Nematicity from incipient electron flat bands in a kagomé metal

Here, we present evidence for nematicity arising from incipient flat electronic bands away from the Fermi level in a kagomé metal CoSn. We use a combination of techniques probing thermodynamic (specific heat, magnetostriction), structural (neutron scattering) and electronic (resonant x-ray scattering) properties of the system to probe even minute distortions. The rotational symmetry breaking exists in a finite temperature range centered near T* = 225 K and is accompanied by strong magnetocaloric anomalies. We attribute the observed phenomena to the pronounced enhancement of system's susceptibility to rotational symmetry-breaking driven by the thermally-activated electron flat band carriers. Our findings point towards the possibilities realization new phases of matter with non-degenerate, thermally activated flat-band electrons.