Fragmented Electronic Spins with Quantum Fluctuations in Organic Mott Insulators Near a Quantum Spin Liquid

Magnetic structures of organic Mott insulators X[Pd(dmit)(2)](2) (X = Me4P, Me4Sb), of which electronic states are located near a quantum spin liquid (X = EtMe3Sb), are demonstrated by C-13 nuclear magnetic resonance. Antiferromagnetic spectra and nuclear relaxations show two distinct magnetic moments within each Pd(dmit)(2) molecule, which cannot be described by single band dimer-Mott model and requires intramolecular electronic correlation. This unconventional fragmentation of S = 1/2 electron spin with strong quantum fluctuation is presumably caused by nearly degenerated intramolecular multiple orbitals, and shares a notion of quantum liquids where electronic excitations are fractionalized and S = 1/2 spin is no longer an elementary particle.