Open AccessRole of Optical Phonons and Anharmonicity in the Appearance of the Heat Capacity Boson Peak-like Anomaly in Fully Ordered Molecular Crystals
Author(s) -
А. И. Кривчиков,
Andrezj Jeżowski,
Daria Szewczyk,
Oxsana A. Korolyuk,
O. O. Romantsova,
L. M. Buravtseva,
Claudio Cazorla,
J. Ll. Tamarit
Publication year - 2022
Publication title -
the journal of physical chemistry letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.563
H-Index - 203
ISSN - 1948-7185
DOI - 10.1021/acs.jpclett.2c01224
Subject(s) - anharmonicity , phonon , condensed matter physics , heat capacity , anomaly (physics) , crystal (programming language) , density functional theory , physics , molecular vibration , materials science , optics , quantum mechanics , raman spectroscopy , computer science , programming language
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.