Evidence for Itinerant Ferromagnetic Flat Bands Producing Large Transverse Responses

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 $E_F$ . To date, such an itinerant flat band at $E_F$ has been reported for electronic states without symmetry breaking. If a flat band arises at $E_F$ 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 $d$ -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 $E_F$ generating large Berry curvature in the ferrimagnet ${\mathrm{GdCo}}_5$ 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 $m^{-1}$ $K^{-1}$ at room temperature. Our study paves a path for developing itinerant magnetic flat bands and to their spintronic and thermoelectric applications.