Transverse excitations in liquid Fe, Cu and Zn

Abstract

Transverse acoustic (TA) excitation modes were observed in inelastic x-ray scattering spectra of liquid Fe, Cu and Zn. From the analysis of current correlation functions, we concluded that TA excitation modes can experimentally be detected through the quasi-TA branches in the longitudinal current correlation spectra in these liquid metals. The microscopic elastic constants are estimated and a characteristic difference from macroscopic polycrystalline value was found in Poisson's ratio of liquid Fe, which shows an extremely softer value of ∼0.38 compared with the macroscopic value of ∼0.275. The lifetime of the TA modes were determined to be ∼0.45 ps for liquid Fe and Cu and ∼0.55 ps for liquid Zn, reflecting different interatomic correlations between liquid transition metals and non-transition metals. The propagation length of the TA modes are ∼0.85 nm in all of liquid metals, corresponding to the size of icosahedral or similar size of cages formed instantaneously in these liquid metals.

Structure and dynamics of liquid Zn: an analysis of ab-initio simulations

“…This prompted the experimentalists to suggest the existence of a second propagating mode of energy lying within this region, and showed that a t based on two propagating modes produces better results than a t based on just one of them. Specically, they used a Lorentzian function to account for the central peak and a damped harmonic oscilator (DHO) for each of the propagating modes [4]. The variation with q of the frequencies of the modes obtained from the ts leads to a dispersion relation (DR) with two branches, a high frequency one that displays a positive dispersion with respect to the hydrodynamic sound velocity, and that is similar to the usual DR in liquid systems, plus a lower energy branch, which has been interpreted as a ngerprint of tranverse waves in the dynamics of density uctuations.…”

“…The variation with q of the frequencies of the modes obtained from the ts leads to a dispersion relation (DR) with two branches, a high frequency one that displays a positive dispersion with respect to the hydrodynamic sound velocity, and that is similar to the usual DR in liquid systems, plus a lower energy branch, which has been interpreted as a ngerprint of tranverse waves in the dynamics of density uctuations. The existence of these two modes was rst suggested for liquid Ga and later also for other liquid metals (Sn, Fe, Cu, Na) [4,13], based on the similarity between the frequencies observed and those of transverse excitations either in the corresponding solid or in the liquid. However, at present no theoretical approach has been able to explain their appearance.…”

“…shown in the legend. Circles: experimental results [4]. Lines: AIMD data (modied for proper comparison with experiment, see text).…”

“…The encircled region corresponds to the anomalous shape associated to the low energy excitation. The inset shows the dispersion relations for q < qp=2 as obtained from INS (circles) [5], IXS (squares) [4], and AIMD (triangles). The dashed line corresponds to the hydrodynamic sound velocity (2780 m/s) [12].…”

“…This asymmetry decreases upon increasing temperature. Finally, concerning its dynamic properties, recent measurements performed using either inelastic x-ray scattering (IXS) at 713 K or inelastic neutron scattering (INS) at 773 K, have concluded that the data is compatible with the existence of two types of propagating excitations in the density uctuations for wavevectors around q p =2, where q p denotes the position of the main peak of S(q) [4,5].…”