  Recently, the crystal symmetry-protected topological semimetals have aroused
extensive interests, especially for the nonsymmorphic symmetry-protected one.
We list the possible nonmagnetic topological semimetals and develop their k.p
Hamiltonian in all layer groups with multiple screw axes in the absence of
spin-orbital coupling. We find a novel cat's cradle-like topological semimetal
phase, which looks like multiple hourglass-like band structures staggered
together. Furthermore, we propose the monolayer borophene and borophane with
pmmn layer group as the first material class to realize such novel semimetal
phase. A pair of tilted anisotropic Dirac cones at the Fermi level is revealed
in the two-dimensional boron-based materials and the low-energy effective
models are given. Moreover, akin to three-dimensional Weyl semimetal, the
topological property of these cat's cradle-like Dirac semimetals can be
verified by the calculation of quantized Berry phase and the demonstration of
flat Fermi-arc edge state connecting two Dirac points as well. Our finding that
borophene and borophane are cat's cradle-like Dirac semimetals is of great
interest for experiment and possible applications in the future.
