Ph.D. thesis (pdf) - dirac

154 Boson Peak
Predictions/explanations by the models
The most detailed prediction regarding the peak position is given for the SPM
model by Schober and coworkers in terms of the position of the peak in the rDOS.
The prediction is that the peak position should follow a (1 +P/P 0 ) (1/3) -dependence
[Gurevich et al., 2005]. The best fit to our data (inset of figure 8.5), is not convincing,
as we find no significant deviation from linear pressure dependence in the studied
pressure range. However, the change in elastic constants is not considered in the
model and this obscures the comparison. Moreover, it is important to note that
we compressed the sample in the melt and not in its glassy state. A different
path of compression would lead to a different density change and the boson peak
position would be different too [Chauty-Cailliaux, 2003]. Another complication when
comparing to the SPM prediction, is that it does not take the shift in the sound
speeds into account.
There are no explicit predictions regarding the pressure dependence of the boson
peak for other models. Phonons localized in nanoscale domains (blobs) yield the
relation ω BP = av/L where v is the sound speed and L is the size of the blobs
[Duval et al., 1990; Schroeder et al., 2004; Quitmann and Soltwisch, 1998]. We find
that ω BP increases more with pressure than the sound speed. In order for the above
picture to be correct it is therefore required that the domain size decreases with
pressure. There is no experimental evidence for the existence of blobs and it is
therefore difficult to anticipate their dependence on pressure.
In the FEC model of Schirmacher and coworkers, in which the boson peak is due to
the fluctuation of the elastic constants, it is predicted that the boson peak position
shifts to higher energies if the sample gets more ordered in the sense that the amplitude
of the fluctuations in the elastic constant is decreased [Maurer and Schirmacher,
2004]. This means that the shift in the boson peak, which we report should be due
to a decrease in disorder when the liquid is compressed. It is easy to imagine that
the compression and the resulting change in the packing of the molecules will lead
to more structural order and therefore also less fluctuations of the elastic constants.
8.2.3 Boson peak intensity
The shift in energy should both for the SPM and the FEC model be accompanied
with a decrease in boson peak intensity. The decrease of the boson peak is also seen
in the raw data (figure 8.5). However, the Debye level also changes as a function of
pressure. This means the that intensity of the rDOS intrinsically decreases at low
energies. This effect is not considered in the models. Comparing data to models, it