Unusual semimetal-semimetal transition of layer-stacked borophene under pressure

The 8-Pmmn borophene, a boron analogue of graphene, hosts tilted and anisotropic massless Dirac fermion quasiparticles owing to the presence of the distorted graphene-like sublattice. First-principles calculations reveal that the bulk layer-stacked 8-Pmmn borophene inherits the Dirac band dispersion resulting in the topological nodal-line semimetal phase. At a pressure 3 GPa, the stacked 8-Pmmn borophene is transformed into the fused three-dimensional borophene that preserves the graphene-like substructure, and thereby leading to an unusual semimetal-semimetal transition, which contrasts greatly to the transformation of graphite into diamond associated with the semimetal-insulator transition.