  A structurally stable carbon allotrope with plentiful topological properties
is predicted by means of first-principles calculations. This novel carbon
allotrope possesses the simple space group C2/m, and contains simultaneously
sp, sp2 and sp3 hybridized bonds in one structure, which is thus coined as
carboneyane. The calculations on geometrical, vibrational, and electronic
properties reveal that carboneyane, with good ductility and a much lower
density 1.43 g/cm3, is a topological metal with a pair of nodal lines
traversing the whole Brillouin zone, such that they can only be annihilated in
a pair when symmetry is preserved. The symmetry and topological protections of
the nodal lines as well as the associated surface states are discussed. By
comparing its x-ray diffraction pattern with experimental results, we find that
three peaks of carboneyane meet with the detonation soot. On account of the
fluffy structure, carboneyane is shown to have potential applications in areas
of storage, adsorption and electrode materials.
