The coherent dynamic structure factor $(Q, co) of liquid Cs has been measured by inelastic neutron scattering near the melting point at 308 K. Using triple-axis spectrometers at the Institut Laue-Langevin in Grenoble and at the Forschungs-Reaktor Munchen the scattering law was determined for energy transfers %co from -2 to 10 meV and for momentum transfers AQ between 0.2 and 2.55 A . The measurement has been corrected for all significant effects, including multiple and incoherent scattering as well as resolution broadening. In this paper we present mainly experimental results including a table of the measured scattering law. The analysis of the dispersion relation and the full width at half maximum of the longitudinal current correlation function J,(Q,co) reveals an anomalous dispersion due to shear relaxation in the liquid. In the vicinity of the structure-factor maximum the measured half-width of the coherent central peak of $(Q, co) confirms recent theoretical assumptions of a collective-diffusion-like structural relaxation process in dense liquids near the melting point. PACS number(s): 62.15.+i, 67.40.Fd
The paper presents the first neutron scattering investigation of the static structure factor S(Q) of expanded liquid cesium from the melting point up to its critical point at conditions near the liquid‐vapour coexistence curve (critical data of cesium: Tc = 1924 K, pc = 92.5 bar, dc = 0. 379 g/cm3). The characteristic changes of the microscopic structure — such as the distance and number of nearest neighbours — are derived as a function of density. The reported results give useful information about the density dependent changes in the electronic structure in course of the transition from a metallic to a nonmetallic state which occurs in the expanded metal. In particular, the results provide direct experimental evidence for the limitation of the nearly‐free‐electron (NFE) model in describing the electrical transport properties of low density fluid cesium.
The paper presents a neutron diffraction investigation of the static structure factor S(Q) of liquid cesium expanded by heating toward conditions close to its liquid-vapor critical point. The experiment was designed to obtain the isothermal density derivative of S(Q) which is related to the triplet correlation function. The data give the first information on the density-dependent changes in the interatomic forces as the metal-nonmetal transition is approached in the expanded liquid. In particular, the results provide direct experimental evidence for the limitation of the uniform fluid model.
In a neutron scattering experiment on liquid caesium a positive dispersion of the collective modes has been observed, indicating the existence of shear relaxations in the liquid. The measured solid-like high-frequency sound propagation with velocity cx>cs-with cs being the well known adiabatic velocity of sound-yields the experimental determination of the shear modulus Gx in the fluid for the first time and confirms the evidence that the observed positive dispersion of collective modes is due to viscous shear relaxation beyond the hydrodynamic limit of a dense liquid near the melting point.
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