Generation of High-Order Van Hove Singularity and Pomeranchuk Instability on the Distorted Surface of a Kagome Metal

The band structure of Kagome metals hosts a flat band and a Dirac node with two saddle points flanking it. While an ideal Kagome flat band maximizes the density of state enhancement of a two dimensional band structure, nothing protects it from dispersing in the presence of material perturbations. In contrast, saddle points, that give rise to logarithmic divergence in the density of states, cannot be easily removed. We investigate a Kagome ferromagnetic metal using scanning tunneling spectroscopy. We identify a triangular distortion on its Kagome surface termination that considerably flattens the saddle point dispersion and induces a high-order van Hove singularity pinned to the Fermi energy. We capture the process of its generation both in ab initio calculation and with a minimal tight binding model that elucidates its origin. We find that the strong interactions of the dense electrons result in a d-wave Pomeranchuck instability. We visualize both in real- and in reciprocal-space a cascade of nematic wavefunction distributions that spontaneously break the rotational symmetry imposed by the underlying distorted Kagome lattice without generating any additional translational symmetry breaking. The evolution of the wave function across the Fermi energy further suggests the spontaneously deformed Fermi surface gives rise to a sort of charge pumping. The form of Kagome distortion that generates the higher order van Hove singularity may be common to other Kagome materials, where it can result in other electronic instabilities.