J. L. Martínez

Coherent Inelastic Neutron Scattering Response from Liquid Methanol

Inelastic spectra of deuteromethanol at 200 K measured by triple-axis neutron spectroscopy are reported. A reconstruction of the dynamical structure factor from the observed spectral intensity has been achieved by means of a deconvolution procedure. A complementary analysis of the spectra in terms of simple phenomenological models (hydrodynamic and viscoelastic) is also presented. The experimental data indicates the existence of a propagating mode which becomes overdamped for wave vectors ≥ 0.6 A−1.

Microscopic dynamics of ortho-terphenyl in its polycrystalline and glassy forms

The microscopic dynamics of polycrystalline ortho-terphenyl are investigated by means of the concurrent use of harmonic lattice dynamics calculations and Raman spectroscopy. In order to reproduce the spectra and thermodynamics of the crystal up to about 70 K, the motions of several internal modes are considered in detail. An empirical force field, able to account for the observed crystal structure, thermodynamics and Raman spectra up to such temperature, is then developed. Estimates of the anharmonic effects are derived from the temperature dependence of vibrational frequencies of those modes of collective or hybrid character (below 6 THz). The Raman spectra of the glass are finally discussed by comparison with the polycrystalline case. The implications of the present findings regarding studies on the glass transition of complex materials are discussed.

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.

Repeatability of Feather Mite Prevalence and Intensity in Passerine Birds

Understanding why host species differ so much in symbiont loads and how this depends on ecological host and symbiont traits is a major issue in the ecology of symbiosis. A first step in this inquiry is to know whether observed differences among host species are species-specific traits or more related with host-symbiont environmental conditions. Here we analysed the repeatability (R) of the intensity and the prevalence of feather mites to partition within- and among-host species variance components. We compiled the largest dataset so far available: 119 Paleartic passerine bird species, 75,944 individual birds, ca. 1.8 million mites, seven countries, 23 study years. Several analyses and approaches were made to estimate R and adjusted repeatability (Radj) after controlling for potential confounding factors (breeding period, weather, habitat, spatial autocorrelation and researcher identity). The prevalence of feather mites was moderately repeatable (R = 0.26–0.53; Radj = 0.32–0.57); smaller values were found for intensity (R = 0.19–0.30; Radj = 0.18–0.30). These moderate repeatabilities show that prevalence and intensity of feather mites differ among species, but also that the high variation within species leads to considerable overlap among bird species. Differences in the prevalence and intensity of feather mites within bird species were small among habitats, suggesting that local factors are playing a secondary role. However, effects of local climatic conditions were partially observed for intensity.

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.

Temperature dependence of internal modes and local symmetry of PO4 tetrahedra in RbH2PO4 (RDP) and RbD2PO4 (DRDP) by Raman scattering

Abstract The internal modes of PO4 tetrahedra in RbH2PO4 (RDP) and RbD2PO4 (DRDP) have been observed above and below Tc in x(zz)y geometry. The ν3 and ν4 internal modes are clearly observed far above Tc in this geometry. Thus it is confirmed the analysis of Tominaga et al. suggesting that the local and momentary site symmetry of PO4 tetrahedra is C2 in the paraelectric phase for KDP isomorph. From this results, the mechanism of phase transition can be considered to be of order-disorder type of PO4 tetrahedra, instead of being interpreted in terms of proton-tunneling dynamics alone, as usual.

Lattice dynamics of ortho-terphenyl: Implications for the current tests of mode coupling theories of the liquid-glass transition

A lattice dynamical calculation for the known crystal structure of orthoterphenyl has been carried out under the rigid molecule approximation. The optimized parameters for the intermolecular potential reproduce adequately both the equilibrium crystal structure as well as the experimental heat capacity up to 35 K. The values for the mean-square-displacement as calculated from the computed vibrational density of states show that for temperatures well above 50 K most of the motions contributing to the atomic displacements as measured by neutron quasielastic scattering arise from the thermally excited internal molecular modes. The implications for the recent tests of kinetic theory predictions regarding the liquid-glass transitions using this material are briefly discussed.

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.

On the origin of the low-frequency excitations in glassy selenium

Abstract The microscopic low-frequency dynamics of glassy selenium is investigated by means of the concurrent use of neutron inelastic scattering and computer simulations. A separation of the dynamic response in terms of intra-and interchain processes is achieved from the analysis of the simulation results. The low-frequency peak in the dynamic structure factor S ( Q , E ) as well as the tail of the vibrational density of states Z ( E ) are then shown to be originated mostly by interchain interactions. The implications of these findings for the anomalous thermal behaviour of glasses are finally discussed.