Neutron scattering measurements of cesium chlorocuprate CsCuCl, a spin-1/2 antiferromagnet on a triangular lattice, displayed diffuse scattering. This was attributed to spinons arising from a 2D RVB state.  Later theoretical work challenged this picture, arguing that all experimental results were instead consequences of 1D spinons confined to individual chains.
Afterwards, it was observed in an organic Mott insulator (κ-(BEDT-TTF)Cu(CN)) by Kanoda's group in 2003. It may correspond to a gapless spin liquid with spinon Fermi surface (the so-called uniform RVB state). The peculiar phase diagram of this organic quantum spin liquid compound was first thoroughly mapped using muon spin spectroscopy.