High-frequency acoustic modes in liquid gallium at the melting point

Experimental Confirmation of the Universal Law for the Vibrational Density of States of Liquids

An analytical model describing the vibrational density of states (VDOS) of liquids has long been elusive, owing to the complexities of liquid dynamics. Nevertheless, Zaccone and Baggioli have recently developed such a model which was proposed to be the universal law for the vibrational density of states of liquids. Distinct from the Debye law, g(ω) ∝ ω², for solids, the universal law for liquids reveals a linear relationship, g(ω) ∝ ω, in the low-energy region. We have confirmed this universal law with experimental VDOS measured by inelastic neutron scattering on real liquid systems including water, liquid metal, and polymer liquids, and have applied this model to extract the effective relaxation rate for the short time dynamics for each liquid. The model has also been further evaluated in the prediction of the specific heat with comparison to existing experimental data as well as with values obtained by different approaches.

Dynamics on next-neighbour distances in liquid and undercooled gallium

Density fluctuations of liquid and 20 K undercooled gallium have been studied by neutron spectroscopy. The decay of density fluctuations has been recorded at the structure factor maximum over a wide temperature range up to twice the melting temperature. The amplitude of the scattering function falls off with rising temperature in a nonlinear way with a changing slope around [Formula: see text]. The derived generalized longitudinal viscosity shows an upturn with decreasing temperature in the same temperature range. This increase in viscosity can be understood that liquid gallium transforms from a more fluid liquid metal to a more viscous liquid metal in that temperature range upon cooling. The change in the amplitude shows a remarkable agreement with results from liquid aluminium, lead and rubidium. This study suggests a universal crossover in dynamics of liquid monatomic metals, despite the many peculiar properties of gallium.

Evolution of atomic structure in Al75Cu25 liquid from experimental and ab initio molecular dynamics simulation studies

X-ray diffraction and electrostatic levitation measurements, together with the ab initio molecular dynamics simulation of liquid Al(75)Cu(25) alloy have been performed from 800 to 1600 K. Experimental and ab initio molecular dynamics simulation results match well with each other. No abnormal changes were experimentally detected in the specific heat capacity over total hemispheric emissivity and density curves in the studied temperature range for a bulk liquid Al(75)Cu(25) alloy measured by the electrostatic levitation technique. The structure factors gained by the ab initio molecular dynamics simulation precisely coincide with the experimental data. The atomic structure analyzed by the Honeycutt-Andersen index and Voronoi tessellation methods shows that icosahedral-like atomic clusters prevail in the liquid Al(75)Cu(25) alloy and the atomic clusters evolve continuously. All results obtained here suggest that no liquid-liquid transition appears in the bulk liquid Al(75)Cu(25) alloy in the studied temperature range.

Transverse acoustic excitations in liquid Ga

The transverse acoustic excitation modes were detected by inelastic x-ray scattering in liquid Ga in the Q range above 9 nm(-1) although liquid Ga is mostly described by a hard-sphere liquid. An ab initio molecular dynamics simulation clearly supports this finding. From the detailed analysis for the S(Q,omega) spectra with a good statistic quality, the lifetime of 0.5 ps and the propagating length of 0.4-0.5 nm can be estimated for the transverse acoustic phonon modes, which may correspond to the lifetime and size of cages formed instantaneously in liquid Ga.

The possibility of transverse excitation modes in liquid Ga

The dynamic structure factor S(Q,ω) of liquid Ga was measured at 100 °C using a high resolution inelastic x-ray scattering (IXS) spectrometer at 3-ID-C/APS. The spectra obtained clearly demonstrate the existence of longitudinal propagating modes at small Q values, like a previous IXS result at 42 °C obtained by Scopigno et al and an inelastic neutron scattering (INS) one at 47 °C obtained by Bove et al, but unlike an INS study at 57 °C by Bermejo et al. The dispersion relation of the excitations deviates positively from the hydrodynamic prediction by about 13%. There are two new findings from this experiment. Firstly, an additional lower energy excitation is necessary to reproduce S(Q,ω) spectra in the Q range beyond 10 nm(-1), in agreement with the result of a first-principles molecular dynamic simulation, which may indicate a transverse acoustic mode in this peculiar liquid metal. Secondly, the quasielastic line comprises a Gaussian contribution at Q values near the first maximum in S(Q), which may indicate the existence of short-lived covalent correlation in liquid Ga with a lifetime of 0.39 ps.

Longitudinal excitations in Mg–Al–O refractory oxide melts studied by inelastic x-ray scattering

The dynamic structure factor S(Q,omega) of the refractory oxide melts MgAl2O4 and MgAl4O7 is studied by inelastic x-ray scattering with aerodynamic levitation and laser heating. This technique allows the authors to measure simultaneously the elastic response and transport properties of melts under extreme temperatures. Over the wave vector Q range of 1-8 nm-1 the data can be fitted with a generalized hydrodynamic model that incorporates a slow component described by a single relaxation time and an effectively instantaneous fast component. Their study provides estimates of high-frequency sound velocities and viscosities of the Mg-Al-O melts. In contrast to liquid metals, the dispersion of the high-frequency sound mode is found to be linear, and the generalized viscosity to be Q independent. Both experiment and simulation show a weak viscosity maximum around the MgAl4O7 composition.

