Topology enabled unconventional superconductivity in a time-reversal symmetry-breaking bulk superconductor

Unconventional superconductors continue to challenge our understanding of condensed matter systems. These condensates have Cooper pairs with lower symmetries than in conventional superconductors. In most unconventional superconductors, the additional symmetry breaking occurs in relation to typical ingredients such as strongly correlated Fermi liquid phases, magnetic fluctuations, or strong spin-orbit coupling in noncentrosymmetric structures. In this presentation, I will show that the time-reversal symmetry breaking in the unconventional superconductor LaNiGa$_2$ is enabled by its previously unknown topological electronic band structure. Our single crystal diffraction experiments indicate a nonsymmorphic crystal structure, in contrast to the previously reported symmorphic structure. The nonsymmorphic symmetries transform the $k_z=\pi/c$ plane of the Brillouin zone boundary into a node-surface. Through band-structure calculations we reveal that distinct Fermi surfaces become degenerate on the node-surface and form Dirac lines and a Dirac loop at the Fermi level. Within the Dirac loop are two symmetry related Dirac points which remain degenerate under spin-orbit coupling. ARPES measurements confirm the calculations and provide evidence for the Fermi surface degeneracies on the node-surface. These unique topological features enable an unconventional superconducting gap in which time-reversal symmetry can be broken in the absence of other typical ingredients. LaNiGa$_2$ provides the first example of a topological crystalline superconductor that breaks time-reversal symmetry without any overlapping magnetic ordering or fluctuations. Our findings will enable future discoveries of additional topological superconductors.