Quantum Matter Seminar

The kagome lattice structure of corner-sharing triangles hosts a variety of electronic motifs that challenge Fermi liquid theory and give rise to exotic responses. On the heavy side, the kagome band structure hosts a pair of saddle points that correspond to van Hove singularities with a diverging density of states. Destructive interference further yields a flat band of localized moments. On the light side, it hosts a symmetry-protected Dirac band of massless electrons that, under certain layer stacking schemes, gives rise to three-dimensional massless Weyl electrons. We studied the three motifs on the atomic scale using scanning tunneling spectroscopy. We visualized Weyl electrons on the surface of the kagome-stacked ferromagnet Co3Sn2S2. On its kagome Co3Sn termination, we identified a new mechanism under which a breathing distortion turns the kagome van Hove singularity into a higher-order one at the Fermi energy. The strong interactions result in a spontaneous formation of a nematic state under a Pomeranchuk instability. In the kagome metal Ni3In, the kagome flat band is tuned to the Fermi energy, resulting in the breakdown of Fermi liquid theory signified by a strange metal non-linear temperature dependence of resistivity. We identify spectroscopically the localized degrees of freedom and their strong interactions with Dirac light bands pertaining to heavy-Fermion systems. Our studies thus reveal the exciting opportunities put forward by kagome metals for the investigation of exotic electronic states ranging from massless Dirac-particles, through moderately heavy states exhibiting spontaneous symmetry breaking, to super-heavy ones at which the interaction energy surpasses the bandwidth extending the universality of the heavy Fermion paradigm to kagome flat bands.