  Charge-density-wave (CDW) instability and pressure-induced superconductivity
in bulk 1T-NbS2 are predicted theoretically by first-principles calculations.
We reveal a CDW instability towards the formation of a stable commensurate CDW
order, resulting in a sqart(13)*sqart(13) structural reconstruction featured
with star-of-David clusters. The CDW phase exhibits one-dimensional metallic
behavior with in-plane flat-band characteristics, and coexists with an
orbital-density-wave order predominantly contributed by 4d_(z^2-r^2 ) orbital
from the inner Nb atoms of the star-of-David cluster. By doubling the cell of
the CCDW phase along the layer stacking direction, a metal-insulator transition
may be realized in the CDW phase in case the interlayer antiferromagnetic
ordering and Coulomb correlation effect have been considered simultaneously.
Bare electron susceptibility, phonon linewidth and electron-phonon coupling
calculations suggest that the CDW instability is driven by softened phonon
modes due to the strong electron-phonon coupling interactions. CDW order can be
suppressed by pressure, concomitant with appearance of superconductivity. Our
theoretical predictions call for experimental investigations to further clarify
the transport and magnetic properties of 1T-NbS2. Furthermore, it would also be
very interesting to explore the possibility to realize the CDW order coexisting
with the superconductivity in bulk 1T-NbS2.
