Gapless quantum spin liquid in an organic spin-1/2 triangular-lattice κ- H3 (Cat-EDT-TTF)2

We report the results of SQUID and torque magnetometry of an organic spin-1/2 triangular-lattice κ-H3(Cat-EDT-TTF)2. Despite antiferromagnetic exchange coupling at 80-100 K, we observed no sign of antiferromagnetic order down to 50 mK owing to spin frustration on the triangular lattice. In addition, we found nearly temperature-independent susceptibility below 3 K associated with Pauli paramagnetism. These observations suggest the development of gapless quantum spin liquid as the ground state. On the basis of a comparative discussion, we point out that the gapless quantum spin liquid states in organic systems share a possible mechanism, namely the formation of a band with a Fermi surface possibly attributed to spinons.