Inducing $\mathbb{Z}_2$ Topology in Twisted Nodal Superconductors

Abstract: Twisted nodal superconductors have been shown to exhibit chiral topological superconductivity under broken time-reversal symmetry. Here we show how a time-reversal preserving topological superconductivity can be induced in nodal triplet superconductor multilayers. For a bilayer system, the application of a Josephson spin current in triplet superconductors induces a non-zero spin Chern number per node in momentum space. However, we show that stabilizing a nontrivial global $\mathbb{Z}_2$ invariant requires an odd number of layers. As a specific example we consider trilayers with three forms of twist: chiral, alternating and single-layer. For chiral and single-layer case, we find that a gap opening in the dispersion leads to a non-trivial $\mathbb{Z}_2$ topological invariant. For single layer twists, we show how this invariant is non-trivial when extended to an arbitrary odd number of layers.