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 <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msub><mi>E</mi> <mi>F</mi></msub> <annotation>$E_{\rm F}$</annotation></semantics> </math> . To date, such an itinerant flat band at <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msub><mi>E</mi> <mi>F</mi></msub> <annotation>$E_{\rm F}$</annotation></semantics> </math> has been reported for electronic states without symmetry breaking. If a flat band arises at <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msub><mi>E</mi> <mi>F</mi></msub> <annotation>$E_{\rm F}$</annotation></semantics> </math> 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 <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mi>d</mi> <annotation>$d$</annotation></semantics> </math> -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 <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msub><mi>E</mi> <mi>F</mi></msub> <annotation>$E_{\rm F}$</annotation></semantics> </math> generating large Berry curvature in the ferrimagnet <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msub><mi>GdCo</mi> <mn>5</mn></msub> <annotation>${\rm GdCo}_5$</annotation></semantics> </math> 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 <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msup><mi>m</mi> <mrow><mo>-</mo> <mn>1</mn></mrow> </msup> <annotation>${\rm m}^{-1}$</annotation></semantics> </math> <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msup><mi>K</mi> <mrow><mo>-</mo> <mn>1</mn></mrow> </msup> <annotation>${\rm K}^{-1}$</annotation></semantics> </math> at room temperature. Our study paves a path for developing itinerant magnetic flat bands and to their spintronic and thermoelectric applications.