Phys. Rev. Lett. 112, 165901 (2014)

The low-temperature behaviour of amorphous, glassy materials differs markedly from their ordered, crystalline counterparts. A celebrated example of this difference is the so-called boson peak, an enhancement in the density of vibrational states measurable in the specific heat of glasses. Its precise origin is a matter of some debate. Put simply, are anomalous features such as the boson peak intrinsic to glasses, or are they eventually supressed by sufficiently strong annealing or ageing processes? Tomás Pérez-Castañeda and colleagues attempt to address this question by studying a model glass that has aged far longer than any system accessible in the laboratory: 110-million-year-old amber from the cave of El Soplao in northern Spain. They measured the specific heat of pristine amber, and examined how this changes when it is thermally annealed in different ways, a process that in effect erases its 110 million years of cooling history. Within experimental error, all the amber samples display the same specific heat below 1 K, suggesting that the boson peak is indeed an intrinsic property of the glassy state.