Low-temperature specific heat measurements have been performed in porous si
lica xerogels with densities varying from 670 to 1730 kg m(-3) to study the
low-energy vibrational dynamics. The specific heat, C-p, shows a bump in t
he temperature range above 4 K, when reported in a plot of C-p/T-3 against
the temperature, T. The bump is almost independent of the sample density an
d is close to the boson peak observed in melt-quenched amorphous silica (a-
SiO2). At temperatures <4 K, an additional contribution to that predicted b
y the Debye theory is observed. It follows an approximately linear temperat
ure dependence (C-exc = a T (1+nu), nu being equal to about 0.25). In the x
erogel with the largest density, specific heat of about a factor 5 larger t
han that of a-SiO2 is measured, which increases with decreasing sample dens
ity. By comparison with the corresponding properties of a-SiO2, we conclude
that the disorder introduced by the presence of pores does not measurably
affect the excess density of vibrational states in a frequency range of the
boson peak (BP), but increases the density of the two-level systems (TLS).
(C) 2001 Elsevier Science B.V. All rights reserved.