  Two-dimensional transition metal dichalcogenides (TMDs) have been attracting
significant interest due to a range of properties, such as layer-dependent
inversion symmetry, valley-contrasted Berry curvatures, and strong spin-orbit
coupling (SOC). Of particular interest is niobium diselenide (NbSe2), whose
superconducting state in few-layer samples is profoundly affected by an unusual
type of SOC called Ising SOC. Combined with the reduced dimensionality, the
latter stabilizes the superconducting state against magnetic fields up to ~35 T
and could lead to other exotic properties such as nodal and crystalline
topological superconductivity. Here, we report transport measurements of
few-layer NbSe$_2$ under in-plane external magnetic fields, revealing an
unexpected two-fold rotational symmetry of the superconducting state. In
contrast to the three-fold symmetry of the lattice, we observe that the
magnetoresistance and critical field exhibit a two-fold oscillation with
respect to an applied in-plane magnetic field. We find similar two-fold
oscillations deep inside the superconducting state in differential conductance
measurements on NbSe$_2$/CrBr$_3$ superconductor-magnet junctions. In both
cases, the anisotropy vanishes in the normal state, demonstrating that it is an
intrinsic property of the superconducting phase. We attribute the behavior to
the mixing between two closely competing pairing instabilities, namely, the
conventional s-wave instability typical of bulk NbSe$_2$ and an unconventional
d- or p-wave channel that emerges in few-layer NbSe2. Our results thus
demonstrate the unconventional character of the pairing interaction in a
few-layer TMD, opening a new avenue to search for exotic superconductivity in
this family of 2D materials.
