Low-temperature thermal properties of a hyperaged geological glass

Abstract

We have measured the specific heat of amber from the Dominican Republic, an ancient geological glass about 20 million years old, in the low-temperature range 0.6 K ≤ T ≤ 26 K, in order to assess the effects of its natural stabilization (hyperageing) process on the low-temperature glassy properties, i.e. boson peak and two-level systems. We have also conducted modulated differential scanning calorimetry experiments to characterize the thermodynamic state of our samples. We found that calorimetric curves exhibit a huge ageing signal ΔH ≈ 5 J g(-1) in the first upscan at the glass transition Tg = 389 K, that completely disappears after heating up (rejuvenating) the sample to T = 395 K for 3 h. To independently evaluate the phonon contribution to the specific heat, Brillouin spectroscopy was performed in the temperature range 80 K ≤ T ≤ 300 K. An expected increase in the Debye level was observed after rejuvenating the Dominican amber. However, no significant change was observed in the low-temperature specific heat of glassy amber after erasing its thermal history: both its boson peak (i.e., the maximum in the Cp/T(3) representation) and the density of tunnelling two-level systems (i.e., the Cp ∼ T contribution at the lowest temperatures) remained essentially the same. Also, a consistent analysis using the soft-potential model of our Cp data and earlier thermal-conductivity data found in the literature further supports our main conclusion, namely, that these glassy 'anomalous' properties at low temperatures remain essentially invariant after strong relaxational processes such as hyperageing.

Do two-level systems and boson peak persist or vanish in hyperaged geological glasses of amber?

“…In this respect, our experimental results are conclusive [24,25]: pristine and rejuvenated glasses from both kinds of amber exhibit the same specific heat below 1 K, within experimental error, especially if the small differences in their Debye contributions are discounted.…”

“…1 of Ref. [24]) with T g  380 K. in all cases at rates ±1 K/min and modulating signal ±0.5 K every 80 s. (a) Typically obtained curves on a pristine amber sample corresponding to the first three consecutive heating runs. The first heating always shows a huge endothermic signal corresponding to the strong stabilization occurred after (110 million years) hyperaging.…”

“…Linear extrapolation of the mass densities to zero temperature were performed by using the Lorenz-Lorentz relation between mass density ρ(T) and refractive index n(T) for a transparent medium, from our HRBS measurements in the range 80 K < T < 300 K [24,25].…”

Do tunneling states and boson peak persist or disappear in extremely stabilized glasses?

“…Notice that its position does not vary with rejuvenation, though a modest increase of its height is observed following the increase of the elastic Debye level. In addition, a strong boson peak in C p /T 3 had already been observed by us in (20 million years old) Dominican amber [10], though apparent residual curing or repolymerization, occurring around the glass transition temperature when rejuvenating those amber samples, hindered a reliable quantitative investigation. Although some authors have tried to correlate the boson peak feature in glasses [38], and even in crystals [39], with transformations of the elastic continuum only, such a Debye-scaling rule is not hold quantitatively in our case.…”

“…Of interest here, amber is a unique example of a (polymer) glass that has aged far longer than any system accessible in the laboratory, thus reaching a state (of lower enthalpy and entropy) which is not accessible under normal experimental conditions. In other words, it is an amorphous solid or glass which has experienced an extreme thermodynamic stabilization process (hyperageing) [10]. Second, we have also studied the specific heat of ultrastable glasses of indomethacin [9].…”