Giant thermal Hall effect in topological magnon insulator Cr₃Se₄ monolayer

The thermal Hall effect, the thermal analogue of the Hall effect, has emerged as a key probe of charge-neutral excitations in topological magnons. Therefore, achieving the giant thermal Hall effect in a topological magnon insulator (TMI) is highly desirable, but the underlying origin has been under active debate. Here, through a Heisenberg-Dzyaloshinskii-Moriya (DM) model combined with first-principles calculations that explicitly include both nearest- and next-nearest-neighbor DM interactions, we show that the usually neglected next-nearest-neighbor DM interactions are crucial for the magnonic thermal Hall effect and can even induce a sign change of the magnonic Chern number. Remarkably, based on the Chern number and edge states analyses, the Cr₃Se₄ monolayer is identified as an experimentally feasible candidate of the TMI with a sign-changed magnonic Chern number and a giant thermal Hall effect, with thermal Hall conductivity reaching as much as |κxyTH| = 3.82 × 10^-11 W K^-1. Moreover, the Cr₃Se₄ monolayer simultaneously realizes the Chern insulator with quantum anomalous Hall effect, giving rise to dual Chern-insulating behavior in which electronic and magnonic edge states emerged within a single ferromagnet. These findings identify a prototypical platform for intrinsic topological magnonic phenomena, offering strong prospects for experimental exploration.