Low-temperature specific heat of compacted vitreous silica

We present low-temperature specific-heat measurements of two types of irreversibly compacted vitreous silica (Suprasil W and Suprasil I, differing in their OH⁻ content). In the whole temperature range investigated (0.1 K<T<35 K), the specific heatC is reduced by up to 50% compared to that of uncompacted samples but exhibits a similar temperature dependence, with the characteristic maximum inC/T ³ shifted to higher temperatures. This coherent change ofC, which is roughly the same for both types of vitreous silica, gives strong evidence that the low-energy excitations characteristic of amorphous materials have a common structural origin. Possible relations to the microscopic changes upon compaction are discussed, giving support to the coupled-rotation model of SiO₄ tetrahedra. Phenomenologically, the change ofC upon compaction can be understood by the soft-potential model. The relationship between height and position of theC/T ³ maximum observed in normal and compacted states for both types of vitreous silica and, surprisingly, also for α-quartz hints at a correlation between the mechanisms leading to theC/T ³ maximum for amorphous and crystalline materials with similar short-range order.