  The past decade has witnessed numerous discoveries of two-dimensional (2D)
semimetals and insulators, whereas 2D metals are rarely identified. Borophene,
a monolayer boron sheet, has recently emerged as a perfect 2D metal with unique
structure and electronic properties. Here we study collective excitations in
borophene, which exhibit two major plasmon modes with low damping rates
extending from infrared to ultraviolet regime. The anisotropic 1D plasmon
originates from electronic excitations of tilted Dirac cones in borophene,
analogous to that in heavily doped Dirac semimetals. These features make
borophene promising to realize directional polariton transportation and
broadband optical communications for next-generation optoelectronic devices.
