Dynamic polarization and plasmons in kekul\'e-patterned graphene: Signatures of the broken valley degeneracy

TL;DR

This paper investigates how Kekulé lattice distortions in graphene break valley degeneracy, affecting its dielectric, plasmonic, and optical properties, revealing new signatures detectable via optical or electrical measurements.

Contribution

It demonstrates how Kekulé patterning causes valley-dependent splitting in graphene's dielectric spectrum, plasmon dispersion, and optical conductivity, providing new signatures of valley symmetry breaking.

Findings

Valley-dependent splitting of dielectric spectrum

Emergence of a second plasmon branch in Landau damping

Splitting of optical conductivity steps

Abstract

The dynamic polarization for kekul\'e-patterned graphene is studied within the Random Phase Approximation (RPA). It is shown how the breaking of the valley degeneracy by the lattice modulation is manifested through the dielectric spectrum, the plasmonic dispersion, the static screening and the optical conductivity. The valley-dependent splitting of the Fermi velocities due to the kekul\'e distortion leads to a similar splitting in the dielectric spectrum of graphene, introducing new characteristic frequencies, which are given in terms of the valley-coupling amplitude. The valley coupling also splits the plasmonic dispersion, introducing a second branch in the Landau damping region. Finally, the signatures of the broken valley degeneracy in the optical conductivity are studied. The characteristic step-like spectrum of graphene is split into two half steps due to the onset of absorption in each valley occurring at different characteristic frequencies. Also, it was found an absorption phenomenon where a resonance peak related to intervalley transport emerges at a "beat frequency", determined by the difference between the characteristic frequencies of each valley. Some of these mechanisms are expected to be present in other space-modulated 2D materials and suggest how optical or electrical response measurements can be suitable to detect spatial modulation.