Ph.D. thesis (pdf) - dirac

Chapter 4
Experimental techniques and
observables
We have so far discussed dynamics in rather loose terms as the relaxations and
vibrations taking place in the liquid. This notion will be precised in this chapter,
where we introduce the dynamical variables we have studied, namely the dielectric
response function and the dynamical structure factor. In this chapter we also present
the basic principles for measuring these quantities while the experimental details
related to the specific methods are given in the beginning of the respective chapters.
4.1 Linear response and two-time correlation functions
The dielectric response we study is the linear response and the dynamical structure
factor is a two-time correlation function. Linear response and two time correlation
function are related via the fluctuation dissipation theorem (FD-theorem), and they
therefore probe the same type of information. We shall not consider other types of
dynamical properties, even if non-linear response and four (or more) time correlation
functions are also considered important for understanding the dynamics in viscous
liquids.
4.1.1 Linear response
When a change of conditions is imposed on a system its equilibrium is changed
and other properties will therefore adjust to this new equilibrium. Such a change in
condition is called an input and the time dependent readjustment of other properties
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