  The topological properties of fermions arise from their low-energy Dirac-like
band dispersion and associated chiralities. Initially confined to points,
extensions of the Dirac dispersion to lines and even loops have now been
uncovered and semimetals hosting such features have been identified. However,
experimental evidence for the enhanced correlation effects predicted to occur
in these topological semimetals has been lacking. Here, we report a quantum
oscillation study of the nodal loop semimetal ZrSiS in high magnetic fields
that reveals significant enhancement in the effective mass of the
quasiparticles residing near the nodal loop. Above a threshold field, magnetic
breakdown occurs across gaps in the loop structure with orbits that enclose
different windings around its vertices, each winding accompanied by an
additional \pi-Berry phase. The amplitudes of these breakdown orbits exhibit an
anomalous temperature dependence. These findings demonstrate the emergence of
novel, correlation-driven physics in ZrSiS associated with the Dirac-like
quasiparticles.
