  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 show that the
stacked 8-Pmmn borophene is transformed into the fused three-dimensional
borophene under pressure, being accompanied by the partially bond-breaking and
bond-reforming. Strikingly, the fused 8-Pmmn borophene inherits the Dirac band
dispersion resulting in an unusual semimetal-semimetal transition. A simple
tight-binding model derived from graphene qualitatively reveals the underlying
physics due to the maximum preservation of graphene-like substructure after the
phase transition, which contrasts greatly to the transformation of graphite
into diamond associated with the semimetal-insulator transition.
