Specific heat at low temperatures of ultrastable glasses of TPD

Glasses exhibit thermal, vibrational and acoustic properties at low temperatures anomalously different from those found in crystalline solids, and with a remarkable degree of universality [1].
Interestingly, our earlier low-temperature specific-heat measurements on (highly anisotropic) ultrastable glasses of indomethacin prepared by Physical Vapour Deposition (PVD) [2] showed the suppression of the ubiquitous linear term below 1-2 K traditionally ascribed to the existence of tunneling two-level systems (TLS) in glasses [3]. This finding apparently challenged the opposite behavior found in hyperaged glasses of geological amber [4].
Here we will present the results of our ongoing experiments of low-temperature specific heat, as well as of Brillouin Scattering, on a different ultrastable glass, TPD, whose degree of anisotropy can be controlled. We aim at comparing its behavior at low temperatures to that of its conventional glass and crystalline states.
[1] W. A. Phillips, ed., Amorphous Solids, Low-Temperature Properties, Springer (1981)
[2] M. D. Ediger, J. Chem. Phys. 147, 210901 (2017)
[3] T. Pérez-Castañeda, C. Rodríguez-Tinoco, J. Rodríguez-Viejo, M. A. Ramos, PNAS 111, 11275 (2014)
[4] T. Pérez-Castañeda, R. J. Jiménez-Riobóo, M. A. Ramos, Phys. Rev. Lett. 112, 165901 (2014)