Collective acoustic modes as renormalized damped oscillators: unified description of neutron and x-ray scattering data from classical fluids

In the Q range where inelastic x-ray and neutron scattering are applied to the study of acoustic collective excitations in fluids, various models of the dynamic structure factor S(Q, omega) generalize in different ways the results obtained from linearized-hydrodynamics theory in the Q-->0 limit. Here we show that the models most commonly fitted to experimental S(Q, omega) spectra can be given a unified formulation. In this way, direct comparisons among the results obtained by fitting different models become now possible to a much larger extent than ever. We also show that a consistent determination of the dispersion curve and of the propagation Q range of the excitations is possible, whichever model is used. We derive an exact formula which describes in all cases the dispersion curve and allows for the first quantitative understanding of its shape, by assigning specific and distinct roles to the various structural, thermal, and damping effects that determine the Q dependence of the mode frequencies. The emerging picture describes the acoustic modes as Q-dependent harmonic oscillators whose characteristic frequency is explicitly renormalized in an exact way by the relaxation processes, which also determine, through the widths of both the inelastic and the elastic lines, the whole shape of collective-excitation spectra.

Fluctuating magnetic moments in liquid metals

We reanalyze literature data on neutron scattering by liquid metals and show that there is an additional broad (in energy) quasielastic mode present that is absent in x-ray scattering. This mode cannot be accounted for by the standard coherent and incoherent scattering mechanisms. We argue that this mode indicates that nonmagnetic liquid metals possess a magnetic moment which fluctuates on a picosecond time scale. This time scale is the same as the time scale of the cage-diffusion process in which an ion rattles around in the cage formed by its neighbors. We find that these fluctuating magnetic moments are present in liquid Hg, Al, Ga, and Pb and possibly also in the alkali metals.

Hard-sphere-like dynamics in a non-hard-sphere liquid

The collective dynamics of liquid gallium close to the melting point has been studied using inelastic x-ray scattering to probe length scales smaller than the size of the first coordination shell. Although the structural properties of this partially covalent liquid strongly deviate from a simple hard-sphere model, the dynamics, as reflected in the quasielastic scattering, are beautifully described within the framework of the extended heat mode approximation of Enskog's kinetic theory, analytically derived for a hard-sphere system. The present work demonstrates, therefore, the applicability of Enskog's theory beyond simple liquids.

Role of many-body correlations in dynamics of liquids

A time correlation function is written exactly in terms of infinite series with each term containing contributions separately due to two, three, and higher body static correlations. For a time correlation function of force acting on a tagged particle, it is found that contributions due to two and three body static correlation functions are sufficient to understand dynamics of dense gases whereas at the triple point and in the glassy phase it is necessary to include contributions due to a four body correlation function.

Reply to "Comment on 'Collective dynamics in liquid lithium, sodium, and aluminum' "

Phys. Rev. E 70, 013201 (2004)]] have raised certain objections to physical interpretation of the parameters of the model proposed by us earlier [Phys. Rev. E 67, 012201 (2003)]]. We have found that heat diffusion term enters into processes which are responsible for the quasielastic peak of the dynamical structure factor. An attempt has been made to study the role played by atomic and electronic contributions to thermal conductivity for studying atomic density-density correlation function.

Comment on "collective dynamics in liquid lithium, sodium, and aluminum"

In a recent paper, Phys. Rev. E 67, 012201 (2003)] proposed a new interpretation of the collective dynamics in liquid metals and, in particular, of the relaxation mechanisms ruling density fluctuations propagation. At variance with both the predictions of the current literature and the results of recent inelastic x-ray scattering experiments, ST associate the quasielastic component of the S (Q,omega ) to the thermal relaxation, as it holds in ordinary adiabatic hydrodynamics valid for nonconductive liquids and in the Q-->0 limit. We show here that this interpretation leads to a nonphysical behavior of different thermodynamic and transport parameters.

Atomic dynamics in liquids with competing interactions

The influence of the type of atomic interaction on the atomic dynamics is studied for liquid Na(x)Sn(1-x) (x = 0.9, 0.77, 0.57, 0.5, 0.33) alloys by cold neutron inelastic scattering. The dispersions obtained from the longitudinal current correlation function J(l)(Q,omega) show clear evidence for the dependence of the dynamics on the type of interaction (metallic, ionic, partly covalent) tuned by changing the composition of the alloy. For the first time, a second dispersion branch is observed in the total J(l)(Q,omega) around Q(p), the position of the principal peak of S(Q), for the Sn-rich compositions. The dynamic properties are discussed and compared to results of recent ab initio molecular dynamics simulations.

Collective dynamics in molten potassium: An inelastic x-ray scattering study

The high-frequency collective dynamics of molten potassium has been investigated by inelastic x-ray scattering, disclosing an energy/momentum transfer region unreachable by previous inelastic neutron scattering (INS) experiments. We find that a two-step relaxation scenario, similar to that found in other liquid metals, applies to liquid potassium. In particular, we show how the sound velocity determined by INS experiments, exceeding the hydrodynamic value by approximately 30%, is the higher limit of a speedup, located in the momentum region 1 < Q < 3 nm(-1), which marks the departure from the isothermal value. We point out how this phenomenology is the consequence of a microscopic relaxation process that, in turn, can be traced back to the presence of "instantaneous" disorder, rather than to the crossover from a liquid to solidlike response.