Neutron scattering and molecular correlations in a supercooled liquid

TL;DR

This paper develops a molecular correlation expansion for neutron scattering functions in supercooled liquids, demonstrating its convergence and revealing the importance of translational and orientational coupling, with implications for understanding intermediate range order.

Contribution

It introduces a new expansion of neutron scattering functions based on molecular correlations, providing a detailed analysis of its convergence and physical implications in supercooled liquids.

Findings

The expansion converges well for the model system.

Translational and orientational degrees of freedom are strongly coupled in supercooled liquids.

A prepeak in the static structure factor indicates intermediate range orientational order.

Abstract

We show that the intermediate scattering function $S_n(q,t)$ for neutron scattering (ns) can be expanded naturely with respect to a set of molecular correlation functions that give a complete description of the translational and orientational two-point correlations in the liquid. The general properties of this expansion are discussed with special focus on the $q$-dependence and hints for a (partial) determination of the molecular correlation functions from neutron scattering results are given. The resulting representation of the static structure factor $S_n(q)$ is studied in detail for a model system using data from a molecular dynamics simulation of a supercooled liquid of rigid diatomic molecules. The comparison between the exact result for $S_n(q)$ and different approximations that result from a truncation of the series representation demonstrates its good convergence for the given model system. On the other hand it shows explicitly that the coupling between translational (TDOF) and orientational degrees of freedom (ODOF) of each molecule and rotational motion of different molecules can not be neglected in the supercooled regime.Further we report the existence of a prepeak in the ns-static structure factor of the examined fragile glassformer, demonstrating that prepeaks can occur even in the most simple molecular liquids. Besides examining the dependence of the prepeak on the scattering length and the temperature we use the expansion of $S_n(q)$ into molecular correlation functions to point out intermediate range orientational order as its principle origin.