Experimental observation of bulk nodal lines and electronic surface states in ZrB${}_2$

Abstract

Topological nodal-line semimetals are characterized by the line-contact bulk band crossings and the topological surface states. Breaking certain protecting symmetry turns this system into a Dirac semimetal or Weyl semimetal that hosts zero-dimensional isolated nodal points. Recent advances in band theory predicted a topological nodal-line semimetal state possessing a new type of nodal line in AlB${}_2$-type diborides. Here, we report an experimental realization of nodal-line fermions and associated surface states near the Fermi energy in ZrB${}_2$ by angle-resolved photoemission spectroscopy combined with first-principles calculations. The Dirac nodal lines in ZrB${}_2$ wind into two groups of nodal rings, which are linked together along the $\hat{I}“$-$K$ direction. We further observe a distinct surface state connecting to each nodal line, indicative of the nontrivial topological nature of the bulk nodal lines. Therefore, our results provide convincing experimental evidence of the nodal-line semimetal states in ZrB${}_2$ both in the bulk and on the surface, suggesting ZrB${}_2$ as a remarkable platform for discovering unique phenomena induced by nodal-line fermions.

24 pages, 4 figures