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Using ab initio methods, the topological and optical properties of surface-functionalized XN sheets (X = Mo, W) were investigated. Based on first principles calculations and the K·p effective model, the existence of topological nodal-line states in potassium-functionalized XN sheets (KMoN and KWN) is reported. This study shows that a nodal line ring exists near the Fermi level in the absence of spin-orbit coupling (SOC). When SOC is included, the band-crossing points are gapped, giving rise to a new nodal ring along Γ-K. This band-crossing is protected due to the existence of mirror reflection and time-reversal symmetry. These calculations demonstrate the inclusion of electron-hole (e-h) interactions, which were further confirmed through the optical absorption of functionalized MoN, revealing the presence of strongly bound excitons below the absorption onset where they depend strongly on the terminated surface groups. Moreover, the surface terminated groups change the energy distribution range of the exciton, which can be used to tune the absorption of infrared (IR) and visible light. Interestingly, FMoN has several strongly bound excitons, with the first exciton having a binding energy of 1.35 eV, larger than the corresponding one in the transition metal dichalcogenide MoS.

The spin Hall effect of light occurring in topological semimetals provides unprecedented opportunities to exploit novel photonic properties and applications. In particular, pristine α-Li3N-type crystal has recently been predicted to...

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Discovering the physical requirements 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. By utilizing density functional theory and solving the Bethe-Salpeter equation (BSE), we find that the layered crystal structure and symmetry imposed constraints in Li3N gives rise to an exceedingly strong anisotropy in optical responses along in-and out-of-plane directions of the crystals making it a natural hyperbolic in a broad spectral range from the visible spectrum to the ultraviolet. 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 broad band hyperbolicity but also offers an approach to search for new hyperbolic materials.