  The transition between gapped (semiconducting) and gapless (metallic) phases
and tunability of bandgap in materials is a very lucrative yet considerably
challenging goal for new-age device preparation. For bulk materials and for
two-dimensional layered systems, this is a rapidly expanding field. We
theoretically propose a one-dimensional pure carbon material with a tunable
bandgap. We find that two parallel coupled polyyne chains show metallic
behaviour with bands crossing on the Fermi level, unlike the single
semiconducting chain. The number of nodal points (two) is robust under
transverse and longitudinal strain, indicating the symmetry-protected nature of
the metallic phase. Sliding one chain with respect to the other breaks
reflection symmetry and a clear bandgap opens up at the nodes, leading to a
gapped phase. By varying the slide parameter, the bandgap can be tuned
efficiently. This work initiates and indicates possible topological phases of
real one-dimensional materials without the involvement of edge modes.
