Observation of Effective Pseudospin Scattering in ZrSiS

3D Dirac semimetals are an. emerging class of materials that possess topological electronic states with a Dirac dispersion in their bulk. In nodal-line Dirac sethimetalS, the conductance and Valenee,bands connect along a closed path in momentum space, Jeading to the prediction of pseudospin vortex rings and pseudospin, skyrmions; Here, we use Fourier transform scanning tunneling spectroscopy (FT-STS) at 4.5 K to resolve quasiparticle interference (QPI) patterns rat single defect centers on the surface of the line, nodal semimetal zirconiumsilicon sulfide (ZrSiS). Our QPI measurements show pseudospin conservation at energies close to the line node. In addition, we determine the Fermi velocity to be hv(F) = 2.65 +/- 0.10 eV angstrom in the Gamma-M direction similar to 300 meV above the Fermi energy EF 40 meV above EF. More importantly, we find that certain scatterers can, introduce energy -dependent nonpreservation of pseudospin, giving rise to effective scattering between states with opposite pseudospin deep inside' valence and conduction bands. Further investigations of quasiparticle interference at the atomic level will aid defect engineering at the synthesis level, needed for the development of lower -power electronics via. dissipationless electronic transport in the future.