Microscopic self-dynamics in liquid Ne-D_{2} mixtures: Quantum features and itinerant oscillators reexamined.

In this paper, we report the results of a centroid molecular dynamics (CMD) study of the canonical velocity autocorrelation functions (VACFs) in liquid Ne-D_{2} mixtures at a temperature of T=30K and in the full D_{2}-concentration range (0%≤x_{D_{2}}≤100%). This binary system was selected because of its moderate, although sizable, quantum effects which, as far as its equilibrium properties are concerned, are fully described by the path integral Monte Carlo (PIMC) simulations that have been also implemented. A comprehensive test of the VACF spectral moments carried out using three physical quantities (namely, mean kinetic energy, Einstein frequency, and mean-squared force) obtained from PIMC was performed revealing the potentialities, as well as the limitations, of the CMD approach to the single-particle dynamics in these low-T liquid mixtures.

Large-cage occupation and quantum dynamics of hydrogen molecules in sII clathrate hydrates.

Hydrogen clathrate hydrates are ice-like crystalline substances in which hydrogen molecules are trapped inside polyhedral cages formed by the water molecules. Small cages can host only a single H2 molecule, while each large cage can be occupied by up to four H2 molecules. Here, we present a neutron scattering study on the structure of the sII hydrogen clathrate hydrate and on the low-temperature dynamics of the hydrogen molecules trapped in its large cages, as a function of the gas content in the samples.

Microscopic dynamics of gas molecules confined in porous channel-like ice structure.

In the rich ice polymorphism landscape, ice XVII, metastable at ambient pressure and at temperatures below 130 K, is surely one of the most interesting from both fundamental and technological perspectives due to its porosity, i.e., its capability to repeatedly absorb and desorb molecular hydrogen by dosing the gas at pressures even below the ambient one.

Role of the single-particle dynamics in the transverse current autocorrelation function of a liquid metal.

A recent simulation study of the transverse current autocorrelation of the Lennard-Jones fluid [Guarini et al., Phys. Rev.

Onset of collective excitations in the transverse dynamics of simple fluids.

A thorough analysis of the transverse current autocorrelation function obtained by molecular dynamics simulations of a dense Lennard-Jones fluid reveals that even such a simple system is characterized by a varied dynamical behavior with changing length scale. By using the exponential expansion theory, we provide a full account of the time correlation at wavevectors Q between the upper boundary of the hydrodynamic region and Q_{p}/2, with Q_{p} being the position of the main peak of the static structure factor. In the Q range studied, we identify and accurately locate the wavevector at which shear wave propagation starts to take place, and show clearly how this phenomenon may be represented by a damped harmonic oscillator changing, in a continuous way, from an overdamped to an underdamped condition.

Microscopic collective dynamics in liquid neon-deuterium mixtures: Inelastic neutron scattering and quantum simulations.

In this paper a combined neutron scattering and quantum simulation study of the collective dynamics in liquid Ne-D_{2} mixtures, at a temperature of T=30K and in the wave-vector transfer range 4 nm^{-1} View Article and Find Full Text PDF

Proton Dynamics in Palladium-Silver: An Inelastic Neutron Scattering Investigation.

Proton dynamics in PdAg membranes is investigated by means of various neutron spectroscopic techniques, namely Quasi Elastic Neutron Scattering, Incoherent Inelastic Neutron Scattering, Neutron Transmission, and Deep Inelastic Neutron Scattering. Measurements carried out at the ISIS spallation neutron source using OSIRIS, MARI and VESUVIO spectrometers were performed at pressures of 1, 2, and 4 bar, and temperatures in the 330-673 K range. The energy interval spanned by the different instruments provides information on the proton dynamics in a time scale ranging from about 10 to 10 ps.

Time dependence of quantum correlation functions.

In the past few years, the exponential expansion analysis of time autocorrelation functions has provided profound insight into the leading microscopic processes driving the atomic-scale dynamics and has made it possible to highlight the presence and the role of various relaxation channels through which the fundamental correlation functions decay with time. Here we apply this method to the determination of the full time dependence of a correlation function c(t) in a quantum system at nonzero temperature, by making explicit its relationship with its Kubo transform c_{K}(t), which in some cases can be approximately computed with the presently available quantum simulation techniques. We obtain an exact expression for c(t) in terms of the exponential modes that describe the time behavior of c_{K}(t).

Density dependence of the dynamical processes governing the velocity autocorrelation function of a quantum fluid.

