The Interplay of Long Range Interactions and Electron Density Distributions in Polar and Strongly Correlated Materials

Abstract

In the present contribution, we summarize the recent theoretical results on the physics of the electron-TO-phonon (el-TO-ph) interaction in polar materials. Within the frameworks of dipole approximation, we linked the interaction of electrons with the long-wavelength transverse optical vibrations to the long-range dipole-dipole interaction. We suggested a macroscopic parameterization of the el-TO-ph coupling constants in terms of experimental material parameters relating to phonon and electron characteristics of the system. The parametrization enabled us to introduce a novel relation that may measure a degree of polarity, and may indicate how close the dielectric behavior of the system is to a structural instability. The ferroelectric tendencies of known crystalline compounds were reproduced. Further considerations were focused on an integrated analysis of charge, electronic, and vibrational properties of iron-based narrow-gap systems FeSi and FeAs₂. Similarly to ferroelectrics, these materials demonstrate a strong interplay of electronic and polarization degrees of freedom, which is mediated by the dynamic hybridization of the relevant frontier orbitals.

Appendix

Computational details on electron structure calculations of the bulk FeAs₂ the crystalline FeAs₂ has the marcasite type crystal structure (P n n m) with the lattice parameters [27] a = 5.3013 Å, b = 5.9858 Å, and c = 2.8822 Å. The DFT calculations were performed with the use of the VASP code within PAW-GGA functional scheme [28–30]. A plane-wave basis set with 500 eV cutoff, and a Γ point centered mesh for the k-point sampling were chosen. The Kohn-Sham eigenstates were determined by using the Perdew-Burke-Ernzerhof GGA exchange-correlation functional [31]. Electron correlations were considered within approach of Dudarev et al. [32]. The effective value of the on-site d−d Coulomb repulsion U _{e f f} = 1.6 eV was taken from [25]. The phonon properties were calculated within the harmonic approximation using PHONOPY package [33]; the elements of dynamical matrix were determined by employing finite displacement method [34, 35]. The forces were evaluated for 2×2×4 supercell and 3×3×2k-points set. A Bader analysis of the valence electron distributions, which implements the AIM (“atoms in molecules”) approach [26], was performed by using the method of ref. [36, 37]. For visualization of the actual phonon modes the VESTA program [38] was applied.