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Paper   IPM / Physic / 16323
School of Physics
  Title:   Natural hyperbolicity in layered hexagonal crystal structures
1.  A. Ebrahimian
2.  R. Asgari
  Status:   Published
  Journal: Phys. Rev. B
  Vol.:  103
  Year:  2021
  Pages:   035425
  Supported by:  IPM
Unrevealing the required physical conditions for meeting the indefinite permittivity in natural material as well as proposing a new natural hyperbolic media offer a possible route to significantly improve our knowledge and ability to confine and controlling light in optoelectronic devices. We demonstrate the hyperbolicity in a class of materials with hexagonal P6/mmm and P63/mmc layered crystal structures and its physical origin is thoroughly investigated. Making use of density functional theory and solving the Bethe-Salpeter equation, we find an exceedingly strong anisotropy in optical responses along the in- and out-of-plane directions of the crystals. This is owing to the weak Coulombic interactions along layers stacking direction and furthermore symmetries imposed constraints in the hexagonal crystal structure. These, therefore, cause the system a natural hyperbolic in a broad spectral range from the visible to the ultraviolet spectrums. More excitingly, the hyperbolicity relation to anisotropic interband absorption in addition to the impressive dependency of the conduction band to the lattice constant along the out-of-plane direction provide the hyperbolicity tunability in these hexagonal structures under strain, doping, and alloying. Our findings not only suggest a large family of real hexagonal compounds as a unique class of materials for realization of the highly tunable broadband hyperbolicity but also offers an approach to search for new hyperbolic materials.

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