Ph.D. thesis (pdf) - dirac

8.3. Boson Peak and fragility 163
1.2
1
S(omega)/S(ω BP
)
0.8
0.6
0.4
0.2
P=1400 MPa
Patm
0 1 2 3 4 5
ω / ω BP
Figure 8.15: The boson peak of PIB3580 at atmospheric pressure and at 1.4 GPa
at T ≈ T g (P). The axis are scaled with the boson peak position and intensity. The
data are corrected for the temperature dependent bosefactor.
from this scaling that R is independent of pressure. Two systems are not sufficient
to state that this is a universal behavior of R. However if the finding is general, then
it strongly supports the suggestion that the correlation between R and the isochoric
fragility m ρ , as the latter also stays constant as a function of pressure along the
T g -line (chapter 3).
8.3.3 The quasi-elastic intensity
The quantity R is a measure of the boson peak intensity relative to the minimum
intensity at the low energy side of the peak. This latter is sometimes clearly influenced
by quasi elastic scattering. It is therefore not a priori the boson peak intensity
that determines R. In figures 8.18 and 8.19 we show the S(ω) of two liquids which
both follow the correlations seen in figures 8.11 and 8.12 namely DBP and DHIQ.
For both samples we show the data at T = 100 K and at T = T g = 180 K, (all
data at atmospheric pressure). The spectra deep in the glass at T = 100 K are
quite similar for the two liquids, while this is not the case at T g . For the less fragile
DBP the main effect is an increase in intensity due to the temperature dependence
of the bose factor. In the case of the very fragile DHIQ on the other hand, there is
a quasi-elastic signal at T g which totally swarms the boson peak. Hence, it is this
quasi-elastic signal, rather than the nature of the boson peak which gives DHIQ its
large value of R. This observation questions the proposed relation between R and