We present an exponential mode analysis of the dynamical processes determining the time behavior of the Kubo velocity autocorrelation function (KVAF) of fluid para-H_{2}, as obtained by ring polymer molecular dynamics simulations at various fluid densities. The mechanisms contributing to the decay of the KVAF are thoroughly characterized at a slightly supercritical temperature, in a density interval ranging from the critical point to the fluid-solid transition. We show that the quantum nature of the system does not influence the specific phenomena and decay channels through which a loss in velocity correlation takes place, since these are the same as found in classical fluids.

Dynamical Origin of the Total and Zero-Point Kinetic Energy in a Quantum Fluid.

By applying an exponential mode analysis to ring polymer molecular dynamics simulations of dense fluid parahydrogen, we find that the dynamical processes establishing the time behavior of the Kubo velocity autocorrelation function display the same nature as those already observed in high-density classical fluids. This result permits us to demonstrate that the exponential mode decomposition is a unique tool to identify which dynamical processes lead to one of the most notable properties of quantum fluids: the large value of the mean kinetic energy per particle and the importance of the zero-temperature quantum effects in determining it.

Density and time scaling effects on the velocity autocorrelation function of quantum and classical dense fluid para-hydrogen.

We report the results of a ring polymer molecular dynamics study of the Kubo velocity autocorrelation function of a quantum fluid as para-hydrogen aimed at the comparison with its classical counterpart. Quite different density conditions were considered for both the classical and quantum cases, in order to compare the two systems before and after the dynamical crossover typically undergone by the velocity autocorrelation function (VAF) of fluids at densities around the triple point, where the shape of the function changes from a monotonic to an oscillatory behavior with a negative minimum. A detailed study of the phase diagram of classical para-hydrogen was necessary for a reasonable choice of the classical states to be taken into consideration, in the spirit of the classical principle of corresponding states.

Hydrogen self-dynamics in diluted liquid mixtures with neon: An inelastic neutron scattering study.

We have measured the dynamic structure factor of liquid neon-hydrogen mixtures (T=30.1 K) at two different H_{2} concentration levels (namely, 3.4% and 10%) making use of inelastic neutron scattering.

VESPA: The vibrational spectrometer for the European Spallation Source.

VESPA, Vibrational Excitation Spectrometer with Pyrolytic-graphite Analysers, aims to probe molecular excitations via inelastic neutron scattering. It is a thermal high resolution inverted geometry time-of-flight instrument designed to maximise the use of the long pulse of the European Spallation Source. The wavelength frame multiplication technique was applied to provide simultaneously a broad dynamic range (about 0-500 meV) while a system of optical blind choppers allows to trade flux for energy resolution.

Impact of the Condensed-Phase Environment on the Translation-Rotation Eigenstates and Spectra of a Hydrogen Molecule in Clathrate Hydrates.

We systematically investigate the manifestations of the condensed-phase environment of the structure II clathrate hydrate in the translation-rotation (TR) dynamics and the inelastic neutron scattering (INS) spectra of an H2 molecule confined in the small dodecahedral cage of the hydrate. The aim is to elucidate the extent to which these properties are affected by the clathrate water molecules beyond the confining cage and the proton disorder of the water framework. For this purpose, quantum calculations of the TR eigenstates and INS spectra are performed for H2 inside spherical clathrate domains of gradually increasing radius and the number of water molecules ranging from 20 for the isolated small cage to more than 1800.

Hydrogen self-dynamics in liquid H(2)-D(2) mixtures studied through inelastic neutron scattering.

We have measured the dynamic structure factor of liquid para-hydrogen mixed with normal deuterium (T=20 K) at two different concentration levels using incoherent inelastic neutron scattering. This choice has been made since the presence of D(2} modifies the self-dynamics of H(2) in a highly nontrivial way, acting both on its pseudophononic and its diffusive parts in a tunable way. After an accurate data reduction, recorded neutron spectra were studied through the modified Young and Koppel model and the H(2) center-of-mass self-dynamics structure factor was finally extracted for the two mixtures.

The HD molecule in small and medium cages of clathrate hydrates: quantum dynamics studied by neutron scattering measurements and computation.

We report inelastic neutron scattering (INS) measurements on molecular hydrogen deuteride (HD) trapped in binary cubic (sII) and hexagonal (sH) clathrate hydrates, performed at low temperature using two different neutron spectrometers in order to probe both energy and momentum transfer. The INS spectra of binary clathrate samples exhibit a rich structure containing sharp bands arising from both the rotational transitions and the rattling modes of the guest molecule. For the clathrates with sII structure, there is a very good agreement with the rigorous fully quantum simulations which account for the subtle effects of the anisotropy, angular and radial, of the host cage on the HD microscopic dynamics.

