Are universal "anomalous" properties of glasses at low temperatures truly universal?
The specific heat C<sub>p</sub> and other properties of glasses (ranging from amorphous solids to disordered crystals) at low temperatures are well known to be markedly different from those in fully-ordered crystals. For decades, this qualitative, and even quantitative, universal behavior of glasses has been thoroughly studied. However, a clear understanding of its origin and microscopic nature, needless to say, a closed theory, is still lacking. To shed light on this matter, I review the situation in this work, mainly by compiling and discussing measured low-temperature C<sub>p</sub> data of many glasses and disordered crystals, as well as highlighting a few exceptions to that “universality rule”. Thus, one can see that, in contrast to other low-temperature properties of glasses, the magnitude of the “glassy” C<sub>p</sub> excess at low temperature is far from being universal. Even worse, some molecular crystals without a clear sign of disorder exhibit linear coefficients in C<sub>p</sub> larger than those found in many amorphous solids, whereas a few of the latter show negligible values.