Prof. Miguel Ángel Ramos^1,2,4

Talk: " Specific heat at low temperatures in quasiplanar molecular crystals: Where do glassy anomalies in minimally disordered crystals come from?"

Co-workers: Daria Szewczyk^1,2,3, Manuel Moratalla^1,2,4, Grzegorz Chajewski³, Jonathan F. Gebbia⁵, Andrzej Jeżowski ³, Alexander I. Krivchikov ^3,5,6, María Barrio ⁵, Josep Ll. Tamarit ⁵,

¹ Laboratorio de Bajas Temperaturas, Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain

² Instituto Nicolás Cabrera (INC), Universidad Autónoma de Madrid, 28049 Madrid, Spain

³ Institute of Low Temperature and Structure Research PAS, 50-422 Wrocław, Poland

⁴ Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain

⁵ Grup de Caracterizació de Materials, Departament de Fisica, EEBE, and Barcelona Research Center in Multiscale Science and Engineering, Universitat Politècnica de Catalunya, 08019 Barcelona, Catalonia, Spain

⁶ B. Verkin Institute for Low Temperature Physics and Engineering, NASU, 61103 Kharkiv, Ukraine

We present low-temperature specific heat (C_p) measurements of a monoclinic P2₁/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 three-fold-like axis perpendicular to the benzene ring. Such a minimal disorder gives rise to typical glassy anomalies, as a linear in contribution in C_p ascribed to two-level systems and a broad maximum around 6.6 K in C_p/T³ (the boson peak). We discuss these results [1] in the framework of other quasiplanar molecular crystals with different accountable number 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_BP) and Debye (Θ_D) temperatures. We find that a linear correlation between T_BP and Θ_D holds for many - but not all - structural glasses and strikingly holds even better for some disordered crystals, including our studied quasiplanar molecular crystals.

• [1] D. Szewczyk, M. Moratalla, G. Chajewski, J. F. Gebbia, A. Jeżowski, A. I. Krivchikov, M. Barrio, J. Ll. Tamarit, and M. A. Ramos, Phys. Rev. B 110, 174204 (2024).