The thermal Hall effect, sometimes also called Righi-Leduc effect, appears as transversal heat currents induced by thermal gradients. Magnons, the magnetic excitations in long-ranged ordered systems, transport spin and heat with no charge currents. In this talk I demonstrate that the transversal transport coefficient in a two-dimensional honeycomb antiferromagnet features clear signs of magnetic and topological phase transitions. An external magnetic field drives the system from the antiferromagnetic phase via the noncollinear spin-flop phase into the field-polarized phase. Additionally, topological phase transitions are found in the spin flop phase. Both types of phase transitions manifest themselves in prominent and distinguishing features in the thermal conductivities; depending on the temperature, the conductivity changes by several orders of magnitude, providing a means to identify experimentally these phase transitions.

![](https://assets.underline.io/uploads/markdown_image/1/image/13ae6acc84046eacddf6d5ad89e32051.jpg)

Thermal Hall Effect of Magnons in Collinear Antiferromagnetic Insulators: Signatures of Magnetic and Topological Phase Transitions

When interacting spins in condensed matter order ferromagnetically, their ground-state wave function is topologically trivial. Nonetheless, in two dimensions, ferromagnets can support spin excitations with nontrivial topology, an exotic state known as topological magnon insulator (TMI). In Ref. [1], we theoretically unveil and numerically confirm a ferromagnetic state in three dimensions dubbed second-order TMI, whose hallmarks are excitations at its hinges, where facets intersect. Since ferromagnetism naturally comes with broken time-reversal symmetry, the hinge magnons are chiral, rendering backscattering impossible. Hence, as shown in the figure, they trace out three-dimensional paths about the sample unimpeded by defects and are topologically protected by the spectral gap. They are remarkably robust against disorder and highly tunable by atomic-level engineering of the sample termination. We predict that a van der Waals heterostructure built from chromium trihalide and transition metal dichalcogenide monolayers exhibits second-order magnon topology. Our findings empower magnonics, the harnessing of spin waves as information carriers, with the tools of higher-order topology, a promising route to combine low-energy information transfer free of Joule heating with three-dimensional vertical integration.

![](https://assets.underline.io/uploads/markdown_image/1/image/3539eda5d4812d50ee16d061b7719c73.jpg)

Chiral Hinge Magnons in Second-Order Topological Magnon Insulators

Alexander Mook

2

5

SHORT BIO

Stay up to date with the latest Underline news!

PRESENTATIONS

CONFERENCES

COMPANY

RESOURCES