Low-Temperature Heat Capacity Anomalies In Ordered And Disordered Phases Of Normal And Deuterated Thiophene

We measured the specific heat C-p of normal (C4H4S) and deuterated (C4D4S) thiophene in the temperature interval of 1 <= T, K <= 25. C4H4S exhibits a metastable phase II2 and a stable phase V, both with frozen orientational disorder (OD), whereas C4D4S exhibits a metastable phase II2, which is analogous to the OD phase II2 of C4H4S and a fully ordered stable phase V. Our measurements demonstrate the existence of a large bump in the heat capacity of both stable and metastable C4D4S and C4H4S phases at temperatures of similar to 10 K, which significantly departs from the expected Debye temperature behavior of C-p approximate to T-3. This case study demonstrates that the identified low-temperature C-p anomaly, typically referred to as a "Boson-peak" in the context of glassy crystals, is not exclusive of disordered materials.