Ph.D. thesis (pdf) - dirac

Chapter 7
Mean squared displacement
In this chapter we report mean squared displacements at the nanosecond timescale
measured by neutron scattering. We study the mean squared displacement as a
function of temperature in 5 different molecular liquids at atmospheric pressure, 3
of these systems are also studied at elevated pressure. The liquids are chosen such
that they span a range from ∼ 45 to ∼ 150 in isobaric fragility. The mean square
displacement at this time scale has been shown to change its temperature dependence
in the vicinity of the calorimetric glass transition temperature. This is one
of the earliest observations of a relation between fast and slow dynamics [Buchenau
and Zorn, 1992], and several different scenarios have been suggested to explain it
[Angell, 1995; Ngai, 2000; Starr et al., 2002; Dyre and Olsen, 2004]. We discuss
these scenarios, the predictions one can draw from them and the agreement with
data in sections 7.3 to 7.5. A common feature of these scenarios is that they predict
a correlation between the temperature dependence of the mean square displacement
and the fragility. We therefore use the arguments developed in section 3.4 in order
to consider the effect of temperature and density.
7.1 Backscattering
7.1.1 Experimentals
The experiments were carried out on the back scattering instrument IN10 at the
ILL. The experiments performed where so called elastic scans, in which the elastic
scattering was measured as a function of temperature. This is done by using the
instrument in a mode where the incoming neutron has a fixed wavelength of 6.27 Å
and one measures the outgoing flux at the same energy. The outgoing neutrons were
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