Al$_2$B$_2$ and AlB$_4$ monolayers: emergence of multiple two-dimensional Dirac nodal line semimetals with novel properties (2110.09136v1)

Abstract: Topological semimetal phases in two-dimensional (2D) materials have gained widespread interest due to their potential applications in developing nanoscale devices. Despite the prediction of the Dirac/Weyl points in a wide variety of 2D candidates, materials featuring topological nodal lines are still in great scarcity. Herein, we predict two stable thinnest films of aluminum diboride with hyper- and hypo-stoichiometries of Al$_2$B$_2$ and AlB$_4$ as new 2D nonmagnetic Dirac nodal line semimetals (NLSMs) which promise to offer many novel features. Our elaborate electronic structure calculations combined with analytical studies reveal that, in addition to the multiple Dirac points, these 2D configurations host various type-I closed nodal lines (NLs) around the Fermi level, all of which are semimetal states protected by the time-reversal and in-plane mirror symmetries. The most intriguing NL in Al$_2$B$_2$ encloses the K point and crosses the Fermi level with a considerable dispersion, thus providing a fresh playground to explore exotic properties in dispersive Dirac nodal lines. More strikingly, in the case of 2D superconductor AlB$_4$ which exhibits a high transition temperature, we provide the first evidence for a set of 2D nonmagnetic open type-II NLs in weak spin-orbit coupling limit, coinciding with closed type-I NLs near the Fermi level. The coexistence of superconductivity and nontrivial band topology in AlB$_4$ not only makes it a promising material to exhibit novel topological superconducting phases, but also the rather large energy dispersion of type-II nodal lines in this configuration, may offer a distinguished platform for realization of novel topological features in two-dimensional limit.