Topological Nodal-Line Fermions in the Non-Centrosymmetric Superconductor Compound PbTaSe2

In a typical three-dimensional metal, the low-energy excitations are found on a two-dimensional closed Fermi surface in momentum space. Topological semimetals, by contrast, can support onedimensional Fermi lines or zero-dimensional Fermi-Weyl points, at locations in momentum space, where the valence and conduction bands touch. While the degeneracy points in Weyl semimetals are protected against any perturbation that preserves translational symmetry, nodal lines are topologically protected by symmetries such as mirror reflection. Here we report on the existence of topological nodal-line states in the non-centrosymmetric compound single-crystalline PbTaSe2 with strong spin-orbit coupling. Remarkably, the spin-orbit nodal lines in PbTaSe2 are not only protected by the reflection symmetry but also characterized by an integer topological invariant as we show here. Our detailed angle-resolved photoemission (ARPES) measurements, first-principles simulations and theoretical analysis illustrate the physical mechanism underlying the formation of the topological nodal-line states (TNLS, in contrast to Weyl Semimetals) and associated surface states for the first time. Therefore, this work paves the way towards exploring the exotic properties of the topological nodal-line fermions in condensed matter systems and, potentially, the rich physics arising from the interplay between the topological nodal-line states and the emergent superconductivity we report in the single crystal phase phase of this compound.