Itinerant and topological excitations in a honeycomb spiral spin liquid candidate

The frustrated insulating magnet can stabilize a spiral spin liquid, arising from cooperative fluctuations among a subextensively degenerate manifold of spiral configurations, with ground-state wave vectors forming a continuous contour or surface in reciprocal space. The atomic-mixing-free honeycomb antiferromagnet GdZnPO has recently emerged as a promising spiral spin-liquid candidate, hosting nontrivial topological excitations. Despite growing interest, the transport and topological properties of spiral spin liquids remain largely unexplored experimentally. Here, we report transport measurements on high-quality, electrically insulating GdZnPO single crystals. We observe a giant low-temperature magnetic thermal conductivity down to $\sim$50 mK, described by $\kappa_{xx}^\mathrm{m}$ $\sim$ $\kappa_0+\kappa_1T$, where both $\kappa_0$ and $\kappa_1$ are positive constants associated with excitations along and off the spiral contour in reciprocal space, respectively. This behavior parallels the magnetic specific heat, underscoring the presence of mobile low-energy excitations intrinsic to the putative spiral spin liquid. Furthermore, the observed positive thermal Hall effect confirms the topological nature of at least some of these excitations. Our findings provide key insights into the itinerant and topological properties of low-lying spin excitations in the spiral spin-liquid candidate.