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Thibault Sohier1, Marco Gibertini1, Matteo Calandra2

  • 1Theory and Simulation of Materials (THEOS), and National Centre for Computational Design and Discovery of Novel Materials (MARVEL), École Polytechnique Fédérale de Lausanne , CH-1015 Lausanne, Switzerland.

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We studied optical phonon modes in 2D materials. The electric field causes a unique zone center slope in their dispersion, influenced by material properties and environment screening.

Keywords:
LO−TO splittingTwo-dimensional materialsoptical phononspolar materials

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Area of Science:

  • Condensed Matter Physics
  • Materials Science
  • Solid-State Physics

Background:

  • Polar two-dimensional (2D) materials, multilayers, and heterostructures exhibit unique electronic and vibrational properties.
  • Understanding phonon modes is crucial for predicting thermal and electrical transport in these systems.

Purpose of the Study:

  • To investigate the long-wavelength dispersion of longitudinal and transverse optical phonon modes in polar 2D materials, multilayers, and heterostructures.
  • To analyze the influence of macroscopic electric fields and environmental screening on phonon dispersion.

Main Methods:

  • Analytical modeling within a 2D framework.
  • Density-functional perturbation theory (DFPT) applied to 2D materials.
  • Calculation of Born effective charges and dielectric properties.

Main Results:

  • Longitudinal and transverse optical phonon modes are degenerate at the zone center in 2D systems, unlike their 3D counterparts.
  • A finite slope at the zone center arises from the macroscopic electric field of longitudinal-optical modes.
  • This slope scales linearly with the number of layers and depends on Born effective charges and environmental dielectric properties.
  • Environmental screening significantly reduces the mode splitting, as demonstrated in boron nitride-graphene heterostructures.
  • Intrinsic screening properties of 2D materials lead to a transition towards 3D-like momentum-independent splitting at higher momenta.

Conclusions:

  • The study reveals key differences in optical phonon dispersion between 2D and 3D polar materials.
  • Environmental screening plays a critical role in tuning phonon mode splitting in 2D heterostructures.
  • These findings are essential for understanding and manipulating electrical transport and optical coupling in 2D systems.