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 $\ensuremath{\kappa}$, and the anomalous temperature dependence of ultrasonic velocity, $\frac{\ensuremath{\Delta}v}{v}$. It is therefore likely that $\ensuremath{\kappa}$ and $\frac{\ensuremath{\Delta}v}{v}$ 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 ${C}_{\mathrm{ex}}$, $\ensuremath{\kappa}$, and $\frac{\ensuremath{\Delta}v}{v}$ 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.