Phonon mode potential and its contribution to anharmonism

Abstract

We present systematic ab-initio study on the phonon mode potential as a source of anharmonicity in the crystal. As an example, the transverse optical (TO) mode potential in PbTe has been fitted to density-functional-theory calculated energies of phonons excited with different amplitudes of mode displacements. The corresponding equation of motion has been analytically and numerically solved in 1D and 2D space, respectively. The solution is used for constructing the ensemble of 10,000 systems with potential and kinetic energies selected according to the thermal equilibrium distributions. The velocity auto-correlation function derived from the computed trajectories is then used to calculate the profile of the phonon spectrum for the TO an LA modes at three temperatures of 100, 300, and 600 K. This technique allows for determination of the contribution of non-quadratic potential of the phonon mode to the anharmonicity in the crystal and its effect on the phonon spectrum.

Magnetic Control of Soft Chiral Phonons in PbTe

“…2(d)[top] ]. This sublinear temperature dependence has been understood by strong anharmonic coupling between TO phonons and LA phonons with first principles calculations beyond the harmonic approximation [12,35]. In this Letter, we take a group theory approach, which complements the first-principles studies in the literature and provides a unified perspective of the magnetic field effect, as we will discuss later.…”

Lattice instability and magnetic phase transitions in strongly correlated MnAs

“…become largely thermal in character. The deviation of U ij from the usual gradual increase with temperature, including their pronounced 'λ'-shaped discontinuity extending between 270 K and T C * , reflects the soft-mode lattice instability in MnAs [37][38][39][40]. The magnitude of the deviation, ∆U (figure 3(d)), may be viewed at as an amplitude of the mode.…”

Nanohardness and Young’s Modulus of Pb1–xCdxTe Crystals Grown by the SSVG and MBE Methods

“…have attracted increasing interest in the last dozen years. Numerous reasons for this include new physical findings, for example, crystalline topological insulators and their properties, − dynamic local symmetry breaking, , a significant modification of the lattice dynamics, − and possible wider applications of PbTe-based materials in thermoelectric energy conversion systems operating in the intermediate temperature range (500–850 K) …”