Reentrant Mott transition from a Fermi liquid to a spin-gapped insulator in an organic spin-1/2 triangular-lattice antiferromagnet

The pressure- induced Mott transition is investigated in a quasi- two- dimensional molecular conductor, EtMe3P[ Pd( dmit)(2)](2) ( P2(1)/ m) with a triangular lattice, which shows a valence bond solid state at ambient pressure. Near the critical pressure of the Mott boundary, an insulator- to- metal transition occurs at 18 K, followed by a first- order reentrant metal- to- insulator transition below 12 K. Finally, a superconducting transition from the insulating state is observed at 5.2 K. The reentrant metal - insulator transition temperature is lowered by application of magnetic field. The result indicates the presence of a spin gap diminished by the Zeeman energy in the reentrant insulating phase neighbouring on the superconducting phase.