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

TL;DR

This paper reports the experimental discovery of bulk nodal lines and topological surface states in ZrB₂, confirming theoretical predictions and highlighting ZrB₂ as a platform for studying nodal-line fermions.

Contribution

First experimental observation of bulk nodal lines and surface states in ZrB₂, validating theoretical models of topological nodal-line semimetals.

Findings

Identification of Dirac nodal lines forming linked rings in ZrB₂

Observation of surface states connecting to nodal lines

Confirmation of nontrivial topological nature of bulk states

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 $\Gamma$-$K$ 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.