Afterwards, it was observed in an organic Mott insulator (κ-(BEDT-TTF) 2 Cu 2 (CN) 3) 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.