  Van der Waals materials provide an ideal platform to explore
superconductivity in the presence of strong electronic correlations, which are
detrimental of the conventional phonon-mediated Cooper pairing in the
BCS-Eliashberg theory1 and, simultaneously, promote magnetic fluctuations.
Despite recent progress in understanding superconductivity in layered
materials, the glue pairing mechanism remains largely unexplored in the
single-layer limit, where electron-electron interactions are dramatically
enhanced. Here we report experimental evidence of unconventional Cooper pairing
mediated by magnetic excitations in single-layer NbSe2, a model strongly
correlated 2D material. Our high-resolution spectroscopic measurements reveal a
characteristic spin resonance excitation in the density of states that emerges
from the quasiparticle coupling to a collective bosonic mode. This resonance,
observed along with higher harmonics, gradually vanishes by increasing the
temperature and upon applying a magnetic field up to the critical values (TC
and HC2), which sets an unambiguous link to the superconducting state.
Furthermore, we find clear anticorrelation between the energy of the spin
resonance and its harmonics and the local superconducting gap({\Delta}), which
invokes a pairing of electronic origin associated with spin fluctuations. Our
findings demonstrate the fundamental role that electronic correlations play in
the development of superconductivity in 2D transition metal dichalcogenides,
and open the tantalizing possibility to explore unconventional
superconductivity in simple, scalable and transferable 2D superconductors.
