Ph.D. thesis (pdf) - dirac

156 Boson Peak
meaning that the Debye level essentially is given by
rDOS D (ω) ∝ 1
ρvt
3 . (8.2.6)
In a Debye situation the hardening of the system (increase of elastic constants)
pushes the Debye density of states. The Debye level gets lower and the cut-off
frequency gets higher, while the total number of states of course stays constant.
The intensity of rDOS will therefore naturally get lower, due to the decrease of the
Debye level.
1
Relative intensity
0.8
0.6
0.4
0.2
BP intensity
ω D
−3
0
0 500 1000 1500
Pressure [MPa]
Figure 8.8: The Debye level (•) and the boson peak intensity () as a function of
pressure.
In figure 8.8 we illustrate the decrease in the Debye level as a function of pressure.
We compare this decrease to the decrease in the boson peak intensity. It is seen that
the amplitude of the boson peak decreases less than the Debye level. Consequently
we conclude that the boson peak intensity relative to the Debye level increases as a
function of pressure. The analysis of the boson peak intensity relative to the Debye
level was also performed in the study of densified glasses by Monaco et al. [2006 b]
and in an earlier study by the same group on a hyper-quenched glass [Monaco et al.,
2006 a]. The finding in these studies was that boson peak intensity relative to the
Debye level was unchanged. The difference between their conclusion and our finding
could (as for the peak position discussed above) be due to the much larger change
of density in the present study. Alternatively it is quite possible that the behavior is
not universal but will depend on specific properties of the glass. Thirdly it is worth