Krivchikov Alexander I, Jeżowski Andrezj, Szewczyk Daria, Korolyuk Oxsana A, Romantsova Olesya O, Buravtseva Lubov M, Cazorla Claudio, Tamarit Josep Ll
Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, 47 Nauky Avenue, Kharkiv 61103, Ukraine.
Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 2 Okólna Strasse, 50-422 Wrocław, Poland.
J Phys Chem Lett. 2022 Jun 9;13(22):5061-5067. doi: 10.1021/acs.jpclett.2c01224. Epub 2022 Jun 2.
We demonstrate that the heat capacity Boson peak (BP)-like anomaly appearing in fully ordered anharmonic molecular crystals emerges as a result of the strong interactions between propagating (acoustic) and low-energy quasi-localized (optical) phonons. In particular, we experimentally determine the low-temperature (<30 K) specific heat of the molecular crystal benzophenone and those of several of its fully ordered bromine derivatives. Subsequently, by means of theoretical first-principles methods based on density functional theory, we estimate the corresponding phonon dispersions and vibrational density of states. Our results reveal two possible mechanisms for the emergence of the BP-like anomaly: (i) acoustic-optic phonon avoided crossing, which gives rise to a pseudo-van Hove singularity in the acoustic phonon branches, and (ii) piling up of low-frequency optical phonons, which are quasi degenerate with longitudinal acoustic modes and lead to a surge in the vibrational density of states at low energies.
我们证明，在完全有序的非谐分子晶体中出现的热容玻色子峰（BP）类异常现象，是由于传播（声学）声子和低能准局域（光学）声子之间的强相互作用所致。具体而言，我们通过实验测定了分子晶体二苯甲酮及其几种完全有序的溴衍生物在低温（<30 K）下的比热容。随后，借助基于密度泛函理论的理论第一性原理方法，我们估算了相应的声子色散和振动态密度。我们的结果揭示了BP类异常现象出现的两种可能机制：（i）声光声子避免交叉，这在声学声子分支中产生了一个赝范霍夫奇点；（ii）低频光学声子的堆积，这些光学声子与纵向声学模式准简并，并导致低能下振动态密度的激增。
