Superconductivity on the border of a spin-gapped Mott insulator: NMR studies of the quasi-two-dimensional organic system EtMe₃P[Pd(dmit)₂]₂

We present ¹³C-NMR studies of the pressure-temperature phase diagram of the quasi-two-dimensional organic system EtMe₃P[Pd(dmit)₂]₂, which has a spin-gapped Mott insulating ground state at ambient pressure. As the system is pressurized, the spin-gapped insulating ground state survives until the insulator-metal Mott transition, just beyond which a superconducting state appears. This demonstrates that the superconducting phase borders the spin-gapped insulating phase. The phase diagram contrasts with those of the large majority of other correlated-electron superconductors in which the superconducting phase borders a magnetically ordered phase. This implies the possibility that the present superconductivity has an exotic origin and nature.