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. The shape of the quantum and classical VAF was thoroughly analyzed, exhibiting at all studied densities clear differences that might be taken as evidence of quantum effects. We show that these differences are substantially reduced by applying a state-dependent time scaling with respect to a reference time identified with the inverse of the collision rate. An even better coincidence in shape is found by comparing the two systems at slightly non-corresponding reduced densities, suggesting that the quantum system behaves almost like the classical one, but at systematically less dense reduced states of the latter. We also find an unexpected and quite interesting density trend of the collision rate of both classical and quantum para-hydrogen, which accounts for the effectiveness of the scaling throughout the explored density range. The mean kinetic energy and the diffusion coefficients are also discussed in some detail.

A high-flux upgrade for the BRISP spectrometer at ILL.

To date, the BRISP spectrometer represents the state-of-the-art for every instrument aiming to perform Brillouin neutron scattering. Exploiting accurate ray-tracing McStas simulations, we investigate an improved configuration of the BRISP primary spectrometer to provide a higher flux at the sample position, while preserving all the present capabilities of the instrument. This configuration is based on a neutron guide system and is designed to fit the instrument platform with no modifications of the secondary spectrometer. These evaluations show that this setup can achieve a flux gain factor ranging from 3 to 6, depending on the wavelength. This can expand the experimental possibilities of BRISP towards smaller samples, possibly using also complex sample environments.

Density of states from mode expansion of the self-dynamic structure factor of a liquid metal.

We show that by exploiting multi-Lorentzian fits of the self-dynamic structure factor at various wave vectors it is possible to carefully perform the Q→0 extrapolation required to determine the spectrum Z(ω) of the velocity autocorrelation function of a liquid. The smooth Q dependence of the fit parameters makes their extrapolation to Q=0 a simple procedure from which Z(ω) becomes computable, with the great advantage of solving the problems related to resolution broadening of either experimental or simulated self-spectra. Determination of a single-particle property like the spectrum of the velocity autocorrelation function turns out to be crucial to understanding the whole dynamics of the liquid. In fact, we demonstrate a clear link between the collective mode frequencies and the shape of the frequency distribution Z(ω). In the specific case considered in this work, i.e., liquid Au, analysis of Z(ω) revealed the presence, along with propagating sound waves, of lower frequency modes that were not observed before by means of dynamic structure factor measurements. By exploiting ab initio simulations for this liquid metal we could also calculate the transverse current-current correlation spectra and clearly identify the transverse nature of the above mentioned less energetic modes. Evidence of propagating transverse excitations has actually been reported in various works in the recent literature. However, in some cases, like the present one, these modes are difficult to detect in density fluctuation spectra. We show here that the analysis of the single-particle dynamics is able to unveil their presence in a very effective way. The properties here shown to characterize Z(ω), and the information in it contained therefore allow us to identify it with the density of states (DoS) of the liquid. We demonstrate that only nonhydrodynamic modes contribute to the DoS, thus establishing its purely microscopic origin. Finally, as a by-product of this work, we provide our estimate of the self-diffusion coefficient of liquid gold just above melting.

Density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the velocity autocorrelation function.

Extending a preceding study of the velocity autocorrelation function (VAF) in a simulated Lennard-Jones fluid [Phys. Rev. E 92, 042166 (2015)PLEEE81539-375510.1103/PhysRevE.92.042166] to cover higher-density and lower-temperature states, we show that the recently demonstrated multiexponential expansion method allows for a full account and understanding of the basic dynamical processes encompassed by a fundamental quantity as the VAF. In particular, besides obtaining evidence of a persisting long-time tail, we assign specific and unambiguous physical meanings to groups of exponential modes related to the longitudinal and transverse collective dynamics, respectively. We have made this possible by consistently introducing the interpretation of the VAF frequency spectrum as a global density of states in fluids, generalizing a solid-state concept, and by giving to specific spectral components, obtained through the VAF exponential expansion, the corresponding meaning of partial densities of states relative to specific dynamical processes. The clear identification of a high-frequency oscillation of the VAF with the near-top excitation frequency in the dispersion curve of acoustic waves is a neat example of the power of the method. As for the transverse mode contribution, its analysis turns out to be particularly important, because the multiexponential expansion reveals a transition marking the onset of propagating excitations when the density is increased beyond a threshold value. While this finding agrees with the recent literature debating the issue of dynamical crossover boundaries, such as the one identified with the Frenkel line, we can add detailed information on the modes involved in this specific process in the domains of both time and frequency. This will help obtain a still missing full account of transverse dynamics, in both its nonpropagating and propagating aspects which are linked through dynamical transitions depending on both the thermodynamic states and the excitation wave vectors.

Time dependence of the velocity autocorrelation function of a fluid: An eigenmode analysis of dynamical processes.

The velocity autocorrelation function (VAF), a key quantity in the atomic-scale dynamics of fluids, has been the first paradigmatic example of a long-time tail phenomenon, and much work has been devoted to detecting such long-lasting correlations and understanding their nature. There is, however, much more to the VAF than simply the evidence of this long-time dynamics. A unified description of the VAF from very short to long times, and of the way it changes with varying density, is still missing. Here we show that an approach based on very general principles makes such a study possible and opens the way to a detailed quantitative characterization of the dynamical processes involved at all time scales. From the analysis of molecular dynamics simulations for a slightly supercritical Lennard-Jones fluid at various densities, we are able to evidence the presence of distinct fast and slow decay channels for the velocity correlation on the time scale set by the collision rate. The density evolution of these decay processes is also highlighted. The method presented here is very general, and its application to the VAF can be considered as an important example.

