X-ray scattering and ab initio calculations

The phonon dispersion relations of bulk hexagonal boron nitride have been determined from inelastic x-ray scattering measurements and analyzed by ab initio calculations. Experimental data and calculations show an outstanding agreement and reconcile the controversies raised by recent experimental data obtained by electron-energy loss spectroscopy and second-order Raman scattering.

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.

Structural and collisional relaxations in liquids and supercritical fluids

The dynamic structure factor S(Q,omega) of both associated (water and ammonia) and simple fluids (nitrogen and neon) has been determined by high-resolution inelastic x-ray scattering in the 2-14 nm(-1) momentum transfer range. A line-shape analysis with a generalized hydrodynamic model was used to study the involved relaxation process and to characterize its strength and time scale. We observe that in the liquid phase such a process is governed by rearrangements of intermolecular bonds, whereas in the supercritical region it assumes a collisional nature.

Liquidlike behavior of supercritical fluids

The high frequency dynamics of fluid oxygen has been investigated by inelastic x-ray scattering, at high pressures and room temperature. In spite of the markedly supercritical conditions (T approximately 2Tc, P>10(2)Pc), the sound velocity exceeds the hydrodynamic value of about 20%, a feature which is the fingerprint of liquidlike dynamics. The comparison of the present results with literature data obtained in several fluids allow us to identify the extrapolation of the liquid-vapor-coexistence line in the (P/Pc, T/Tc) plane as the relevant edge between liquidlike and gaslike dynamics. More interestingly, this extrapolation is very close to the non-metal-metal transition in hot dense fluids, at pressure and temperature values as obtained by shock wave experiments. This result points to the existence of a connection between structural modifications and transport properties in dense fluids.

Probing the coulomb interaction of the unconventional superconductor by phonon spectroscopy

The phonon dispersion curves of the superconductor PuCoGa(5) were studied by inelastic x-ray scattering at room temperature. The experimental data agree well with ab initio lattice dynamics calculations. An accurate description of the phonon spectrum is obtained only when a local Coulomb repulsion U approximately equal 3 eV among 5f electrons is taken into account.

Phonon dispersion of oriented DNA by inelastic x-ray scattering

Films of oriented deoxyribonucleic acid (DNA), prepared by the wet spinning method, have been studied using inelastic x-ray scattering. Spectra were recorded within the range of energy transfers -30< homega <30 meV at momentum transfers homegaQ ranging from 2.5 to 30 nm(-1) whereby the direction of Q essentially coincided with the helical axis. Measurements at ambient temperature cover samples in the A, B, C, and D conformations of DNA. Within the limits of the instrumental resolution, the spectra were analyzed by the response of a damped harmonic oscillator delivering dispersion and damping of modes having displacements with nonzero projections onto Q, i.e., essentially the compression waves traveling along the helical axis. The longitudinal speed of sound resulting from the sinusoidal dispersion varies only weakly with conformation. Our sound speed values are compared to results from Brillouin spectroscopy. The dispersion curves exhibit a minimum at about the inverse rise per residue, which -- together with strong elastic scattering -- reflect the large degree of disorder. Overdamping of the modes is observed for Q>5 nm(-1). The possibility that the observed large damping parameters are due to several contributing modes is discussed in terms of a simple model calculation for an idealized double helix. Whereas the quasicrystalline approximation for an effective disordered chain could well describe the sinusoidal dispersion, it fails to reproduce the observed damping by one order of magnitude. Our results indicate that the high-frequency dynamics of DNA is liquidlike and is most appropriately described by instantaneous normal modes of short correlation length.

Phonon modes at the 2H-NbSe2 surface observed by grazing incidence inelastic x-ray scattering

We have determined the dispersion of acoustic and optical surface phonon modes 2H-NbSe2 at the by inelastic x-ray scattering under grazing incidence conditions. Already, at room temperature, an anomaly is observed close to the charge density wave -vector position located at about one-third along the Gamma-M direction of the Brillouin zone. Our results indicate that the anomaly for the surface mode occurs at a lower energy than that measured in bulk sensitive geometry in the same experiment, showing evidence of a modified behavior in the uppermost layers. We demonstrate that inelastic x-ray scattering in grazing incidence conditions provides a unique tool to selectively study either surface or bulk lattice dynamics in a single experiment.

High-frequency longitudinal and transverse dynamics in water

High-resolution, inelastic x-ray scattering measurements of the dynamic structure factor S (Q,omega) of liquid water have been performed for wave vectors Q between 4 and 30 nm(-1) in distinctly different thermodynamic conditions ( T=263-420 K ; at, or close to, ambient pressure and at P=2 kbar ). In agreement with previous inelastic x-ray and neutron studies, the presence of two inelastic contributions (one dispersing with Q and the other almost nondispersive) is confirmed. The study of their temperature and Q dependence provides strong support for a dynamics of liquid water controlled by the structural relaxation process. A viscoelastic analysis of the Q -dispersing mode, associated with the longitudinal dynamics, reveals that the sound velocity undergoes a complete transition from the adiabatic sound velocity ( c(0) ) (viscous limit) to the infinite-frequency sound velocity ( c(infinity) ) (elastic limit). On decreasing Q , as the transition regime is approached from the elastic side, we observe a decrease of the intensity of the second, weakly dispersing feature, which completely disappears when the viscous regime is reached. These findings unambiguously identify the second excitation to be a signature of the transverse dynamics with a longitudinal symmetry component, which becomes visible in S (Q,omega) as soon as the purely viscous regime is left.

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.

High-frequency acoustic modes in liquid gallium at the melting point

The microscopic dynamics in liquid gallium at melting has been studied by inelastic x-ray scattering. We demonstrate the existence of acousticlike modes up to wave vectors above one-half of the first maximum of the static structure factor, at variance with earlier results from inelastic neutron scattering [F. J. Bermejo et al., Phys. Rev. E 49, 3133 (1994)]. Despite structural (extremely rich polymorphism) and electronic (mixed valence) peculiarities, the collective dynamics is strikingly similar to the one of van der Waals and metallic fluids. This result speaks in favor of the universality of the short time dynamics in monatomic liquids rather than of system-specific dynamics.