Topological phases in $α$-Li$_{\rm 3}$N-type crystal structure of light-element compounds

Abstract

Materials with tunable topological features, simple crystal structure and flexible synthesis, are in extraordinary demand towards technological exploitation of unique properties of topological nodal points. The controlled design of the lattice geometry of light elements is determined by utilizing density functional theory and the effective Hamiltonian model together with the symmetry analysis. This provides an intriguing venue for reasonably achieving various distinct types of novel fermions. We, therefore, show that a nodal line (type-I and II), Dirac fermion, and triple point (TP) fermionic excitation can potentially appear as a direct result of a band inversion in group-I nitrides with α-Li3N-type crystal structure. The imposed strain is exclusively significant for these compounds, and it invariably leads to the considerable modification of the nodal line type. Most importantly, a type-II nodal loop can be realized in the system under strain. These unique characteristics make α-Li3N-type crystal structure an ideal playground to achieve various types of novel fermions well-suited for technological applications.

Natural hyperbolicity in layered hexagonal crystal structures

“…In the same way, a recent study shows that the layered nature of group-I nitrides with hexagonal P6/mmm (αtype) crystal structure gives rise to an exceedingly strong anisotropy in the electronic structure [27]. The α-Li 3 Ntype structure possesses a layered structure composed of alternating planes of the hexagonal Li 2 N and pure Li +ions with weak interactions along the stacking direction.…”

“…As a standard feature of this new class of natural hyperbolic materials [26,29,30], the valence and conduction bandgap of Li 3 N is highly impressionable and susceptible to the lattice constant c. These unique conditions provide the possibility of the hyperbolicity tunability in Li 3 N with strain, doping and alloying. In fact, our calculations show that the hyperbolic window of Li 3 N and the relative intensity of the subdiffraction and diffraction-limited waves in transmission through Li 3 N can be efficiently tuned by applying strain along the z−axis and doping in addition to using its ternary compounds like Li 2 K(Na)N. Although Li 3 N and Li 2 K(Na)N possess the same structure, the larger ionic radius of K(Na) leads to a band crossing along the z−direction [27], which extends the hyperbolic window in the visible regime highly demanding in light device applications. Beside Li 3 N and the related alloys, the anisotropic electronic structure of α and β-type compounds gives rise to hyperbolic frequency windows for both type I and II as well offering hexagonal P6/mmm and P6 3 /mmc layered structures as a unique class of materials for realization of the highly tunable broadband hyperbolicity and relevant device applications.…”