Topological nodal line in superfluid He-3 and the Anderson theorem

Superconductivity and superfluidity with anisotropic pairing-such as d-wave in cuprates and p-wave in superfluid He-3-are strongly suppressed by impurities. Meanwhile, for applications, the robustness of Cooper pairs to disorder is highly desired. Recently, it has been suggested that unconventional systems become robust if the impurity scattering mixes quasiparticle states only within individual subsystems obeying the Anderson theorem that protects conventional superconductivity. Here, we experimentally verify this conjecture by measuring the temperature dependence of the energy gap in the polar phase of superfluid He-3. We show that oriented columnar non-magnetic defects do not essentially modify the energy spectrum, which has a Dirac nodal line. Although the scattering is strong, it preserves the momentum along the length of the columns and forms robust subsystems according to the conjecture. This finding may stimulate future experiments on the protection of topological superconductivity against disorder and on the nature of topological fermionic flat bands. Anderson's theorem states that superconductivity in a conventional superconductor is robust to non-magnetic disorder. Here, the authors demonstrate the protection of Cooper pairs by the extended Anderson theorem in the polar phase of superfluid helium-a spin-triplet superconductor analogue.