Anomalies in the Dirac bands in proximity of correlated electrons

Dirac fermions, particles with zero rest mass, are observed in topological materials and are believed to play key role in the exoticities in fundamental science and advancement of quantum technology. Most of the topological systems studied so far are weakly correlated systems and their properties in the presence of electron correlation is an interesting emerging area of research, where the electron correlation is expected to enhance the effective mass of the particles. Here, we studied the properties of Dirac bands in a nonsymmorphic layered Kondo lattice system, CeAgSb$_2$ employing high-resolution angle-resolved photoemission spectroscopy and first-principles calculations. In addition to the Dirac cones due to non-symmorphic symmetry, this material hosts Dirac fermions in the squarenet layer in proximity of a strongly correlated Ce-layer exhibiting Kondo behavior. Experimental results reveal crossings of the highly dispersive linear bands at the Brillouin zone boundary due to non-symmorphic symmetry. In addition, there are anisotropic Dirac cones constituted by the squarenet Sb 5$p$ states forming a diamond-shaped nodal line. These Dirac bands are linear in a wide energy range with a unusually high slope. Interestingly, near the local Ce 4$f$ bands, these bands exhibit a change in slope akin to formation of a 'kink' observed in other materials due to electron-phonon coupling. Emergence of such exotic properties in proximity to strongly correlated electronic states has significant implication in the study of complex quantum materials including unconventional superconductors.