Effect of neutron irradiation on the density of low-energy excitations in vitreous silica

Abstract

Neutron irradiation of vitreous silica produces changes of the same relative magnitude in the low-temperature excess specific heat $C_{\mathrm{ex}}$, the anomalous thermal conductivity $\kappa$, and the anomalous temperature dependence of ultrasonic velocity, $\frac{\Delta v}{v}$. It is therefore likely that $\kappa$ and $\frac{\Delta v}{v}$ are dominated by the localized excitations responsible for the excess specific heat of vitreous silica. The tunneling-states model with a single energy-dependent density of states adequately accounts for the measured $C_{\mathrm{ex}}$, $\kappa$, and $\frac{\Delta v}{v}$ of unirradiated vitreous silica. Changes in these properties with neutron irradiation can be explained using this model by altering only the density of tunneling states.