Itinerant topological magnons and spin excitons in twisted transition metal dichalcogenides: Mapping electron topology to spin counterpart

Twisted transition metal dichalcogenide (tTMD) provides a highly tunable platform to explore the interplay between strong correlation and topology. Among them, the properties involving the charge degree of freedom have been extensively studied, while those related to spin are much less investigated. Motivated by the recent discovery of integer and fractional quantum anomalous Hall effects in tMoTe2, for which the flat-band ferromagnetism is an essential prerequisite, we investigate theoretically the spin excitations out of the flat-band ferromagnetic ground state in tMoTe2. Remarkably, we identify the itinerant magnons and spin excitons with nontrivial topology. We elaborate that the topology of these itinerant spin excitations, which are described as particle-hole bound states, inherits directly from that of the underlying electrons and is fundamentally different from that in local spin systems. Thus, we establish a direct relationship of the topology between the many-body excitations and their fundamental constituents. We further demonstrate that by tuning the displacement field, a topological transition for these excitations occurs, leading to a step-like change and bifurcation in the thermal Hall conductivity, which could serve as unique and compelling evidence to be tested experimentally. Our work deepens the understanding of spin excitations in itinerant electron systems with flat bands, provides a new paradigm to manipulate the magnon topology by electrical methods, and paves the way towards future investigation of magnonics in tTMD.