Quasi-elastic scattering under short-range order: the linear regime and beyond

Incoherent quasi-elastic scattering of neutrons or gamma rays determines the jump vectors of diffusing single atoms. Coherent quasi-elastic scattering beyond that contains additional information on the interaction between atoms. Here we firstly derive the influence of weak interactions (i.e. the high-temperature limit) on the correlation times taking an analytical approach via transition state theory, different from and, in our opinion, easier to follow than the lattice dynamics approach by Sinha and Ross. As de Gennes had already argued earlier, short-range order increases the correlation time (or equivalently narrows the quasi-elastic line) proportional to the diffuse intensity at the corresponding wavevector transfer. Secondly we show by way of Monte Carlo simulations that for strong interactions the deviations from the analytical theory bear additional information about the jump processes. We compare our theory with the results of a recent XPCS experiment.