Dirac fermions with highly-dispersive linear bands are usually considered weakly correlated, due to relatively large bandwidths (W) compared to Coulomb interactions (U). With the discovery of nodal-line semimetals, the notion of Dirac point has been extended to lines and loops in the momentum space. The anisotropy associated with nodal-line structure gives rise to greatly reduced kinetic energy along the line. However, experimental evidence for anticipated enhanced correlations in nodal-line semimetals is sparse. Through optical studies on the ZrSiSe single crystals provided by 2DCC, Basov’s group at Univ. of Columbia find prominent correlation effects in nodal-line semimetals. They observed spectroscopic hallmarks of electronic correlations: strong reduction of the Drude weight and the Fermi velocity.