Topological nodal-line fermions in spin-orbit metal PbTaSe2

TLDR

Topological semimetals host one‑dimensional Fermi lines or zero‑dimensional Weyl points, but nodal lines require additional crystalline symmetries such as mirror reflection for protection. The study reports the existence of topological nodal‑line states in PbTaSe₂ with strong spin‑orbit coupling, based on systematic theory and detailed experimental characterization. The authors employ angle‑resolved photoemission, first‑principles simulations, and topological analysis to reveal the mechanism forming nodal‑line states and their associated surface states in PbTaSe₂. They find that PbTaSe₂ hosts spin‑orbit nodal lines protected by mirror symmetry, characterized by an integer topological invariant, and that these lines give rise to surface states.

Abstract

Abstract Topological semimetals can support one-dimensional Fermi lines or zero-dimensional Weyl points in momentum space, where the valence and conduction bands touch. While the degeneracy points in Weyl semimetals are robust against any perturbation that preserves translational symmetry, nodal lines require protection by additional crystalline symmetries such as mirror reflection. Here we report, based on a systematic theoretical study and a detailed experimental characterization, the existence of topological nodal-line states in the non-centrosymmetric compound PbTaSe 2 with strong spin-orbit coupling. Remarkably, the spin-orbit nodal lines in PbTaSe 2 are not only protected by the reflection symmetry but also characterized by an integer topological invariant. Our detailed angle-resolved photoemission measurements, first-principles simulations and theoretical topological analysis illustrate the physical mechanism underlying the formation of the topological nodal-line states and associated surface states for the first time, thus paving the way towards exploring the exotic properties of the topological nodal-line fermions in condensed matter systems.