Optical lattice vibrations in monolayer hexagonal boron nitride from macroscopic equations

Area of Science:

• Condensed Matter Physics
• Materials Science
• Theoretical Chemistry

Background:

• Hexagonal boron nitride (BN) exhibits unique optical lattice vibrations.
• Previous models often overestimated phonon frequencies by neglecting key physical effects.

Purpose of the Study:

• To theoretically investigate optical lattice vibrations in monolayer hexagonal boron nitride (BN).
• To elucidate the roles of electronic polarization (EP) and local field effects (LFEs) in lattice dynamics.
• To derive analytical expressions for phonon dispersion, group velocity, and density of states.

Main Methods:

• Utilized a dipole lattice model with two pairs of macroscopic equations.
• Self-consistently incorporated electronic polarization (EP) and local field effects (LFEs).
• Derived analytical expressions for in-plane (longitudinal and transverse optical - LO and TO) and out-of-plane (ZO) phonon modes.

Main Results:

• Analytical phonon dispersion relations accurately reproduce previous numerical results.
• Observed degeneracy of LO and TO modes at the Brillouin zone center and splitting at finite wavevectors.
• Demonstrated that neglecting EP and LFEs significantly overestimates phonon frequencies.
• Quantified the impact of EP and LFEs on dielectric susceptibility: a 2.5-3.5 factor increase for in-plane and a decrease for out-of-plane.

Conclusions:

• Accurate description of BN lattice dynamics necessitates the inclusion of both ionic EP and LFEs.
• The developed model provides a robust framework for understanding vibrational properties in 2D materials.
• Findings highlight the importance of self-consistent treatment of polarization and local fields in condensed matter theory.