F.J. Mompean

Collective excitations in liquid deuterium studied by inelastic neutron scattering

Abstract Well defined collective excitations up to a momentum transfer value of 3.8 A -1 have been found in liquid deuterium 0.5 K above the melting point. The derived dispersion curve shows a remarkable similarity with the one observed for liquid 4 He in the normal liquid phase, with a well defined minimum at a momentum transfer of 2.08 A -1 corresponding to a gap of 30 K. A qualitative comparison is made with recent calculations on liquid para-hydrogen at the triple point.

Collective low-frequency excitations in a molecular glass

The excitations below 40 meV of a glass-former mixture (CD3OD 6% D2O) have been studied by neutron spectroscopy. A well-resolved inelastic peak with a dispersion behaviour reminiscent of those found in metallic glasses has been observed for temperatures above 10 K. The presence of such peak is correlated with an abrupt enhancement of the total vibrational density of states (TDOS). The nature of the excitations is analysed by means of their wave vector dependence. Finally, the TDOS are tentatively interpreted in terms of a coupling between sound waves and localized excitations, which becomes effective for temperatures above 10 K.

Observation of high frequency excitations in a molecular glass

Abstract Neutron inelastic triple-axis spectra of CD 3 OD/6% D 2 O in the glassy and liquid phases are reported. Both the dispersion and damping of the sound modes show characteristic features as the temperature is lowered down into the glassy regime. Also the analysis of the observed scattering intensities evidences the presence in the glass of a second component centered at zero frequency which is not observed in the liquid.

Magnetic Dynamics in Liquid Oxygen

The magnetic and structural dynamics in liquid oxygen are studied by means of high-resolution inelastic neutron scattering and computer molecular-dynamics simulations. The former technique enables an approximate separation between nuclear and magnetic responses from the total, unpolarized neutron double differential cross-sections. The magnetic scattering spectra are then shown to reproduce the wave vector dependence of the inelastic intensities associated with the magnetic excitations measured in a previous study. Finally, the magnetic spectral functions are shown to be well reproduced by a model of exchange-coupled paramagnet and estimates of the exchange parameter and correlation length are obtained from the analysis of the frequency moments.

Observation of propagating collective excitations in liquid SO2

The spectrum of collective excitations in liquid sulphur dioxide at two thermodynamic states has been measured by neutron triple‐axis spectroscopy for momentum transfer ranges of 0.35≤Q≤2.0 A−1. Well‐resolved inelastic peaks indicating the presence of propagating modes are found for Q values below 0.5 A−1 at T=266 K and below 0.7 A−1 at T=210 K. A precise value for the hydrodynamic limit has been found in both cases and the measured ‘‘dispersion relations’’ are discussed in relation to predictions made by recent theoretical approaches.

Magnetic dynamics in the disordered phases of oxygen

Abstract The results of a neutron scattering investigation on the spin and structural dynamics of oxygen in the liquid and orientationally disordered phases are presented. The presence of magnetic excitations is inferred from the analysis of the widths of the unpolarized spectra as well as from the wave vector dependence of the inelastic intensities.

Coherent excitations in liquid deuterium

The coherent dynamical response of liquid deuterium 0.5 K above the melting temperature has been studied by means of triple-axis inelastic scattering and time-of-flight techniques. A model scattering law to represent the single-particle and low energy collective dynamics has been tested by analysing the time-of-flight spectra. On the other hand, well defined collective excitations up to a momentum transfer of 3.8A-1 have been found in the triple-axis experiments. The derived dispersion curve shows a well defined minimum at 2.08A-1 with a gap of 30 K.

Neutron quasielastic scattering from molecular liquids and glasses

An overview of the present status of the theory of neutron scattering from disordered molecular systems (liquids and glasses), with the main emphasis on the theoretical modelling of spectral lineshapes, is presented. Applications to several specific cases are described in order to illustrate the capabilities of Quasielastic Neutron Scattering (QENS) for obtaining reliable information about the stochastic dynamics of these systems.

Inelastic scattering from molecular liquids and glasses

Abstract A brief account of recent studies on the coherent inelastic response from representative examples of molecular systems in liquid and glassy states is presented.

Quasielastic neutron scattering in condensed oxygen

The quasielastic spectra of liquid and plastic-crystalline oxygen are shown to be dominated by the magnetic response. An approximate separation of the nuclear and magnetic responses from the total, unpolarized neutron double differential cross-sections is achieved by means of a subtraction of the structural contributions calculated from a Molecular Dynamics simulation. The magnetic scattering spectra are then shown to reproduce the wavevector dependence of the inelastic intensities associated with the magnetic excitations measured in a previous study, and are analyzed in terms of a model for an exchange-coupled paramagnet. Finally, the close resemblance between the magnetic response of the liquid and of the plastic crystal γ-phase are discussed.