Research interest
Research interest
Topological characters in a magnetic systems can arise in either (1) real space due to topological spin textures such as magnetic skyrmions and magnetic hopfions or (2) in momentum space in a form of topological band structures or (3) both. They have profound effects on the observable quantities of the magnets. For instance, topology of spin textures influences its own dynamics. This is seen in the dynamical equations of motion for a magnetic skyrmion as the skyrmion topological charge appears explicitly in the EOMs, giving rise to physical phenomena such as the skyrmion Hall effect present only when the topological charge is nonzero. Topology of spin textures in metallic magnets also gets imprinted onto electronic wave functions, giving rise to characteristic effects observed in experiments such as the emergent electrodynamics, topological Hall effect and optical Hall resonance. In reciprocal space, magnetic systems can realize topological features in the band structure including Chern bands, Weyl nodes, and Weyl nodal lines. Symmetry can enrich these topological features, as already been seen in topological nodal line semimetals, where spatial symmetries protect nodal lines, endowing them with robustness against symmetry-protecting perturbations.
I am interested in novel physics arising in magnetic systems from the interplay between (i) real-space topology, (ii) reciprocal-space topology, and (iii) novel symmetry.
"Which phenomena emerge due to such an interplay? How? What systems share these properties, as they are broadly dictated by simply topology and symmetry?"
Physical phenomena of interest include dynamics, spectral properties, and linear/nonlinear responses, e.g. transport. Systems of interest include chiral magnets, frustrated magnets, ferromagnets, commensurate antiferromagnets, altermagnets, and systems supporting multipolar moments; these are promising in supporting real-space topology, reciprocal-space topology, and novel symmetry properties.
We recently complete a work on the nonlinear Hall effects from hidden octupolar orders, where we consider a system lacking magnetic dipole moments. The octupolar order parameter has a rather high symmetry, resulting in a vanishing magnetic-dipole-like order parameter. This in turn quenches the linear-response anomalous Hall effect, yet it supports a third-order Hall effect as the leading effect. This provides a physical observable which is tied to the hidden octupolar order, thereby exploitable as a probe of the hidden order. It turns out that RuO2, an altermagnetic candidate, shares similar symmetry properties and similar octupolar characters, so the third-order Hall effect can be used as a probe of the altermagnetism as well. It remains an open questions how to differentiate between altermagnetism and atomic multipolar ordering in terms of their transports, spectral and optical properties.
In a few upcoming projects, we are working on the interplay between symmetry and topology, which turns out to dictate over the dynamics of skyrmions and hopfions. We are also formulating a symmetry-based theory of Hall transports in antiferromagnets, which is expected to provide a guide to engineer Hall phenomena. In related projects, we investigate electronic band structures and how they influence transport properties.