Fingerprints of hydrogen bonding in the terahertz dynamics of ethanol and water: An inelastic x-ray scattering study.

We used inelastic x-ray scattering methods to measure the terahertz spectrum of density fluctuations of ethanol in both liquid and solid phases. The results of a Bayesian inference-based lineshape analysis with a multiple excitation model and the comparison with a previous similar analysis on water indicate that the different structures induced by hydrogen bonds in ethanol and water have a profound influence on the respective dynamic responses, the latter being characterized by longer living and better resolved high-frequency acoustic excitations. In addition, we compare these findings with those obtained with an alternative approach based on the exponential expansion theory and ensuring sum rules fulfillment, demonstrating that the model's choice directly impacts the number of spectral modes detected.

Ice phonon spectra and Bayes inference: A gateway to a new understanding of terahertz sound propagation in water.

Understanding how molecules engage in collective motions in a liquid where a network of bonds exists has both fundamental and applied relevance. On the one hand, it can elucidate the "ordering" role of long-range correlations and inspire new avenues to control such order to implement sound manipulation. Water represents an ideal investigation case to unfold these general aspects, and, across the decades, it has been the focus of thorough scrutiny. Despite this investigative effort, the spectrum of terahertz density fluctuations of water largely remains a puzzle for condensed matter physicists. To unravel it, we compare previous scattering measurements of water spectra with new ones on ice. Owing to the unique asset of Bayesian inference, we draw a more detailed portrayal of the phonon response of ice. The comparison with the one of liquid water challenges the current understanding of density fluctuations in water, or more in general, of any networked liquid.

Nematic Correlation Length in Iron-Based Superconductors Probed by Inelastic X-Ray Scattering.

Nematicity is ubiquitous in electronic phases of high-T_{c} superconductors, particularly in the Fe-based systems. We used inelastic x-ray scattering to extract the temperature-dependent nematic correlation length ξ from the anomalous softening of acoustic phonon modes in FeSe, underdoped Ba(Fe_{0.97}Co_{0.03})_{2}As_{2}, and optimally doped Ba(Fe_{0.94}Co_{0.06})_{2}As_{2}. In all cases, we find that ξ is well described by a power law (T-T_{0})^{-1/2} extending over a wide temperature range. Combined with the previously reported Curie-Weiss behavior of the nematic susceptibility, these results point to the mean-field character of the nematic transition, which we attribute to a sizable nematoelastic coupling that is likely detrimental to superconductivity.

Intrinsic anharmonic localization in thermoelectric PbSe.

Lead chalcogenides have exceptional thermoelectric properties and intriguing anharmonic lattice dynamics underlying their low thermal conductivities. An ideal material for thermoelectric efficiency is the phonon glass-electron crystal, which drives research on strategies to scatter or localize phonons while minimally disrupting electronic-transport. Anharmonicity can potentially do both, even in perfect crystals, and simulations suggest that PbSe is anharmonic enough to support intrinsic localized modes that halt transport. Here, we experimentally observe high-temperature localization in PbSe using neutron scattering but find that localization is not limited to isolated modes - zero group velocity develops for a significant section of the transverse optic phonon on heating above a transition in the anharmonic dynamics. Arrest of the optic phonon propagation coincides with unusual sharpening of the longitudinal acoustic mode due to a loss of phase space for scattering. Our study shows how nonlinear physics beyond conventional anharmonic perturbations can fundamentally alter vibrational transport properties.

Microfocusing options for the inelastic X-ray scattering beamline at sector 3 of the Advanced Photon Source.

Synchrotron radiation from third-generation high-brilliance storage rings is an ideal source for X-ray microbeams. The aim of this paper is to describe a microfocusing scheme that combines both a toroidal mirror and Kirkpatrick-Baez (KB) mirrors for upgrading the existing optical system for inelastic X-ray scattering experiments at sector 3 of the Advanced Photon Source. SHADOW ray-tracing simulations without considering slope errors of both the toroidal mirror and KB mirrors show that this combination can provide a beam size of 4.5 µm (H) × 0.6 µm (V) (FWHM) at the end of the existing D-station (66 m from the source) with use of full beam transmission of up to 59%, and a beam size of 3.7 µm (H) × 0.46 µm (V) (FWHM) at the front-end of the proposed E-station (68 m from the source) with a transmission of up to 52%. A beam size of about 5 µm (H) × 1 µm (V) can be obtained, which is close to the ideal case, by using high-quality mirrors (with slope errors of less than 0.5 µrad r.m.s.). Considering the slope errors of the existing toroidal and KB mirrors (5 and 2.9 µrad r.m.s., respectively), the beam size grows to about 13.5 µm (H) × 6.3 µm (V) at the end of the D-station and to 12.0 µm (H) × 6.0 µm (V) at the front-end of the proposed E-station. The simulations presented here are compared with the experimental measurements that are significantly larger than the theoretical values even when slope error is included in the simulations. This is because of the experimental set-up that could not yet be optimized.

Six-reflection meV-monochromator for synchrotron radiation.

An in-line monochromatization scheme suitable for 10-40 keV synchrotron radiation is presented based on the use of six crystal reflections that achieves meV and sub-meV bandwidths with high efficiency. The theoretical spectral efficiency surpasses all previous multicrystal designs and approaches that of single room-temperature back-reflecting crystals. This article presents the designs of two such devices along with their theoretical and measured performances.

