Ph.D. thesis (pdf) - dirac

6.2. Sound speed and attenuation 99
Int [arb. units]
2000
1500
1000
500
0
−10 −5 0 5 10
ω [meV]
Int [arb. units]
1000
800
600
400
200
0
−10 −5 0 5 10
ω [meV]
Figure 6.2: S coh (Q, ω) of PIB3580 at Q =2 nm −1 at room temperature and ambient
pressure (left) and 300 MPa (right). The full red line illustrates the fit to equation
6.1.2. The black curve shows the inelastic signal before convolution with the
resolution function (second term of equation 6.1.1) .
room temperature. The qualitative behavior is the same at other temperatures and
with samples of other molecular weights. The dispersion is linear up to Q=2 nm −1
where it starts bending slowly off becoming flat around Q =5 nm −1 . The result
corresponds to the dispersion generally seen for disordered materials [Ruocco and
Sette, 2001]: showing a maximum at about Q m /2, with Q m being the position of the
first structure factor maximum. Q m ≈ 10 nm −1 for PIB [Farago et al., 2002] (see also
figure 6.4). The effect of pressure is a shift of the Brillouin lines to higher frequency
(figure 6.2), corresponding to an increase in sound speed. The shift corresponds
to a change in sound speed from 2070 m/s to 2860 m/s for the PIB680 at room
temperature (the dispersion shown in figure 6.3).
10
8
300 MPa
Patm
ω [meV]
6
4
2
0
0 5 10 15 20
Q [nm −1 ]
Figure 6.3: The dispersion of longitudinal sound modes of PIB680 measured by IXS
at room temperature at atmospheric pressure and 300 MPa.
The sound speed in the glass is temperature independent within error-bars, while the