Fig. 3: Dirac magnon nodal lines.

Nodal lines along L for a Heisenberg and b XXZ interlayer couplings (blue/red lines correspond to in-/out-of-plane polarization). Top left insets show the band structure near the nodal points, two doubly-degenerate touching cones in a and momentum-offset pairs of touching cones in b, curly arrows indicate precession of the nodal points along L. In b the diameter of the 'double helix' nodal lines is amplified for visibility by ×3 compared to the XXZΔ model. c Momentum INS intensity map as in Fig. 2d, e, but centred at the nominal nodal energy. Dashed lines show 2D Brillouin zone edges. The intensities are on a linear scale as per the colour bar in Fig. 2g. d Energy scan averaged between all six K-points in c as well as (1/3,1/3), for a cylindrical wavevector range of in-plane radius 0.03 Å⁻¹ (dark grey dot at (1/3,4/3) in c bottom left) and L = [0, 2.3]. Error bars represent one standard deviation, the dashed line is the calculated lineshape for the XXZΔ model (η = 0), and the solid line is a fit that includes an additional exchange anisotropy η = − 1.7 meV, both calculations include instrumental resolution effects.