  Below 1 K, the specific heat Cp of glasses depends approximately linearly on
temperature T, in contrast with the cubic dependence observed in crystals, and
which is well understood in terms of the Debye theory. That linear contribution
has been ascribed to the existence of two-level systems as postulated by the
Tunnelling Model. Therefore, a least-squares linear fit Cp = C_1 T + C_3 T^3
has been traditionally used to determine the specific-heat coefficients, though
systematically providing calorimetric cubic coefficients exceeding the elastic
coefficients obtained from sound-velocity measurements, that is C_3 > C_Debye.
Nevertheless, Cp still deviates from the expected C_Debye proportional to T^3
dependence above 1 K, presenting a broad maximum in Cp/ T^3 which originates
from the so-called boson peak, a maximum in the vibrational density of states
g(f)/f^2 at frequencies around 1 THz. In this work, it is shown that the
apparent contradiction between calorimetric and elastic Debye temperatures long
observed in glasses is due to the neglect of the low-energy tail of the boson
peak (which contribute as Cp proportional to T^5, following the Soft-Potential
Model). If one hence makes a quadratic fit Cp = C_1 T + C_3 T^3 + C_5 T^5 in
the physically-meaningful temperature range, an agreement C_3 = C_Debye is
found within experimental error for several studied glasses.