Electron-phonon coupling and the soft phonon mode in TiSe2.

We report high-resolution inelastic x-ray measurements of the soft phonon mode in the charge-density-wave compound TiSe(2). We observe a complete softening of a transverse optic phonon at the L point, i.e., q=(0.5, 0, 0.5), at T≈T(CDW). Detailed ab initio calculations for the electronic and lattice dynamical properties of TiSe(2) are in quantitative agreement with experimental frequencies for the soft phonon mode. The observed broad range of renormalized phonon frequencies, (0.3, 0, 0.5)≤q≤(0.5, 0, 0.5), is directly related to a broad peak in the electronic susceptibility stabilizing the charge-density-wave ordered state. Our analysis demonstrates that a conventional electron-phonon coupling mechanism can explain a structural instability and the charge-density-wave order in TiSe(2) although other mechanisms might further boost the transition temperature.

Observation of phonons with resonant inelastic x-ray scattering.

Phonons, the quantum mechanical representation of lattice vibrations, and their coupling to the electronic degrees of freedom are important for understanding thermal and electric properties of materials. For the first time, phonons have been measured using resonant inelastic x-ray scattering (RIXS) across the Cu K-edge in cupric oxide (CuO). Analyzing these spectra using an ultra-short core-hole lifetime approximation and exact diagonalization techniques, we can explain the essential inelastic features. The relative spectral intensities are related to the electron-phonon coupling strengths.

Observation of a continuous phase transition in a shape-memory alloy.

Elastic neutron-scattering, inelastic x-ray scattering, specific-heat, and pressure-dependent electrical transport measurements have been made on single crystals of AuZn and Au0.52Zn0.48. Elastic neutron scattering detects new commensurate Bragg peaks (modulation) appearing at Q =(1.33,0.67,0) at temperatures corresponding to each sample's transition temperature (TM = 64 and 45 K, respectively). Although the new Bragg peaks appear in a discontinuous manner in the Au0.52Zn0.48 sample, they appear in a continuous manner in AuZn. Surprising us, the temperature dependence of the AuZn Bragg peak intensity and the specific-heat jump near TM are in favorable accord with a continuous transition. A fit to the pressure dependence of TM suggests the presence of a critical end point in the AuZn phase diagram located at TM* = 2.7 K and p* = 3.1 GPa.

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.

A cryogenically stabilized meV-monochromator for hard X-rays.

The design and performance results for a cryogenically stabilized high-resolution monochromator for 23.880 keV (lambda = 52 pm) X-rays are presented. The four-crystal-reflection monochromator is suitable for nuclear resonant scattering measurements from 119Sn compounds using synchrotron radiation. The design includes a low-vibration cryostat that maintains two of the four crystal reflections at a temperature where the coefficient for thermal expansion of the crystalline material (silicon) vanishes. Test results demonstrate a 1.3 meV bandwidth with negligible broadening due to vibrations and a spectral efficiency of 37% when used with an undulator source.

Excitations of lithium ammonia complexes studied by inelastic x-ray scattering.

We have carried out high-resolution inelastic x-ray scattering measurements of the excitations of lithium dissolved in ammonia. The incident x-ray energy was 21.6 keV and the resolution was about 2 meV. Several different excitations are observed in the energy range of 0-60 meV (0-500 cm(-1)). In addition to acoustic phonons at low energies, we see excitations that are associated with vibrations of Li(NH3)4+ complexes. We examined these excitations as a function of momentum transfer, lithium concentration, temperature, and state of the system (solid versus liquid). Data are compared with Hartree-Fock and density-functional theory calculations of the excitations of this complex, which agree well with the measured excitation energies.

X-ray Bragg diffraction in asymmetric backscattering geometry.

We observe three effects in the Bragg diffraction of x rays in backscattering geometry from asymmetrically cut crystals. First, exact Bragg backscattering takes place not at normal incidence to the reflecting atomic planes. Second, a well-collimated (approximately 1 microrad) beam is transformed after the Bragg reflection into a strongly divergent beam (230 microrad) with reflection angle dependent on x-ray wavelength--an effect of angular dispersion. The asymmetrically cut crystal thus behaves like an optical prism, dispersing an incident collimated polychromatic beam. The dispersion rate is approximately 8.5 mrad/eV. Third, parasitic Bragg reflections accompanying Bragg backreflection are suppressed. These effects offer a radically new means for monochromatization of x rays not limited by the intrinsic width of the Bragg reflection.

Microscopic dynamics of liquid aluminum oxide.

Collective excitations have been observed in liquid aluminum oxide at high temperatures by combining a containerless sample environment with inelastic x-ray scattering. The excitation spectra show a well-defined triplet peak structure at lower wave vectors Q (1 to 6 nanometers-1) and a single quasi-elastic peak at higher Q. The high-Q spectra are well described by kinetic theory. The low-Q spectra require a frequency-dependent viscosity and provide previously unknown experimental constraints on the behavior of liquids at the interface between atomistic and continuum theory.

Evidence for an instability near twice the fermi wave vector in the low electronic density liquid metal Li(NH3)4.

We report high-resolution inelastic x-ray scattering measurements in the metallic liquid Li(NH3)4, which to a good approximation can be treated as a dilute alkali metal. We see a well-defined excitation out to large momentum transfers. This excitation shows a strong softening at wave vectors near the first peak in the structure factor, which occurs near twice the Fermi momentum.