Mott Transition from a Spin Liquid to a Fermi Liquid in the Spin-Frustrated Organic Conductor $\kappa \mathrm{\text{−}}(\mathrm{ET}{)}_2{\mathrm{Cu}}_2(\mathrm{CN}{)}_3$

The pressure-temperature phase diagram of the organic Mott insulator $\kappa \mathrm{\text{−}}(\mathrm{ET}{)}_2{\mathrm{Cu}}_2(\mathrm{CN}{)}_3$, a model system of the spin liquid on triangular lattice, has been investigated by ${}^1\mathrm{H}$ NMR and resistivity measurements. The spin-liquid phase is persistent before the Mott transition to the metal or superconducting phase under pressure. At the Mott transition, the spin fluctuations are rapidly suppressed and the Fermi-liquid features are observed in the temperature dependence of the spin-lattice relaxation rate and resistivity. The characteristic curvature of the Mott boundary in the phase diagram highlights a crucial effect of the spin frustration on the Mott transition.

Figure 1

The pressure-temperature phase diagram of $\kappa \mathrm{\text{−}}(\mathrm{ET}{)}_2{\mathrm{Cu}}_2(\mathrm{CN}{)}_3$, constructed on the basis of the resistance and NMR measurements under hydrostatic pressures. The Mott transition or crossover lines were identified as the temperature where $1/T_{1}T$ and $dR/dT$ show the maximum as described in the text. The upper limit of the Fermi-liquid region was defined by the temperatures where $1/T_{1}T$ and $R$ deviate from the Korringa’s relation and $R_0+A{T}^2$, respectively. The onset superconducting transition temperature was determined from the in-plane resistance measurements.

Figure 2

(a) Temperature differential of the in-plane resistance of $\kappa \mathrm{\text{−}}(\mathrm{ET}{)}_2{\mathrm{Cu}}_2(\mathrm{CN}{)}_3$ at various hydrostatic pressures and a zero magnetic field. The inset shows temperature dependence of the resistance. (b) The $T^2$ dependence of the resistance of $\kappa \mathrm{\text{−}}(\mathrm{ET}{)}_2{\mathrm{Cu}}_2(\mathrm{CN}{)}_3$ under hydrostatic pressures. The circles mark the points where the resistance deviates from the equation $R=R_0+A{T}^2$.

Figure 3

${}^1\mathrm{H}$ NMR spectra of $\kappa \mathrm{\text{−}}(\mathrm{ET}{)}_2{\mathrm{Cu}}_2(\mathrm{CN}{)}_3$ at 1.4 K under various hydrostatic pressures. The observed resonance frequency $f_0$ was 156 MHz.

Figure 4

The ${}^1\mathrm{H}$ NMR spin-lattice relaxation rate divided temperature, $1/T_{1}T$, of $\kappa \mathrm{\text{−}}(\mathrm{ET}{)}_2{\mathrm{Cu}}_2(\mathrm{CN}{)}_3$ at various pressures.

Figure 5

The pressure dependence of $A$ and $1/T_{1}T$ at the low-temperature region. Closed circles and triangles represent the $A$ and $(T_{1}T{)}^{-1}$ values, respectively.