Topological Magnons and Giant Orbital Nernst Effect in a Zigzag Kitaev Antiferromagnet

Abstract

The exploration of topological and transport properties of collinear antiferromagnets and the role of Kitaev interactions in realising topological states therein have rarely been systematically addressed in literature. In this context, we consider a zigzag-ordered antiferromagnet with both extended Kitaev and Dzyaloshinskii-Moriya interactions (DMI) in presence of an external magnetic field to focus on the topological phases demonstrated by the magnon band structure and validated by the transport properties. The hybridization between the up- and down-spin sectors carries evidences of opening bulk gaps in the magnon band structure, giving rise to nontrivial topological phases characterized by finite Chern numbers, chiral edge modes, and a nonzero thermal Hall conductivity. Furthermore, generally speaking, a finite magnon orbital moment can exist and contribute to the Nernst response even when the net spin moment vanishes owing to the fundamental independence of the spin and orbital magnetizations. This motivates us to investigate the magnon orbital moment, orbital Berry curvature, and the resulting orbital Nernst conductivity associated with the magnon bands. We find that a giant orbital Nernst conductivity emerges even in the absence of an external magnetic field. Moreover, the distinction between different topological phases is more lucidly manifested via the orbital Nernst conductivity, thereby highlighting an enhanced sensitivity of the orbital transport to the underlying band topology. For completeness, we briefly discuss the scenario corresponding to a N\'eel-ordered spin alignment, which leads to a vanishing Chern number and consequently suppressed thermal Hall and orbital Nernst conductivities compared to the zigzag-ordered case, even in the presence of DMI and Kitaev interactions.