Trophic activity of human P2X7 receptor isoforms A and B in osteosarcoma.

The P2X7 receptor (P2X7R) is attracting increasing attention for its involvement in cancer. Several recent studies have shown a crucial role of P2X7R in tumour cell growth, angiogenesis and invasiveness. In this study, we investigated the role of the two known human P2X7R functional splice variants, the full length P2X7RA and the truncated P2X7RB, in osteosarcoma cell growth.

Experimental inelastic neutron scattering spectrum of hydrogen hexagonal clathrate-hydrate compared with rigorous quantum simulations.

We have performed high-resolution inelastic neutron scattering (INS) measurements on binary hydrogen clathrate hydrates exhibiting the hexagonal structure (sH). Two samples, differing only in the ortho/para fraction of hydrogen, were prepared using heavy water and methyl tert-butyl ether as the promoter in its perdeuterated form. The INS spectrum of the translation-rotation (TR) excitations of the guest H2 molecule was obtained by subtracting the very weak signal due to the D2O lattice modes.

Neutron scattering measurements and computation of the quantum dynamics of hydrogen molecules trapped in the small and large cages of clathrate hydrates.

We report inelastic neutron scattering (INS) measurements on molecular hydrogen trapped in simple (D2O) and binary (D2O plus perdeuterated tetrahydrofuran) clathrate hydrates, performed at a low temperature using two different neutron spectrometers to probe both energy and momentum transfer. The INS spectra of binary clathrate samples exhibit a rich structure containing sharp bands arising from both the rotational transitions and the rattling modes of the guest H2 molecule. They agree well with the rigorous fully quantum simulations, which account for the subtle effects of the anisotropy, angular and radial, of the host cage on the H2 microscopic dynamics and the resulting spectra.

Proton vibrational dynamics in lithium imide investigated through incoherent inelastic and Compton neutron scattering.

In the present study we report neutron spectroscopic measurements on polycrystalline lithium imide, namely, incoherent inelastic neutron scattering at 20 K, and neutron Compton scattering from 10 K up to room temperature. From the former technique the H-projected density of phonon states up to 100 meV is derived, while the latter works out the spherically averaged single-particle (i.e.

Raman spectra of ammonia borane: low frequency lattice modes.

We have measured the Raman spectrum of ammonia borane at low temperature (T = 15 K) and across the orthorhombic-to-tetragonal phase transition at T = 225 K. A comprehensive study of the low frequency lattice modes using Raman spectroscopy has been carried out. Data analysis has been complemented by a density functional theory calculation of which the results have been used for a detailed assignment of the Raman active modes.

Raman and inelastic neutron scattering study on a melt-infiltrated composite of NaAlH4 and nanoporous carbon.

Inelastic neutron scattering and Raman scattering spectra of a melt-infiltrated composite of NaAlH(4) and active carbon fibers have been measured at low temperature for two sample conditions: as prepared and subjected to hydrogen desorption-absorption cycling. After a careful data analysis, the present experimental results have been compared to the corresponding spectroscopic data taken from bulk NaAlH(4) and Na(3)AlH(6). Evident signatures induced by infiltration process onto the NaAlH(4) phonon bands have been detected, showing up as a strong peak broadening and smoothing together with, in some cases, an energy shift.

Temperature behavior of the AlH3 polymorph by in situ investigation using high resolution Raman scattering.

A Raman investigation of the AlH(3) polymorph has been carried out at a low temperature (20 K) under helium atmosphere (2 bar). The pristine material was composed of three polymorphs, namely, the α, β, and γ phases. The β phase has been removed by warming the sample to 70 °C, while further heating at 100 °C was used to remove the γ phase.

High resolution Raman and neutron investigation of Mg(BH4)2 in an extensive temperature range.

Raman spectra of Mg(BH(4))(2) have been measured in an extensive temperature range, from 15 to 473 K. Taking into account the high temperature conversion from the alpha to the beta phase, we have observed evident signatures of this phase transition and determined the Raman vibrational spectrum of each phase. The neutron scattering spectra of the beta phase sample were also recorded.

First principle calculations of alkali hydride electronic structures.

Electronic structure, volume optimization, bulk moduli, elastic constants, and frequencies of the transversal optical vibrations in LiH, NaH, KH, RbH, and CsH are calculated using the full potential augmented plane wave method, extended with local orbitals, and the full potential linearized augmented plane wave method. The obtained results show some common features in the electronic structure of these compounds, but also clear differences, which cannot be explained using simple empirical trends. The differences are particularly prominent in the electronic distributions and interactions in various crystallographic planes.