Thermal Kharkiv Seminars: "Heat Capacity and Thermal Conductivity of Solids at Low Temperatures"
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Specific heat at low temperatures in quasiplanar molecular crystals: Where do glassy anomalies in minimally disordered crystals come from?
Prof. Miguel Ángel Ramos
Universidad Autónoma de Madrid
The seminar will be held remotely via the Zoom application. To participate in the event use the following data: Zoom ID: 815 1184 7591 & Password: 664133.
Co-workers: Daria Szewczyk1,2,3, Manuel Moratalla1,2,4, 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 (Cp) 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 Cp ascribed to two-level systems and a broad maximum around 6.6 K in Cp/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 (TBP) and Debye (ΘD) temperatures. We find that a linear correlation between TBP and ΘD 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).
