A coherent-inelastic-neutron-scattering experiment was performed on liquid D20 at room temperature. We observe for the first time the collective high-frequency sound mode as predicted by computer molecular-dynamics simulations. We interpret the excitation to be a mode propagating within the hydrogen-bonded patches existing in liquid water, and distinct from the ordinary sound wave.PACS numbers: 61.12.Fy, 61.25.Em Short-wavelength collective excitations in dense atomic liquids are in general difficult to observe by coherent inelastic neutron scattering. We may define the evidence of the collective excitations by the existence of a peak or a shoulder in the dynamic structure factor S(Q, co) on each side of the central line, in the Q range of, say, 0& Q & 1.2 A '. The neutronscattering experiments performed so far indicate that, for neon at a liquid density 1.2 g cm 3 and T= 26.5 K, the excitation exists up to Qo-~1. 0, and for liquid rubidium and lead near their melting points up to Qo.~4. 0, 2 where a-is the effective hard-core diameter of the atoms. From the computer moleculardynamics simulations of hard spheres by Alley and coworkers, 4 it was established that the collective excitation exists up to Qo.~0.5. It was generally believed that the damping of the short-wavelength excitations depends critically on both the steepness of the repulsive part and the depth of the attractive part of the potential. For liquid metals, the repulsive part is generally softer and the attractive well deeper. This may be the reason why the excitation is observable up to higher Q. More recently, the existence of a collective mode in simple fluids has been discussed explicitly in terms of the kinetic theory of dense fluids. s These authors claimed that collective excitations can be defined in dense fluids up to Qo-~30 and that the behavior of S(Q, co) is dominated by these modes for all Q in which they exist. 5 In this paper we report for the first time an observation of new collective modes in a molecular fluid: water. From a point of view of the intermolecular interaction, water is a rather special case. Take, for instance, the ST2 model potential which has been shown to be successful in predicting various properties of water. 6 This potential consists of superposition of a Lennard-Jones potential between the molecular centers and directional electrostatic interactions which mimic the hydrogen bonding. A new feature of this potential, as compared with that of the simpler liquids, is the presence of a directional and strong attraction between molecules. Our experimental results lead us to conjecture the existence of a new collective mode which we shall call schematically the "high-frequency mode, " which is different from the ordinary sound wave in water. This kind of mode was already predicted in an earlier computer molecular-dynamics simulation (CMD) of Rahman and Stillinger and more recently also by Impey, Madden, and McDonald. We shall, in the following, compare our experimental results with these CMD predictions.The experiment...

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

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The magnetic structure in the ordered phase of the nearly one-dimensional Heisenberg antiferromagnet has been measured using elastic neutron scattering. crystallizes in the orthorhombic Pnma space group with spin chains running along the crystallographic b-direction. Below the ordering temperature the magnetic structure is incommensurate along the chain direction with a temperature-independent ordering wavevector q = (0, 0.472, 0) (rlu}). The occurrence of an incommensurate structure is shown to be the consequence of frustration on the spins induced by the exchange interaction between chains. Group theory is used to determine the possible magnetic structures compatible with the symmetry of the crystal. The results show that at T = 0.3 K the spin ordering is cycloidal with spins rotating in a plane that contains the propagation direction b. A mean-field calculation of the magnetic ground-state energy including exchange anisotropy effects is used to study the stability of the observed structure. Values for the interchain exchange constants that are consistent with the features of the magnetic structure are proposed.

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