Anharmonic effects and the lattice dynamics of insulators

Particlelike Phonon Propagation Dominates Ultralow Lattice Thermal Conductivity in Crystalline Tl_{3}VSe_{4}

We investigate the microscopic mechanisms of ultralow lattice thermal conductivity (κ_{l}) in Tl_{3}VSe_{4} by combining a first principles density functional theory based framework of anharmonic lattice dynamics with the Peierls-Boltzmann transport equation for phonons. We include contributions of the three- and four-phonon scattering processes to the phonon lifetimes as well as the temperature dependent anharmonic renormalization of phonon energies arising from an unusually strong quartic anharmonicity in Tl_{3}VSe_{4}. In contrast to a recent report by Mukhopadhyay et al. [Science 360, 1455 (2018)SCIEAS0036-807510.1126/science.aar8072] which suggested that a significant contribution to κ_{l} arises from random walks among uncorrelated oscillators, we show that particlelike propagation of phonon excitations can successfully explain the experimentally observed ultralow κ_{l}. Our findings are further supported by explicit calculations of the off-diagonal terms of the heat current operator, which are found to be small and indicate that wavelike tunneling of heat carrying vibrations is of minor importance. Our results (i) resolve the discrepancy between the theoretical and experimental κ_{l}, (ii) offer new insights into the minimum κ_{l} achievable in Tl_{3}VSe_{4}, and (iii) highlight the importance of high order anharmonicity in low-κ_{l} systems. The methodology demonstrated here may be used to resolve the discrepancies between the experimentally measured and the theoretically calculated κ_{l} in skutterides and perovskites, as well as to understand the glasslike κ_{l} in complex crystals with strong anharmonicity, leading towards the goal of rational design of new materials.

Vibrational dynamics of hydrogen in proteins

Biological macromolecules expand with increasing temperature and this dynamic expansion is associated with the onset of function. The expansion is typically characterized by the mean square vibrational displacement (MSD), <u²> of specific constituents such as hydrogen within the macromolecules. The <u²> increases with increasing temperature and the slope of <u²> versus temperature can increase significantly at a temperature T{D} identified as a dynamical transition. We illustrate that the observed expansion and change in slope of <u²> with temperature at T{D} can be reproduced within a simple model of the vibration, an atom in an anharmonic potential, V(u). Given V(u), only the temperature is varied in the model. A simple Gaussian potential or a potential containing a hard wall is particularly effective is reproducing the observed change in the slope of <u²> with temperature around T{D}.

Calculations for displacive ω-phase transformations in Ti-Al alloys with Nb additions at finite temperature

We examine by means of first-principles calculations the bcc-like (bcc: body centered cubic) to ω-like phase transformations in Ti-Al alloys with Nb additions at finite temperature. To simulate the alloy we use different discrete atomic configurations in a six atom unit cell of the stoichiometry Ti(3)Al(2)Nb. Calculated ground state energies show an instability in the ternary Ti(3)Al(2)Nb alloy against the ω structure type atomic displacement. To better understand the role of entropy in the stability/instability of these systems, the first-principles calculations are extended to finite temperature by including various contributions to the free energy. In particular, the vibrational free energy is calculated within a quasiharmonic approximation. It is shown that the bcc structure is stabilized by the contribution of the low energy modes to the lattice entropy against ω type atomic displacements. We find that configurational entropy plays a major role in the ω to B8(2) transformation. Calculated lattice parameters and transition temperatures are found to be in excellent agreement with experiment.