Experimental Observation of Dirac Nodal Links in Centrosymmetric Semimetal TiB$_2$

TL;DR

This study provides experimental evidence of Dirac nodal links in the centrosymmetric semimetal TiB₂, revealing rich topological features and linking configurations that could enable novel physical phenomena and applications.

Contribution

First experimental observation of Dirac nodal links in TiB₂ using ARPES and first-principles calculations, highlighting its potential as a platform for topological physics.

Findings

Observation of two groups of Dirac nodal rings in TiB₂

Nodal rings linked along the Γ–K direction forming a nodal-link configuration

Dirac nodal rings with linear dispersion up to 2 eV

Abstract

The topological nodal-line semimetal state, serving as a fertile ground for various topological quantum phases, where a topological insulator, Dirac semimetal, or Weyl semimetal can be realized when the certain protecting symmetry is broken, has only been experimentally studied in very few materials. In contrast to discrete nodes, nodal lines with rich topological configurations can lead to more unusual transport phenomena. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, here, we provide compelling evidence of nodal-line fermions in centrosymmetric semimetal TiB$_2$ with a negligible spin-orbit coupling effect. With the band crossings just below the Fermi energy, two groups of Dirac nodal rings are clearly observed without any interference from other bands, one surrounding the Brillouin zone (BZ) corner in the horizontal mirror plane $\sigma_h$ and the other surrounding the BZ center in the vertical mirror plane $\sigma_v$. The linear dispersions forming Dirac nodal rings are as wide as 2 eV. We further observe that the two groups of nodal rings link together along the $\Gamma$-$K$ direction, composing a nodal-link configuration. The simple electronic structure with Dirac nodal links mainly constituting the Fermi surfaces suggests TiB$_2$ as a remarkable platform for studying and applying the novel physical properties related to nodal-line fermions.