  When certain elemental metals like Cu, Sr and Nb are intercalated between the
layers of Bi$_2$Se$_3$, a topological insulator, the intercalated systems
superconduct with critical temperatures around 3 K. Naturally, in all these
cases, the possibility of topological superconductivity was suggested and
explored. However, in cases of Cu and Sr intercalated systems, the
low-temperature scanning tunneling microscopy (STM) experiments revealed fully
formed gaps where no signature of low-energy states, a requisite for
topological superconductivity, was found. Here, through STM spectroscopy down
to 400 mK we show that in Nb$_x$-Bi$_2$Se$_3$ ($x$ = 0.25), the spectra deviate
from a BCS-like behavior and the spectral weight at low-bias is large. Our
observations are consistent with the idea that the order parameter of
Nb$_x$-Bi$_2$Se$_3$ is nodal. Therefore, our results conclude that compared to
other members of the family, Nb$_x$-Bi$_2$Se$_3$ has a stronger possibility of
being a topological superconductor.
