Superconductivity on the border of a spin-gapped Mott insulator: NMR studies of the quasi-two-dimensional organic system ${\text{EtMe}}_3\text{P}{\left[\text{Pd}{\left(\text{dmit}\right)}_2\right]}_2$

We present ${}^{13}\text{C}$-NMR studies of the pressure-temperature phase diagram of the quasi-two-dimensional organic system ${\text{EtMe}}_3\text{P}{\left[\text{Pd}{\left(\text{dmit}\right)}_2\right]}_2$, 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.

Figure 1

$\text{Pd}{\left(\text{dmit}\right)}_2$ molecule with selective substitution of ${}^{13}\text{C}$ isotope. The carbon atoms at both ends of the molecule are enriched.

Figure 2

${}^{13}\text{C}$-NMR spectra for randomly oriented samples of ${\text{EtMe}}_3\text{P}{\left[\text{Pd}{\left(\text{dmit}\right)}_2\right]}_2$ at several pressures.

Figure 3

(Color online) Temperature dependence of the spectral intensity multiplied by temperature. The shaded area indicates expected values when all nuclei contribute to the spectrum without the skin effect.

Figure 4

(Color online) (a) Temperature dependence of ${}^{13}\text{C}$ nuclear spin-lattice relaxation rate of ${\text{EtMe}}_3\text{P}{\left[\text{Pd}{\left(\text{dmit}\right)}_2\right]}_2$ at several pressures. The rate is determined by stretched-exponential analysis. The solid lines in the right figure are fits to the Korringa relation ${\left(T_{1}T\right)}^{-1}=\text{const}$. (b) Temperature dependence of stretching exponent $\beta$ at several pressures. The shaded area indicates expected values when the system is homogeneous.

Figure 5

(Color online) Temperature dependence of the resonance frequency of the NMR tank circuit at zero magnetic field. It reflects the ac susceptibility of the samples. The steep changes in the frequency indicate transitions to the superconducting phase.

Figure 6

(Color online) Schematic phase diagram of ${\text{EtMe}}_3\text{P}{\left[\text{Pd}{\left(\text{dmit}\right)}_2\right]}_2$ at 7.6 T. Circles: spin-gap transition temperatures determined by the behavior of $T_1^{-1}$. Triangles: superconducting transition temperatures at zero magnetic field determined by the onset of the diamagnetic signal. Squares: Mott transition (or rapid crossover) temperatures determined by the decrease in the NMR intensity. According to Ref. 7, the Mott transition is of first order below about 40 K, while it becomes crossover at higher temperatures. The hatched area around the first-order transition indicates the region where the coexistence of the metallic and insulating phases is expected.