The electronic structure of palladium diselenide (PdSe2) in its monolayer form is analyzed from the topological band theory perspective. Employing first-principles calculations, effective models and symmetry indicators we found that the low-lying conduction bands are topologically nontrivial, protected by time reversal and crystalline symmetries. Numerical evidence supporting the nontrivial character of the bands is presented and examples of physical responses are discussed.

Two-dimensional pentagonal structures based on the Cairo tiling are the basis of a family of layered materials with appealing physical properties. In this work we present a theoretical study of the symmetry-based electronic and optical properties of these pentagonal materials. We provide a complete classification of the space groups that support pentagonal structures for binary and ternary systems. By means of first-principles calculations, their electronic band structures and the local spin textures in momentum space are analyzed. Our results show that pentagonal structures can be realized in chiral and achiral lattices with Weyl nodes pinned at high-symmetry points and nodal lines along the Brillouin zone boundary; these degeneracies are protected by the combined action of crystalline and time-reversal symmetries. Additionally, we discuss the linear and nonlinear optical features of some penta-materials, such as the shift current, which shows an enhancement due to the presence of nodal lines and points, and their possible applications.I.