Ph.D. thesis (pdf) - dirac

152 Boson Peak
In figure 8.6 we show the relative peak shift with pressure of transverse sound modes,
longitudinal sound modes at low and high Qs and of the boson peak, as a function
of pressure. All modes are shifted to higher frequency with increasing pressure. We
have calculated the change in density with pressure using the equation of state of
Sanchez and Cho [1995] in the melt and the expansion coefficient α P = 10 −4 K −1 at
all pressures in the glass. Defining the Gruneisen parameter as dlog ω peak
d log ρ
we find a
value of 2.7 and 3.7 for the IXS mode at 2 nm −1 and the boson peak respectively.
This clearly shows that the boson peak is more sensitive to pressure than any of
the other modes. This result is consistent with the result of Schroeder et al. [2004]
who find that changes of the boson peak frequency in Raman spectra under pressure
were stronger than variations of the sound velocities. On the contrary it is found
in a recent study by Monaco et al. [2006 b] of a permanently compressed inorganic
glass that the shift in the boson peak corresponds to that of the sound speeds (more
precisely they compare to the Debye-frequency, see below for the definition). In the
study by Monaco et al. [2006 b] the density changes were quite small (∼ 6%), while
considerably larger variations (∼ 20%) are achieved in our measurements. This
could be part of the explanation of the different result as it is apparent from figure
8.6 that the differences increase with increasing pressure.
The sound modes measured by BLS are at much smaller frequencies than the boson
peak, while the sound modes measured by IXS are at frequencies of the same order of
magnitude as the boson peak frequency. It could therefore be argued that the boson
peak should be compared to the latter. In line with this, it has been suggested,
based on temperature dependences in silica glass, that the boson peak position is
stronger coupled to the behavior of the high frequency IXS sound modes than the
low frequency BLS sound modes [Masciovecchio et al., 1999]. Our results on PIB
do not support this view as we find that the longitudinal sound speed measured by
BLS agrees with the sound speed found by IXS (figure 8.7). Moreover, the pressure
dependence of the two is the same in the limited pressure range where both are
studied. Based on this we assume that the IXS and the BLS modes have similar
pressure dependences in the whole range, and conclude that the boson peak position
has a more dramatic pressure dependence than the sound modes in the same energy
range.
Note also in figure 8.7 that the position of the boson peak seen in the rDOS is in
the regime where the dispersion of the longitudinal sound is linear.