Specific heat at low temperatures in quasiplanar molecular crystals: Origin of glassy anomalies in minimally disordered crystals

We present low-temperature specific heat (${C}_{\mathrm{p}}$) measurements of a monoclinic $P{2}_{1}/c$ crystal formed by quasiplanar molecules of tetrachloro-$m$-xylene. The dynamic disorder frozen at low temperature of the asymmetric unit (formed by a half molecule) consists of reorientation around a threefold-like axis perpendicular to the benzene ring. Such a minimal disorder gives rise to typical glassy anomalies, as a linear contribution in ${C}_{\mathrm{p}}$ ascribed to two-level systems and a broad maximum around 6.6 K in ${C}_{\mathrm{p}}/{T}^{3}$ (the boson peak). We discuss these results in the framework of other quasiplanar molecular crystals with different accountable numbers of in-plane molecular orientations. We find that the density of two-level systems does not correlate with the degree of orientational disorder. Rather, it is the molecular asymmetry that seems to play a relevant role in the thermal anomalies. Furthermore, we discuss the suggested correlation between the boson peak (${T}_{\mathrm{BP}}$) and Debye (${\mathrm{\ensuremath{\Theta}}}_{D}$) temperatures. We find that a linear correlation between ${T}_{\mathrm{BP}}$ and ${\mathrm{\ensuremath{\Theta}}}_{D}$ holds for many---but not all---structural glasses and strikingly holds even better for some disordered crystals, including our studied quasiplanar molecular crystals.