Evidence for Itinerant Ferromagnetic Flat Bands Producing Large Transverse Responses

ABSTRACT Interference of electronic wavefunctions may result in the emergence of a flat band, and trigger nontrivial correlated phenomena and phase formations when the flat band crosses the Fermi energy . To date, such an itinerant flat band at has been reported for electronic states without symmetry breaking. If a flat band arises at even with broken symmetry, the ordered state may offer a distinct platform for studying novel phases and spontaneous responses. Here we report experimental and theoretical evidence for itinerant ferromagnetic flat bands formed by spin‐polarized ‐electron orbitals on a stacked honeycomb–kagome lattice, hosting the alternating stack of honeycomb and kagome sublattices. Our theory together with angle‐resolved photoemission spectroscopy and magneto–thermoelectric measurements finds multiple topological flat bands at generating large Berry curvature in the ferrimagnet below its Curie temperature of 940 K. We observe large transverse responses, in particular, gigantic anomalous Nernst effect producing the largest transverse thermoelectric conductivity over 10 A at room temperature. Our study paves a path for developing itinerant magnetic flat bands and to their spintronic and thermoelectric applications.