Effective field theory and thermal Hall effect of magnons in square-lattice antiferromagnets

Abstract: Thermal Hall transport has emerged as a powerful probe of neutral quasiparticles and associated gauge fields in insulating materials. Although the emergence of a thermal Hall effect is known to be sensitive to lattice geometry and gauge structures, an intuitive understanding of the conditions for its emergence remains limited, especially for edge-shared lattice geometries such as square and triangular lattices. Here, we develop an effective field theory of magnons in square-lattice antiferromagnets to establish the intuitive picture that elucidates the conditions for a finite thermal Hall response. By constructing an effective field theory from a spin model on the square lattice, we show that its low-energy excitations can be described by magnons with an effective SU(2) gauge field and Zeeman field that couple to magnon's pseudospins, which reflect the two-sublattice degrees of freedom in the antiferromagnets. The field strength associated with the SU(2) gauge field acts as a pseudospin-dependent magnetic field, bending the magnon's trajectories in opposite directions depending on their pseudospin. In addition, the effective Zeeman field induces an imbalance between pseudospin up and down magnons, and the combination of these two fields gives rise to the thermal Hall effect of magnons. This intuitive picture provides a systematic classification of magnetic orders in square-lattice antiferromagnets based on the presence or absence of the thermal Hall effect. We expect that our framework can be extended to various other spin models.