In frustrated magnetic systems, long-range ordering is forbidden and degeneracy of energy states persists, even at extremely low temperatures. Under certain conditions, these systems form an exotic quantum spin-liquid ground state, in which strongly correlated spins fluctuate in the spin lattices. Here we investigate the thermodynamic properties of an anion radical spin liquid of EtMe(3)Sb[Pd(dmit)(2)](2), where dmit represents 1,3-dithiole-2-thione-4,5-dithiolate. This compound is an organic dimer-based Mott insulator with a two-dimensional triangular lattice structure. We present distinct evidence for the formation of a gapless spin liquid by examining the T-linear heat capacity coefficient, γ , in the low-temperature heat capacity. Using comparative analyses with κ-(BEDT-TTF)(2)Cu(2)(CN)(3), a generalized picture of the new spin liquid in dimer-based organic systems is discussed. We also report anomalous enhancement of γ, produced by a kind of criticality inherent to the Pd(dmit)(2) phase diagram.
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