We performed the single-crystal ${}^{13}\mathrm{C}$ NMR studies on a quasi-two-dimensional system, deuterated $\kappa -\left(\mathrm{BEDT}-\mathrm{TTF}{)}_2\mathrm{Cu}\right[\mathrm{N}\left(\mathrm{CN}{)}_2\right]\mathrm{Br}$, which is just on the border of the Mott transition. The NMR spectra are separated into two parts coming from the metallic (superconducting) and insulating phases due to the phase separation at low temperature. The examination of the separated spectra revealed that the Mott transition in this system is characterized by the first-order transition between the pseudogapped superconductor and the simplest commensurate antiferromagnet with a moment of $0.26{\mu }_{\mathrm{B}}/\mathrm{dimer}$.

• Received 16 August 2001

DOI:https://doi.org/10.1103/PhysRevLett.89.017003