Gapped nodal planes and large topological Nernst effect in the chiral lattice antiferromagnet CoNb₃S₆

The electronic structure of compensated antiferromagnets (CAF) creates large functional responses, reminiscent of ferromagnets and suitable for data storage and readout, despite (nearly) net-zero spontaneous magnetization. Many experimental signatures of CAF - such as giant thermoelectric Nernst effects - should be enhanced when two or more electronic bands are nearly degenerate in vicinity of the Fermi energy. Here, we report a zero-field, thermoelectric Nernst effect  >1 μV/K in the CAF CoNb₃S₆ despite its tiny net magnetization  ~2 milli - μ_B. As drivers of the functional Nernst and Hall effects, we identify near-degeneracies of electron bands at the upper and lower boundaries of the first Brillouin zone, which are vestiges of nodal planes enforced by a screw axis symmetry in the paramagnetic state. Hot spots of emergent, or fictitious, magnetic fields are formed at the slightly gapped nodal planes. Taking into account more than six hundred Wannier orbitals, our theoretical model reproduces the observed spontaneous Nernst effect, emphasizes the role of proximate spin-space group symmetries and nodal planes for the electronic structure of CAF, and demonstrates the promise of ab-initio search for functional responses in a wide class of materials with reconstructed unit cells (supercells) due to spin or charge order.