Intrinsic anomalous Hall effect in altermagnets

We study the anomalous Hall effect arising from the altermagnetic order and spin-orbit interaction in doped FeSb$_2$. To investigate the anomalous transport, we have constructed a tight-binding model of FeSb$_2$. We separately considered the constraints imposed on the model parameters by the spin symmetry group and magnetic symmetry group at zero and finite spin-orbit interaction, respectively. The resulting model includes the effect of exchange splitting and is applicable at both zero and finite spin-orbit interaction. In the case of spin symmetry, the analysis covers the spin-only subgroup arising from collinear magnetism, as well as non-trivial symmetry elements. This allows us to explore changes in the hopping amplitudes as symmetry is reduced by spin-orbit interaction from the spin group to the magnetic group. While the anomalous Hall effect is forbidden by spin symmetry, it is allowed by the symmetries of the magnetic group. The intrinsic Hall conductivity is shown to vanish linearly with spin-orbit interaction. This non-analytic behavior is universal to altermagnets. It originates from the singularity of the Berry curvature localized along lines on a Fermi surface confined to symmetry planes. These planes host spin degeneracy protected by spin symmetry, which is lifted by spin-orbit interaction.