APL Materials (Apr 2024)

Type-II Dirac phonons in a two-dimensional phononic crystal

  • Changqing Xu,
  • Jun Mei,
  • Guancong Ma,
  • Ying Wu

DOI
https://doi.org/10.1063/5.0189354
Journal volume & issue
Vol. 12, no. 4
pp. 041128 – 041128-6

Abstract

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We explore the distinctive properties associated with a type-II Dirac point in a simply structured phononic crystal with a lattice deformation. This type-II Dirac point emerges at the Brillouin zone boundary, resulting from the lifting of two degenerate bands and featuring a conical-like Fermi surface in the equi-frequency curve. A practical implementation of such a phononic crystal is achieved with LEGO bricks. Upon introducing a periodic parity-time (PT) symmetric non-Hermitian perturbation, the phononic crystal undergoes a transition from PT-symmetric phase to PT-broken phase, causing the deformation of type-II Dirac point into an oval of exceptional points in the band structure. Based on the eigenmodes of the type-II Dirac point, a k ⃗ ⋅ p ⃗ perturbation theory can be used to characterize these systems before and after the phase transition. Using a scattering matrix, we analyze the symmetric and broken phases and demonstrate that broadband unidirectional transparency and a coherent perfect absorber and laser can be realized with such a phononic crystal slab.