Evidence of chiral fermion edge modes through geometric engineering of thermal Hall in $\alpha$-RuCl$_3$

The experimental observation of half-integer-quantized thermal Hall conductivity in the Kitaev candidate material $\alpha$-RuCl$_3$ has served as smoking-gun signature of non-Abelian anyons through an associated chiral Majorana edge mode. However, both the reproducibility of the quantized thermal Hall conductivity and the fundamental nature of the associated heat carriers, whether bosonic or fermionic, are subjects of ongoing and vigorous debate. In a recent theoretical work, it was proposed that varying the sample geometry through creating constrictions can distinguish between different origins of the thermal Hall effect in magnetic insulators. Here, we provide experimental evidence of chiral fermion edge modes by comparing the thermal Hall effect of a geometrically constricted $\alpha$-RuCl$_3$ sample with that of an unconstricted bulk sample. In contrast to the bulk crystals where the thermal Hall signal fades below 5\,K, the constricted crystals display a significant thermal Hall signal that remains measurable even at 2\,K. This sharp difference agrees well with the theoretical prediction and provides compelling evidence for the contribution of chiral fermion edge modes to the thermal Hall effect in $\alpha$-RuCl$_3$. More broadly, this work confirms that the geometry dependence of the thermal Hall effect can help identify chiral spin liquids in candidate materials like $\alpha$-RuCl$_3$ and paves the way for the experimental realization of thermal anyon interferometry.