Fingerprints of order and disorder on the high-frequency dynamics of liquids

Abstract

It is largely accepted that liquids are characterized by a short-range order usually corresponding to that of the solid phase at the same density. It is less clear to what extent dynamic properties of liquids and crystals can be compared. In particular, high-frequency collective excitations reminiscent of phonons in solids exist as well in liquids. They are however traditionally discussed in terms of relaxation processes characteristic of the liquid phase. We report here on a quantitative comparison of the collective excitations in liquid and polycrystalline sodium. We show that liquid sodium exhibits acoustic excitations of both longitudinal and transverse polarization at frequencies strictly related to those of the corresponding crystal. The only relevant difference between the liquid and the polycrystal appears in the broadening of the excitations: An additional disorder-induced contribution comes into play in the case of the liquid, which we show to be related to the distribution of local structures around the average one. These results establish a direct connection between structural and dynamic properties of liquids, with shortrange order and overall structural disorder leaving very specific fingerprints.vibrational dynamics | X-ray scattering T he short-range order in liquids, characterized by parameters like, e.g., the first neighbors distance and number, is usually similar to that of the corresponding solids (1, 2). Ab initio calculations of liquid structures spanning a large density range do support this idea (3). For what concerns dynamic properties, the situation is more complex. Local vibrations appear in the liquid as in the corresponding solid, although they are usually much broader (1). This broadening is generally related to the distribution of local structures that is present in the liquid phase (4) and that is, e.g., characterized by the distribution of first neighbors distances. Collective excitations exist as well in liquids, and their broadening is usually attributed to relaxation processes (5-7). In fact, the response of a crystal to an external perturbation can be formally expressed in terms of phonons, the modes of lattice vibration that can be defined in presence of a long-range, periodic structure. In liquids this description is instead formally not appropriate, and the concept of relaxation is usually introduced with reference to a dynamic process that controls the reequilibration of the system after it has been perturbed (8). Several distinct relaxation processes can be introduced to describe the dynamics of liquids, each one characterized by a characteristic time, τ, that sets the timescale over which it is active and observable. Indeed, if the perturbation frequency, ω, is such that ωτ ≪ 1, the relaxation is active, whereas when ωτ ≫ 1 the relaxation process does not have the time to take place and is frozen. In the infinite frequency limit, where all relaxations are frozen, the liquid behaves like a solid: This limit corresponds to the elastic regime, where only the instantaneous ...

Collective modes and thermodynamics of the liquid state

“…(b): longitudinal (filled black bullets) and transverse (filled red bullets) excitations in liquid Na. Open diamonds correspond to longitudinal (black) and transverse (red) excitations in polycrystalline Na, and dashed-dotted lines to longitudinal (black) and transverse (red) branches along [111] direction in Na single crystal [70]. (c): longitudinal (black bullets) and transverse (red bullets) excitation in liquid Ga.…”

“…(51), (54), (60)) and highlyviscous liquids including in the glass transformation range (see Eqs. (69), (70), (73)) and is implicitly present throughout our discussion.…”

Boson peak and Ioffe-Regel criterion in amorphous siliconlike materials: The effect of bond directionality

“…In the mobility edge, at higher frequencies, eigenmodes have a multifractal shape characteristic of a localized behavior. We will now study the dynamical structure factor, in order to relate these observations to the study of densitydensity correlation functions, as can be tracked in neutron diffraction experiments for example [12,13]. The analysis of the dynamical structure factor allows also to discuss the Ioffe-Regel criterion for waves scattering.…”

Structural Evolution of Supercritical CO2across the Frenkel Line

“…Jj,5.70.Fh, 05.70.+a Unlike the long-range order of ideal crystalline structures, local order is an intrinsic characteristic of soft matter materials and often serves as the key to the tuning of their properties and their possible applications [1][2][3][4][5][6][7][8]. Liquid materials often exhibit medium and/or short range order, such as nanoscale structural motifs, clustering, or local ordering at the level of the atomic coordination spheres.…”