  The Dirac nodal line (DNL) is a novel form of massless Dirac fermions that
reside along lines in momentum space. Here, we verify genuine DNLs in the
molecular material, [Ni(dmdt)$_2$], with the combined NMR experiments and
numerical simulations. The NMR spectral shift and spin-lattice relaxation rate
divided by temperature, $1/T_1T$, decrease linearly and quadratically with
temperature, respectively, and become constant at low temperatures, consistent
with slightly dispersive DNLs with small Fermi pockets. Comparison of these
results with model simulations of DNLs reveals the suppression of the Fermi
velocity and the enhancement of antiferromagnetic fluctuations due to electron
correlation as well as the influence of the Landau quantization. The present
study offers a demonstration to identify the DNL and evaluate the correlation
effect with NMR.
