Non-equilibrium topology in magnonic systems

Abstract

In this Thesis, we study the observable consequences of topological magnetic excitations and how they can be used for applications on the long term. Since direct detection is not feasible, we study a series of alternative approaches to access the effects of topology in magnetic systems, with a focus on the effect of topology on the transport properties of spin wave systems. We first study the topological effects rooted in the dipole-dipole interaction – an interaction which is present in any magnon systems. We study the propagation of spin waves in obliquely magnetized thin films, where the dipole-dipole interaction breaks the inversion symmetry. We show that a single spin wave excited by a microwave antenna will induce a transverse spin current, which can be detected by a normal metal lead. Next, we study how the magnon Hall effect can be detected through electrical injection and detection. We show that the Hall coefficient, when taking dipole-dipole interactions into account, can be measured. We then turn to topological magnon phases, which have topologically protected edge states, addressing a major problem in the field of magnon topology: the high frequencies of the chiral edge modes in magnon Chern insulators. We propose a strategy to lower these chiral edge states to zero frequency, by considering a non-equilibrium setup. We show that the spin waves can be excited in the GHz-range, making them directly experimentally detectable. We finally consider non-Hermitian topological magnon phases. In a non-Hermitian extension of a one-dimensional topological magnon insulator we show the edge excitations to be robust against non-linearities and stochastics fluctuations. Within a non-Hermitian extension of the magnon Chern insulator we show that the chiral edge states can be topologically amplified or damped. This model also exhibits a novel non-Hermitian hybrid skin effect, where the edge modes are localized on specific sides of the lattice, and we propose an experiment to detect this skin effect.