  The electronic structure of compensated antiferromagnets (CAF) has drawn
attention for its ability to create large responses, reminiscent of
ferromagnets and suitable for data storage and readout, despite (nearly)
net-zero spontaneous magnetization. Many of the striking experimental
signatures predicted for CAF, such as giant thermoelectric Nernst effects, are
enhanced when two or more electronic bands are nearly degenerate in vicinity of
the Fermi energy. Here, we use thermoelectric and electric transport
experiments to study the electronic structure of the layered, chiral metal
CoNb3S6 in its all-in-all-out CAF ground state and report near-degeneracies of
electron bands at the upper and lower boundaries of the first Brillouin zone.
Considering non-symmorphic spin-space group symmetries in the non-relativistic
approximation for the ordered phase, these near-degeneracies are approximately
protected by a lattice translation combined with spin rotation, and 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 plane, generating the spontaneous Hall and Nernst effects
in this CAF. Taking into account more than six hundred Wannier orbitals, our
model quantitatively reproduces the observed spontaneous Nernst effect,
emphasizes the role of proximate symmetries in the emergent responses of CAF,
and demonstrates the promise of ab-initio search for functional responses in a
wide class of materials with reconstructed unit cells due to spin or charge
order.
