Composite Topological Weyl Nodal lines

Abstract: Nodal lines are one-dimensional topological features of semi-metal band structures along which two bands are degenerate as a result of non-accidental symmetry-protected crossings, and behave topologically as $k$-space vortices in the Berry connection. Here, we present a new class of tilted nodal lines, protected by mirror symmetry, formed from the intersection of three band crossings at a set of critical points. One crossing is gapped out, fusing the remaining two crossings at the shifted critical points to form composite nodal lines. We demonstrate these composite nodal lines are capable of supporting fundamentally different Berry curvature textures than the typical two-band case, despite having a simple ring topology. In addition, we present a realistic model based on cubic, forced-ferromagnetic, EuTiO$_3$, where the spin and orbital degrees of freedom are plentiful enough to allow the material realization of such composite nodal lines. In this system, the composite nature of the nodal line results in a spin Hall conductivity with a non-monotonic dependence on carrier concentration.