Spiral spin liquid on a honeycomb lattice*

The spiral spin liquid state initially proposed by Bergman D. et al. in Nat. Phys. 3, 487 (2007) has become a new paradigm to achieve exotic spin correlations through competing interactions instead of the conventional geometrical frustration. In the spiral spin liquid state, spins fluctuate collectively as spirals, and their propagation vectors form a continuous ring or surface in reciprocal space. Due to this enormous degeneracy, spiral spin liquids present a novel route to realize quantum spin liquids and topological spin textures. Experimental realization of spiral spin liquids, however, is challenging as the competition between interactions is often too weak in real materials. Until now, the diamond-lattice compound MnSc₂S₄ studied in Nat. Phys. 13, 157 (2017) has remained as the only host of a spiral spin liquid. According to theoretical calculations, spiral spin liquids should be generally existent on the bipartite lattices, which include the prototype bipartite honeycomb lattice. Realizing the spiral spin liquid state on the honeycomb lattice is highly desirable as the reduced dimension may enhance quantum fluctuation in favor of the quantum spin liquid state and allow more convenient manipulation of the topological textures for spintronics applications.  We present evidence for the existence of the spiral spin liquid state in the honeycomb magnet FeCl₃.  Using diffuse neutron scattering, we observed a continuous ring of scattering in reciprocal space, which provides evidence for the existence of a spiral spin liquid state. The spiral correlations can be ascribed to the competition between the exchange interactions within the honeycomb layer, which is further corroborated through inelastic neutron scattering in the long-range ordered phase.