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.
Highlights
Topological materials with symmetry-protected nodes have recently attracted much attention in condensed matter physics
The simple electronic structure with Dirac nodal links mainly constituting the Fermi surfaces suggests TiB2 as a remarkable platform for studying and applying the novel physical properties related to nodal-line fermions
The mirrorlike (001) surface is illustrated in the inset of Fig. 1(b)
Summary
Topological materials with symmetry-protected nodes have recently attracted much attention in condensed matter physics. Perturbation that preserves a certain symmetry cannot remove the nodes by opening a full direct gap in these materials When such nodes are close to the Fermi level (EF), the low-energy quasiparticle excitations are drastically different from the usual Schrödinger-type fermions. These nodes can be classified by their dimensionality [1,2]. Without interfering with other bands, the dispersions forming the nodal rings exhibit linearly in a wide energy range of approximately 2 eV These two groups of nodal rings link together along the Γ‐K direction forming a nodal-link configuration, which goes beyond the isolated nodal line configuration in other systems. The compelling evidences of the nodal-line fermions existing just below EF and the nodal links mainly constituting the Fermi surfaces (FSs) provide an ideal system for further investigations and potential applications on transport phenomena of nodal lines
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