Topology of contact points in Lieb-kagom\'e model

Abstract

We analyse Lieb-kagom\'e model, a three-band model with contact points showing particular examples of the merging of Dirac contact points. We prove that eigenstates can be parametrized in a classification surface, which is a hypersurface of a 4-dimension space. This classification surface is a powerful device giving topological properties of the energy band structure; the analysis of its fundamental group proves that all singularities of the band structure can be characterized by four independent winding (integer) numbers. Lieb case separates: its classification surface differs and there is only one winding number.