Experimental evidence for the presence of nonacoustic excitations in molten Ga

Abstract

The nature of the broad band of microscopic excitations measured in molten gallium is investigated by means of analysis of their mode eigenvectors I͑Q , ͒. The results, derived from neutron spectroscopy experiments, show that excitations with a dominant acoustic character are confined to low energy transfers. In contrast, those appearing at higher frequencies unmistakably exhibit optical character, as evidenced by the out-of-phase relationship shown by the phase of oscillations in I͑Q , ͒ that become significantly shifted relative to those of the static structure factor. Such effects are shown to arise from partial covalency that is already known to be present in gaseous, cluster and crystalline forms.

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

“…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]. In this latter interpretation the broad mode represents localized collective excitations that already herald the onset of the solid phase [5]. However, neither interpretation can explain the observed intensity (Fig.…”

“…Both systems show a broad contribution to the spectra that exceeds the intensity found in computer simulations by at least an order in magnitude. We illustrate this for the case of liquid Ga as measured by Bermejo et al [5] and who determined the intensity of this broad mode as a function of q. First, we note that Bermejo et al do not interpret this broad mode as being caused by cage-diffusion, rather they attribute it to a coherent process.…”

“…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]. In this latter interpretation the broad mode represents localized collective excitations that already herald the onset of the solid phase [5].…”

“…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.…”

“…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].…”