Visualizing near-coexistence of massless Dirac electrons and ultra-massive saddle point electrons

Strong singularities in the electronic density of states aenhance correlation effects and play a key role in determining the ordering instabilities in various materials. Recently high order van Hove singularities (VHS) with diverging power-law scaling have been classified in single-band electron models. We show that the 110 surface of Bismuth exhibits a remarkable power-law divergence of the DOS, with unusually high exponent, arising from high-order VHS presented on this surface. Spectroscopic mapping of the surface band structure using scanning tunneling microscopy and spectroscopy shows that this high-order VHS occurs in conjunction with a highly anisotropic Dirac band crossing located at the center of the surface Brillouin zone. The enhanced power-law divergence is shown to originate from the anisotropic flattening of the Dirac band just above the Dirac node. Such near-coexistence of massless Dirac electrons and ultra-massive saddle points enables to study the interplay of high order VHS and Dirac fermions.