3D Spin-Liquid State in an Organic Hyperkagome Lattice of Mott Dimers
AbstractWe report the first 3D spin liquid state of isotropic organic spins. Structural analysis, and magnetic and heat–capacity measurements were carried out for a chiral organic radical salt, (TBA)1.5[(−)-NDI-Δ] (TBA denotes tetrabutylammonium and NDI denotes naphthalene diimide), in which (−)-NDI-Δ forms a K4 structure due to its triangular molecular structure and an intermolecular π-π overlap between the NDI moieties. This lattice was identical to the hyperkagome lattice of S=1/2 Mott dimers, and should exhibit 3D spin frustration. In fact, even though the high-temperature magnetic susceptibility followed the Curie-Weiss law with a negative Weiss constant of θ=−15 K, the low-temperature magnetic measurements revealed no long-range magnetic ordering down to 70 mK, and suggested the presence of a spin liquid state with a large residual paramagnetism χ0 of 8.5×10−6 emu g−1 at the absolute zero temperature. This was supported by the N14 NMR measurements down to 0.38 K. Further, the low-temperature heat capacities cp down to 68 mK clearly indicated the presence of cp for the spin liquid state, which can be fitted to the power law of T0.62 in the wide temperature range 0.07–4.5 K.