Thermal transport and topological aspects of magnons

In this dissertation, we investigate the thermal transport properties of magnons in several antiferromagnetic systems, focusing on their topological nature and their interplay with electrons and phonons. In the second chapter, we study magnon spin thermal transport using a strong coupling approach in pyrochlore iridate trilayer thin films grown along the [111] direction. As a result of the Dzyaloshinskii-Moriya interaction (DMI), the spin configuration of the ground state is an all-in/all-out (AIAO) ordering on neighboring tetrahedra of the pyrochlore lattice. In such a state, the system has an inversion symmetry and a Nernst-type thermal spin current response is well defined. We calculate the temperature dependence of the magnon Nernst response concerning the magnon band topology controlled by spin-orbit coupling parameters and observe topologically protected chiral edge modes. Our study suggests that the [111] grown thin-film pyrochlore iridates are a promising candidate for thermal spin transport and spin caloritronic devices. In the third chapter, we present a general theory of the longitudinal spin Seebeck effects in the case that the anti-symmetric DMI is considered at the interface. By using an AIAO spin-ordered pyrochlore iridate as the magnetic insulator with a large DMI, we conduct a systematic study of the dependence of the thermally-driven interfacial spin current on the temperature gradient, the interfacial DMI interaction and the crystalline orientation along the interface. Our results show that the spin current injected in the metal is surprisingly sensitive to the orientation of the interface and the direction of the Dzyaloshinskii-Moriya (DM) vectors, offering a route for both probing magnetic properties via a spin-transport measurement and engineering efficient heterostructures. In the fourth chapter, we theoretically study magnon-phonon hybridization in a two-dimensional antiferromagnet on a honeycomb lattice. With an in-plane DMI from mirror symmetry breaking, we find non-trivial Berry curvature around the anti-crossing regions among magnon and both optical and acoustic phonon bands, which gives rise to well-defined Chern numbers. We also show that the Chern numbers of this hybridized system are highly tunable by an external magnetic field. For experiments, we evaluate the thermal Hall conductivity reflecting the non-trivial Berry curvatures of the excitations and propose the possibility of observing valley Hall effects resulted from spin-induced chiral phonons. Our study complements prior work in magnon-phonon hybridized systems without optical phonons and suggests possible applications in spin caloritronics with topological magnons and chiral phonons.