Frequency dependent specific heat of viscous silica

Abstract

We apply the Mori-Zwanzig projection operator formalism to obtain an expression for the frequency dependent specific heat c(z) of a liquid. By using an exact transformation formula due to Lebowitz et al., we derive a relation between c(z) and K(t), the autocorrelation function of temperature fluctuations in the microcanonical ensemble. This connection thus allows to determine c(z) from computer simulations in equilibrium, i.e. without an external perturbation. By considering the generalization of K(t) to finite wave-vectors, we derive an expression to determine the thermal conductivity λfrom such simulations. We present the results of extensive computer simulations in which we use the derived relations to determine c(z) over eight decades in frequency, as well as λ. The system investigated is a simple but realistic model for amorphous silica. We find that at high frequencies the real part of c(z) has the value of an ideal gas. c'(ω) increases quickly at those frequencies which correspond to the vibrational excitations of the system. At low temperatures c'(ω) shows a second step. The frequency at which this step is observed is comparable to the one at which the α-relaxation peak is observed in the intermediate scattering function. Also the temperature dependence of the location of this second step is the same as the one of the $α-$peak, thus showing that these quantities are intimately connected to each other. From c'(ω) we estimate the temperature dependence of the vibrational and configurational part of the specific heat. We find that the static value of c(z) as well as λare in good agreement with experimental data.