Correlated singular flat bands on the surface pentagonal lattice of ferromagnetic CoS2

Flat bands have been proposed in various geometrically frustrated lattices, yet their experimental realization has been limited only to the two-dimensional (2D) kagome and three-dimensional (3D) pyrochlore lattices. While synthesizing bulk crystals with desired lattice geometry is generally challenging, the surfaces of 3D bulk materials sometimes naturally host 2D lattice structures hardly achievable in 2D bulk crystals. Thus, the electrons localized on such 2D surfaces can be a new platform for exotic flat bands residing on the unexplored lattice geometry. Here, we report the first experimental observation of singular flat bands with band crossings in the 2D pentagonal lattice on the surface of the 3D ferromagnetic topological semimetal CoS2. We demonstrate that the coupling between localized surface flat bands and extended bulk topological bands gives rise to the surface non-Fermi liquid behavior characterized by the quasiparticle scattering rate with linear temperature dependence and pronounced quasiparticle broadening. This study of CoS2 highlights metallic pyrite materials as an ideal platform for exploring the exotic properties of spin-polarized singular flat bands localized on the surface pentagonal lattice, which provides a fresh perspective for investigating correlated electron phenomena in pentagon-based materials. Here, the authors report the observation of flat bands on a pentagonal surface lattice of ferromagnetic CoS2, revealing a route to engineering correlated flat-band states in 3D crystals.