Ph.D. thesis (pdf) - dirac

104 High Q collective modes
3200
3000
2800
v l
[m/s]
2600
2400
Patm Q=2nm −1
300MPa Q=2nm −1
2200
Patm Q=4nm −1
300MPa Q=4nm −1
2000
0 50 100 150 200
T [K]
Figure 6.11: Sound speed of cumene calculated at Q=2 nm −1 and Q=4 nm −1 respectively,
as a function of temperature at ambient pressure and at 300 MPa.
pressure dependence of the sound speed in the glass is very weak if present at all.
Γ/Q 2 [meV nm 2 ]
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Patm Q=2nm −1
300MPa Q=2nm −1
Patm Q=4nm −1
300MPa Q=4nm −1
−0.1
0 50 100 150 200
T [K]
Figure 6.12: Sound attenuation of cumene as a function of temperature. The results
are shown in terms of Γ/Q 2 , to illustrate that the data are consistent with a Γ ∝ Q 2
behavior.
The sound attenuation is shown in figure 6.12. We do not determine the Q-dependence
of the sound attenuation directly since we, as in the case of PIB, have few Q values.
In figure 6.16 we plot Γ/Q 2 at Q=2 nm −1 and Q=4 nm −1 respectively, as a function
of temperature and at ambient pressure as well as at 300 MPa. It is seen that Γ/Q 2
is Q-independent, except in the melt at ambient pressure, meaning that the Q dependence
generally is consistent with the Γ ∝ Q 2 behavior which is often found in
glasses [Ruocco and Sette, 2001]. It is moreover seen that the pressure dependence
of Γ is very weak, with only a slight shift to lower values at high temperatures,