Vibrational dynamics of liquid gallium at 320 and970K

Abstract

The microscopic ion dynamics of liquid gallium was investigated at 320 K-that is, just above the melting point-and 970 K by inelastic neutron scattering experiments and molecular dynamics simulations. The high quality of the experimental data allowed the observation of density fluctuation modes extending up to 1.0 Å −1 and existing at both temperatures. At melting, an acousticlike mode propagating with a velocity definitely exceeding the sound velocity was observed, in agreement with the results of a recent inelastic x-ray scattering experiment. The mode velocity and damping were found to be almost temperature independent. The experimental response function was compared with the results of a molecular dynamics simulation, based on a simple model for the effective ion-ion potential which, however, did not contain any temperature-dependent parameter. The result worth noting is that, despite the simple potential, the simulation was capable to reproduce all the observed features of the measured dynamicstructure factor quantitatively and at both the temperatures.

Polarized neutron scattering investigation of excitations at low momentum transfer in liquid Ga: The mystery continues

“…We will revisit this latter interpretation in the later parts of this paper. Irrespective of its interpretation, it can be seen from comparison to the measured static structure factor S(q) (which agrees very well [3] with the simulated static structure factor) that the intensity of the broad mode violates the S(q) sum-rule on the spectra in the low qrange. Thus, a broad mode has been observed in neutron scattering spectra; the line width of this broad mode (of the order of a few meV) allows for it to be interpreted as part of the cage-diffusion process, or alternatively, as a separate coherent excitation [5].…”

“…[5] so that the intensity at the peak of the static structure factor S(q)( q =2.52 Å − 1 ) coincides with the peak intensity of S(q) (shown as the solid line in this panel). Note that the amplitude of the quasi-elastic contribution follows the independently measured [21] and simulated [3] S(q) for wave numbers q N 2Å − 1 , but also that the amplitude of the broad mode for q b 2Å − 1 greatly exceeds S(q). This excess intensity at low q does not have a satisfactory explanation.…”

“…Surprisingly however, it was reported [7] that cage diffusion could be observed directly in neutron scattering experiments at low momentum transfer as a contribution separate from the standard three hydrodynamic modes and extensions thereof to finite q. This separate contribution, directly observable in liquid Hg [7,9] and Ga [3,5] and inferred from scattering data on liquid Pb [14,15], has not been seen in x-ray experiments [6,16,17], nor has it been observed in computer simulations with a strength anywhere near comparable to what has been seen in neutron scattering experiments, as we review in the following. The purpose of this paper is to test whether a new interpretation in terms of shortlived magnetic excitations that appears to provide a way out of this conundrum [9,15] actually works for liquid gallium.…”

“…Published neutron scattering experiments [3,5] showed liquid Ga to be a good candidate for testing the possibility of a magnetically enhanced cage diffusion mode at low q. For instance, the data by Bermejo et al [5] show (Fig.…”

“…Benefitting from the increased capabilities of neutron and X-ray scattering facilities, it is now possible to get highly accurate data on the microscopic origins of the dynamic response unique to liquid metals. Not only has the propagation of short wavelength sound modes been investigated in a series of liquid metals such as Ga [3][4][5][6], Hg [7][8][9][10] and alkali metals [11][12][13], the details of the relaxation mechanism of density fluctuations in general have been probed and modeled [1,2,5]. One of the main results to come out of these experiments was the role that cage diffusion plays in liquid metals for fluctuations over all length scales [1,2].…”