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Collective dynamics of liquid Au: a truly golden neutron study from BRISP measurements and ILL scientific calculations

Caption Fig.1. Dynamic structure factor of gold at four Q values of the NBS experiment performed on BRISP. The experimental data (black dots) are broadened by the instrumental energy resolution. The corresponding fit by means of the Generalized Hydrodynamics model (red solid line) has been carried out taking detailed-balance asymmetry and resolution into account.

Caption Fig.1. Dynamic structure factor of gold at four Q values of the NBS experiment performed on BRISP. The experimental data (black dots) are broadened by the instrumental energy resolution. The corresponding fit by means of the Generalized Hydrodynamics model (red solid line) has been carried out taking detailed-balance asymmetry and resolution into account.

Caption Fig.2: Ratio of damping to frequency of acoustic excitations for the four liquids indicated in the figure. Data for each system have been appositely scaled to the carbon dioxide values and plotted as a function of Q/Qp (Qp being the main maximum in the static structure factor). Note that Cd and Au have the same Qp value (26 nm−1), just as it happens for CD4 and CO2 (19 nm−1). Data for Au (red stars), Cd (green squares), CO2 (pink diamonds), and CD4 (blue dots) substantially follow the same curve. The dashed line shows the hydrodynamic behavior of zs/ωs for CO2.

Caption Fig.2: Ratio of damping to frequency of acoustic excitations for the four liquids indicated in the figure. Data for each system have been appositely scaled to the carbon dioxide values and plotted as a function of Q/Qp (Qp being the main maximum in the static structure factor). Note that Cd and Au have the same Qp value (26 nm−1), just as it happens for CD4 and CO2 (19 nm−1). Data for Au (red stars), Cd (green squares), CO2 (pink diamonds), and CD4 (blue dots) substantially follow the same curve. The dashed line shows the hydrodynamic behavior of zs/ωs for CO2.

Collective dynamics of liquid Au: a truly golden neutron study from BRISP measurements and ILL scientific calculations

The knowledge about the microscopic dynamics of a fluid at equilibrium has, in the years, been increased by neutron or X-rays spectroscopic investigations of liquid metals. Indeed, these are the emblem of "simple fluids", notwithstanding basic differences from rare-gas liquids, such as the existence of an electron gas, the high sound velocity (around and above 2000 m/s, typically), and the greater, sometimes naked-eye, visibility of collective modes in the S(Q,ω) spectra, even at wave vectors near the position of the maximum in the static structure factor. Studying monatomic systems like liquid metals remains a main route to the core of collective dynamics in liquids. We therefore addressed a neutron study of a metal still unknown from the dynamic point of view: liquid gold. Experimental difficulties, mainly related to the high melting temperature and the considerable neutron absorption of gold, turned out to be not critical and allowed a first neutron Brillouin scattering experiment (0.6 < Q < 1.6 Å-1) on the BRISP spectrometer. The measurement had golden issues in many respects since it provided clean inelastic spectra rarely gifted by neutron scattering. The quality of the data thus allowed a significant comparison with recent ab initio simulations performed by the ILL Computing for Science group. The agreement found between simulation and experiment justified the further analysis of liquid gold dynamics at the higher Q-values probed by simulations (up to 7 Å-1) and led to conclude that the relevant dynamic features of the system, or of other liquid metals such as cadmium, do not substantially differ from those of insulating liquids, like methane and carbon dioxide. A very different opinion has long been wide-spread in the scientific community.

Reference

E. Guarini, U. Bafile, F. Barocchi, A. De Francesco, E. Farhi, F. Formisano, A. Laloni, A. Orecchini, A. Polidori, M. Puglini, F. Sacchetti, "Dynamics of liquid Au from neutron Brillouin scattering and ab initio simulations: Analogies in the behavior of metallic and insulating liquids", PRB 88, 104201 (2013).