Comment on 'An effective fitting scheme for the dynamic structure of pure liquids'.

The occurrence of a propagation gap in the acoustic excitations of a liquid is excluded by Wax and Bryk (2013 J. Phys.: Condens. Matter 25 325104). The requirement of a finite second frequency moment for the dynamic structure factor is used to come to this conclusion. We show here that this requirement does not conflict with the existence of overdamped, non-propagating modes which give rise to spectra that do not contain inelastic components. Such a behaviour has indeed been detected in the analysis of the collective dynamics of several liquids, carried out by using well-established sum-rule-compliant S(q, ω) models.

Orientational and translational correlations of liquid methane over the nanometer-picosecond scales by molecular dynamics simulation and inelastic neutron scattering.

Five models for the site-site intermolecular pair interactions of methane are compared in some detail and used to investigate both structural and dynamical properties of the dense liquid deuteromethane by means of molecular dynamics (MD) simulations. The orientational distribution probabilities of molecular pairs are carefully analyzed for each anisotropic potential model. We propose a revision of existing classification methods used to group the innumerable relative orientations of methane-methane pairs into six basic geometries. With this new approach, our results for the probability of the six basic categories as a function of the intermolecular distance are different from the ones present in the literature, where the role of the angular spread on the anisotropic interaction energy is not taken in full consideration and certain configurations with no significant change in the pair-potential are assigned to different categories. The analysis of the static orientational correlations in liquid methane and the prevalence of certain configurations in different ranges guide the subsequent discussion of the MD model-dependent results for the dynamic structure factor. Comparison with our inelastic neutron scattering results for liquid CD(4) at the nanometer and picosecond space and time scales allows us to confirm the full adequacy of the Tsuzuki, Uchimaru and Tanabe model of 1998 with respect to more recent potentials.

Inelastic neutron scattering and molecular dynamics determination of the interaction potential in liquid CD4.

Anisotropic interactions of liquid CD4 are studied in detail by comparison of inelastic neutron Brillouin scattering data with molecular dynamics simulations using up to four different models of the methane site-site potential. We demonstrate that the experimental dynamic structure factor S(Q,omega) acts as a highly discriminating quantity for possible interaction schemes. In particular, the Q evolution of the spectra enables a selective probing of the short- and medium-range features of the anisotropic potentials. We show that the preferential configuration of methane dimers at liquid densities can thus be discerned by analyzing the orientation-dependent model potential curves, in light of the experimental and simulation results.

Experimental studies of the ortho-toluidine glass transition.

The thermodynamic and dynamics proprieties of ortho-toluidine, in the vicinity of a glass transition, have been studied by calorimetric and by two light scattering techniques, depolarized light scattering and time-resolved optical Kerr effect. Differential scanning microcalorimetry clearly detects a glass transition in o-toluidine and it measures some thermodynamics critical parameters, in particular, the transition temperature. The light scattering data have been analyzed according to the mode-coupling theory. This theory gives a good interpretation of our data and it allows to extract safely the critical parameters of the o-toluidine dynamics. We found a fair agreement between the analysis outputs performed in the frequency domain and in the time domain. Finally, we compared the glass transition features of o-toluidine with that of its isomer meta-toluidine, looking for some general idea about the molecular aspects of the glass transition.

Neutron investigation of the ion dynamics in liquid mercury: evidence for collective excitations.

The ion dynamics of liquid mercury was investigated by inelastic neutron scattering. By exploiting an optimized high-resolution ( approximately 1 meV) experimental configuration, the dynamic response function was accurately measured. Collective excitations extending up to 0.6 A(-1) were observed with an associated velocity of 2100+/-80 m/s. This value is notably greater than the sound velocity, but it is provided by a simple Bohm-Staver calculation. The latter finding emphasizes those electron-related features in the ion dynamics, which are common to systems as different as polyvalent and alkali metals.

Many-body interaction effects on the low-k structure of liquid Kr.

Neutron diffraction measurements and theoretical calculations of the structure factor S(k) of liquid Kr are extended to small k values (k<4 nm(-1)). The results show that many-body interaction contributions have an increasing effect on S(k) as k-->0, reaching at least 40% of the measured intensity. Both the phase diagram and the low-k structural data of dense Kr turn out to be closely reproduced by the hierarchical reference theory if additional many-body forces are taken into account by an augmented strength of the Axilrod-Teller triple-dipole potential. The experimental density derivative of S(k) is also used for a very sensitive test of the theories and interaction models considered here.

Neutron Brillouin scattering study of collective dynamics in a dense He-Ne gaseous mixture.

The dynamic structure factor S(k,omega) of a 77% He and 23% Ne gaseous mixture at T = 39.3 K and total number density n = 15.8 nm(-3) has been measured by inelastic neutron scattering at small angles. In the range of wave vectors studied, 0.7

Structure of gaseous Kr in the low-q region by neutron scattering and interaction potentials.

Accurate experimental information on the long-range triplet interactions in noble fluids, as well as on the two-body potential, can be obtained from the low-density behavior of the static structure factor S(q) in the small-q region. The results here reported of a recent low-q neutron diffraction investigation in Kr, devoted to undercritical densities in the range 2.4