Long optical lattice vibrations and dielectric function of monolayer hexagonal boron nitride

Abstract

Using two pairs of lattice equations deduced from a microscopic dipole lattice model taking into account electronic polarization (EP) of ions and local field effects (LFEs) self-consistently, in-plane and out-of-plane optical vibrations in two-dimensional (2D) hexagonal BN are studied theoretically. The three mutually independent coefficients of either pair of lattice equations are determined by a set of three generally known quantities such as the 2D electronic and static susceptibilities and phonon frequency, making the lattice equations very useful for calculating the lattice dynamical properties. Explicit expressions are obtained for lattice vibrational energy density, and phonon dispersion, group velocity and density of states. The transparent phonon dispersion relations describe the previous numerical calculations very well, and the longitudinal optical (LO) phonon dispersion relation is identical to the analytical expression of Sohier {\it et al}. [Nano Lett. {\bf 17}, 3758 (2017)]. The out-of-plane phonon frequency is finite owing to ionic EP. A 2D lattice dielectric function $ε(k,ω)$ is derived--due solely to the LO vibrations--which also allows the LO phonon dispersion to be rederived simply from $ε(k,ω)=0$, similar to the bulk case. A 2D Lyddane--Sachs--Teller relation and a frequency--susceptibility relation are obtained for the in-plane and out-of-plane vibrations, respectively, connecting the phonon frequencies to the 2D dielectric functions or susceptibilities. Using three first-principles calculated parameters, the lattice dynamical properties are studied comprehensively, particular attention being paid to the EP and LFEs. The ionic EP and LFEs should be included simultaneously, but otherwise neglecting either or both causes large discrepancies to the calculated dynamical properties.