  Condensed matter systems in low dimensions exhibit emergent physics that does
not exist in three dimensions. When electrons are confined to one dimension
(1D), some significant electronic states appear, such as charge density wave,
spin-charge separations and Su-Schrieffer-Heeger (SSH) topological state.
However, a clear understanding of how the 1D electronic properties connects
with topology is currently lacking. Here we systematically investigated the
characteristic 1D Dirac fermion electronic structure originated from the
metallic NbTe$_2$ chains on the surface of the composition-tunable layered
compound NbSi$_x$Te$_2$ ($x$ = 0.40 and 0.43) using angle-resolved
photoemission spectroscopy. We found the Dirac fermion forms a Dirac nodal line
structure protected by the combined $\widetilde{\mathcal{M}}{\rm_y}$ and
time-reversal symmetry T and proves the NbSi$_x$Te$_2$ system as a topological
semimetal, in consistent with the ab-initio calculations. As $x$ decreases, the
interaction between adjacent NbTe2 chains increases and Dirac fermion goes
through a dimension-crossover from 1D to 2D, as evidenced by the variation of
its Fermi surface and Fermi velocity across the Brillouin zone in consistence
with a Dirac SSH model. Our findings demonstrate a tunable 1D Dirac electron
system, which offers a versatile platform for the exploration of intriguing 1D
physics and device applications.
