Incipient nematicity from electron flat bands in a kagomé metal

Abstract Engineering new quantum phases requires fine tuning of the electronic, orbital, spin, and lattice degrees of freedom. To this end, kagomé lattice with flat bands has garnered great attention by hosting various topological and correlated phases. However, the impact of flat bands in materials’ macroscopic properties has been elusive. Here we discover the unconventional nematicity in kagomé metal CoSn. Thermodynamic, dilatometry, resonant X-ray scattering, inelastic neutron scattering, Lamour diffraction, and thermoelectric measurements consistently hint rotational symmetry-breaking and nematic order that is pronounced only near T* = 225 K. The observations, principally the nematic's finite temperature stability--incipience-- can be explained by a phenomenological model which reveals that thermally excited flat bands become unstable and promote symmetry breaking at a characteristic temperature determined by their energy distance from the Fermi level, qualitatively and quantitatively agreeing with the experiments. Our work shows that that thermal fluctuations, which are typically detrimental for correlated electronic phases, can induce new ordered states of matter.