Flat bands and non-Fermi liquid behaviour in a Weyl ferromagnet*

There is an enormous literature dealing with the Anderson and Kondo lattices where dispersive bands coexist with bands which are flat on account of small underlying atomic orbitals. Kagome systems represent venues for interaction effects occurring when dispersive bands cross bands which are flat for reasons of lattice geometry rather than small orbital extent. We have performed density functional theory (DFT), transport, angle resolved photoemission (ARPES) and resonant inelastic X-ray scattering (RIXS) to map the spin and charge quasiparticles in the bilayer Kagome ferromagnet Fe3Sn2, well known for enigmatic electrical and magnetic properties. The findings start with numerous Weyl nodes, close to the Fermi surface, and strongly dependent on the magnetization direction which is readily steerable with modest external fields. At the lowest temperatures, high resolution laser-based ARPES displays sharp quasiparticles at the Fermi surface which are in accord with the DFT+ U calculations, and are also strongly scattered, in linear marginal Fermi liquid fashion, as the binding energy is increased from zero. In addition, there is an even sharper band, not predicted by DFT, which crosses the Fermi surface nearby and appears to arise from hybridization with a flat band, placed by DFT+U just above the Fermi surface, where carriers can be localized due to strong Coulomb repulsion. RIXS validates this picture by revealing spin waves with unusually strong damping, implying very strong coupling between the charge degrees of freedom and longer-lived magnetization fluctuations.