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Topological semimetal phases in two-dimensional (2D) materials have gainedwidespread interest due to their potential applications in developing nanoscaledevices. Despite the prediction of the Dirac/Weyl points in a wide variety of2D candidates, materials featuring topological nodal lines are still in greatscarcity. Herein, we predict two stable thinnest films of aluminum diboridewith hyper- and hypo-stoichiometries of Al$_2$B$_2$ and AlB$_4$ as new 2Dnonmagnetic Dirac nodal line semimetals (NLSMs) which promise to offer manynovel features. Our elaborate electronic structure calculations combined withanalytical studies reveal that, in addition to the multiple Dirac points, these2D configurations host various type-I closed nodal lines (NLs) around the Fermilevel, all of which are semimetal states protected by the time-reversal andin-plane mirror symmetries. The most intriguing NL in Al$_2$B$_2$ encloses theK point and crosses the Fermi level with a considerable dispersion, thusproviding a fresh playground to explore exotic properties in dispersive Diracnodal lines. More strikingly, in the case of 2D superconductor AlB$_4$ whichexhibits a high transition temperature, we provide the first evidence for a setof 2D nonmagnetic open type-II NLs in weak spin-orbit coupling limit,coinciding with closed type-I NLs near the Fermi level. The coexistence ofsuperconductivity and nontrivial band topology in AlB$_4$ not only makes it apromising material to exhibit novel topological superconducting phases, butalso the rather large energy dispersion of type-II nodal lines in thisconfiguration, may offer a distinguished platform for realization of noveltopological features in two-dimensional limit.

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