Abstract
Based on the temperature response of a relaxation type calorimeter, the heat capacity of vitreous silica has been determined between 0.35 K and 30 K and in magnetic fields up to 8 T, for several high purity glasses with different hydroxyl contents and native defects. It is found that the heat capacity of synthetic silica glasses without native defects is composed of contributions due to phonons and intrinsic low-energy excitations, which are unaffected by magnetic fields. A thermodynamically consistent description of the low-energy excitations can be given in terms of the tunneling state (TS) model. The intrinsic value of the TS density of state for vitreous silica is determined to be \(\bar P\) = 0.92 · 1045J−1m−3 and increases with increasing hydroxyl content. The superlinear temperature dependence of the heat capacity found to be valid down to 15 mK is phenomenologically described by a minimal energy splitting of Δ0,min/kB ≈ 2 mK. Magnetic contributions to the specific heat observed in Suprasil W are interpreted as excited triplet states of native point defects and have been investigated in fields up to 8 T.
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