Ph.D. thesis (pdf) - dirac

150 Boson Peak
comparison of the evolution in the boson peak intensity as compared to the intensity
of the Debye level. The aim of this section is to better understand the origin of the
boson peak. The question of whether there is a relation between the boson peak
and the fragility is attacked in the following section (8.3).
8.2.1 Boson Peak as a function of pressure
In figure 8.5 we show the rDOS of PIB3580 measured at 4 different pressures ranging
from atmospheric pressure to 1.4 GPa. A well resolved boson peak is seen at all
pressures. The peak position increases as a function of pressure with a slope of ∼
0.15 meV / 100 MPa (inset of fig. 8.5) in the whole range. The peak position shifts
by a factor 2 from atmospheric pressure to 1.4 GPa.
The second striking effect of pressure is that it reduces the boson peak intensity.
However, at high frequencies, from above roughly 8 meV there are no significant
changes in the spectra.
g(ω)/ω 2 [arb. unit]
1.8
1.6
1.4
1.2
1
0.8
0.6
ω BP
[meV]
5
4
3
2
0 500 1000 1500
Pressure [MPa]
0.4
0.2
0
0 2 4 6 8 10 12 14 16 18
ω [meV]
Figure 8.5: The rDOS (g(ω)/ω 2 ) of PIB at different pressures; atmospheric pressure
(•), 4 MPa () 8 MPa (), and 14 MPa (). The inset shows the peak positions
as a function of pressure. The full lines is the best straight line. The dashed
line is the best fit to equation 19 of ref. Gurevich et al. [2005], P 0 =204 MPa and
ω BP = 2.13 meV.
The boson peak has mainly been studied at different temperatures and in glasses
with different thermal history, while few studies report the effect of pressure on the
boson peak. Moreover, the existing studies are mainly performed by Raman scattering
[Andrikopoulos et al., 2006; Schroeder et al., 2004; Yamaguchi et al., 1999; Sugai