Strain effects on anharmonic lattice dynamics from first-principle caculations

Abstract

Lattice dynamics describes the vibration of atoms at their equilibrium positions and their effects on other physical properties, such as lattice stability, thermal expansion, melting behavior, and thermal transports. Phonon dispersion based on the harmonic approximation can only study the lattice dynamics at 0 K when there are no phonon interactions and the phonon lifetime is infinite. At elevated temperatures, the phonon lifetime is reduced due to increased anharmonic phonon-phonon coupling, accompanied by a frequency shift compared with the harmonic frequency. In this dissertation, anharmonic lattice dynamics at finite temperatures of several interesting materials are investigated from first-principles based on perturbation theory or selfconsistent ab initio lattice dynamics calculations. More importantly, the effects of compressive or tensile strain on anharmonic phonon coupling are also studied. First, the tensile strain effects on the lattice dynamics of graphene and monolayer hexagonal boron nitride (h-BN) is investigated by calculating the bubble diagram phonon self-energy. It is found that the transverse optical (TO) mode at the K point obviously softens at elevated temperatures for graphene and monolayer h-BN under equibiaxial strains, suggesting the failure strain decreases as temperature increases. Second, the anharmonic phonon-phonon coupling of sodium under high pressures is investigated from first-principles. The fcc and cI16 sodium above 90 GPa are found to exhibit abnormal negative thermal expansion, which is mainly attributed to the transverse acoustic (TA) modes with large negative Gr¨uneisen parameters. For the anharmonic phonon spectra of bcc sodium, the TA modes along G-N behave differently than other modes with increasing pressure and temperature. The competition between the pressure-induced softening and temperature-induced stiffening of these TA modes may be related to its anomalous melting behavior. Third, the strain effects on the lattice dynamics of several thermoelectric materials (Te, Fm3m GeTe, PbTe and SnTe) are studied. A counter-intuitive strain-dependent lattice thermal conductivity is found in Te, which can be rationalized by studying the phonon-phonon coupling. In addition, similar to PbTe, the optical modes at the G point show double peaks at elevated temperatures for SnTe at 1 GPa and GeTe at 10 and 15 GPa, originating from phonon dispersion nesting and strong phonon-phonon coupling. However under higher pressure, only minor stiffening of optical modes at the G point can be observed.