Ph.D. thesis (pdf) - dirac

8.3. Boson Peak and fragility 159
1.2
1
g(ω)/ω 2 2
/[g(ω BP
)/ω BP ]
0.8
0.6
0.4
0.2
0
0 1 2 3 4 5 6
ω/ω BP
(P)
Figure 8.10: The boson peak at different pressures atmospheric pressure (•), 4 MPa
() 8 MPa (), and 14 MPa (). The data scaled with the boson peak intensity
and the boson peak position. The dashed line shows a 1/ω-behavior, the full line is
exp(−ω/ω 0 ).
8.3 Boson Peak and fragility
In this section we will discuss the proposed correlation between the relative intensity
of the boson peak and the fragility. The suggestion is that larger fragility should
correlate to smaller relative intensity of the boson peak. This correlation is supported
by data from several glass formers [Sokolov et al., 1993, 1997; Rössler and Sokolov,
1996; Novikov et al., 2005] though it has also been contested [Yannopoulos and
Papatheodorou, 2000].
Two different measures are suggested for quantifying the relative intensity of the
boson peak: (i) by normalizing the boson peak to the Debye density of states [Sokolov
et al., 1997] (see also section 2.7.3) or (ii) by normalizing the boson peak intensity
at T g to the intensity of quasi-elastic scattering at T g [Sokolov et al., 1993]. In the
latter case a parameter, R, is defined as the quasi-elastic intensity divided by the
boson peak intensity evaluated at T g . Of these two measures R is by far the easiest to
evaluate experimentally, because it does not require knowledge of S(Q, ω) in absolute
values, nor does it require knowledge of the sound speeds of the system. Moreover, it
has the advantage of being evaluated in the equilibrium liquid. For these reasons we
will focus on the correlation with R in the main part of this chapter. It is suggested
by Novikov et al. [2005] that the two measures of boson peak intensity essentially