Emmanuel Farhi

Virtual experiments

A new sample component is presented for the Monte Carlo, ray-tracing program, McStas, which is widely used to simulate neutron scattering instruments. The new component allows the sample to be described by its material dynamic structure factor, which is separated into coherent and incoherent contributions. The effects of absorption and multiple scattering are treated and results from simulations and previous experiments are compared.The sample component can also be used to treat any scattering material which may be close to the sample and therefore contaminates the total, measured signal.

Structure and dynamics ofl−Ge: Neutron scattering experiments andab initiomolecular dynamics simulations

We report the first measurements of the dynamics of liquid germanium (l-Ge) by quasielastic neutron scattering on time-of-flight and triple-axis spectrometers. These results are compared with simulation data of the structure and dynamics of l-Ge which have been obtained with ab initio density functional theory methods. The simulations accurately reproduce previous results from elastic and inelastic scattering experiments, as well as the q dependence of the width of the quasielastic signal of the new experimental data. In order to understand some special features of the structure of the liquid we have also simulated amorphous Ge. Overall we find that the atomistic model represents accurately the average structure of real l-Ge as well as the time dependent structural fluctuations. The quasielastic neutron scattering data allows us to investigate to what extent simple theoretical models can be used to describe diffusion in l-Ge.

Observation of macroscopic structural fluctuations in bcc solid {sup 4}He

We report neutron diffraction studies of low density bcc and hcp solid {sup 4}He. In the bcc phase, we observed a continuous dynamical behavior involving macroscopic structural changes of the solid. The dynamical behavior takes place in a cell full of solid, and therefore represents a solid-solid transformation. The structural changes are consistent with a gradual rotation of macroscopic grains separated by low angle grain boundaries. We suggest that these changes are triggered by random momentary vibrations of the experimental system. An analysis of Laue diffraction patterns indicates that in some cases these structural changes, once initiated by a momentary impulse, seem to proceed at a constant rate over times approaching 1 hour. The energy associated with these macroscopic changes appears to be on the order of kT. Under similar conditions (temperature and pressure), these effects were absent in the hcp phase.