  Time-reversal invariant superconductors in three dimensions may contain nodal
lines in the Brillouin zone, which behave exactly as Wilson loops of 3d
momentum-space Chern-Simons theory of the Berry connection. Here we study the
conditions of realizing linked nodal lines (Wilson loops), which yield a
topological contribution to the thermal magnetoelectric coefficient that is
given by the Chern-Simons action. We find the essential conditions are the
existence of torus or higher genus fermi surfaces and spiral spin textures. We
construct such a model with two torus fermi surfaces, where a generic
spin-dependent interaction leads to double-helix-like linked nodal lines as the
superconductivity is developed.
