Inhomogeneous electronic state of organic conductor $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2{\mathrm{Cu}\left[\mathrm{N}\right(\mathrm{CN})}_2]\mathrm{I}$ studied by ${}^{13}\mathrm{C}$ NMR spectroscopy

The physical properties of the organic conductor $\kappa \text{−}\left(\mathrm{BEDT}\text{−}{\mathrm{TTF})}_2{\mathrm{Cu}[\mathrm{N}\left(\mathrm{CN}\right)}_2\right]\mathrm{I}$ $\left(\kappa \text{−}\mathrm{I}\right)$ are inconsistent with the universal phase diagram of isostructural $\kappa$-Br and $\kappa$-Cl. To systematically understand $\kappa$-type salts, we measured and compared ${}^{13}\mathrm{C}$ NMR spectra of $\kappa$-I, $\kappa$-Br, and $\kappa$-Cl. The $\kappa$-I spectra manifested abrupt line broadening below 40 K due to the development of electronic inhomogeneity and an increase in the spin-lattice relaxation rate divided by temperature $1/T_{1}T$ down to the lowest temperature, suggesting an enhanced antiferromagnetic (AF) fluctuation. These results imply that AF fluctuation without long-range order in $\kappa$-I is induced by electronic inhomogeneity, which disturbs the development of coherency. Consequently, $\kappa$-I is an insulator with AF fluctuation at low temperature similar to x-ray-irradiated $\kappa$-Cl. We suggest that not only pressure but also disorder is a key parameter in the physical properties of $\kappa$-type salts.

Figure 1

(a) ${}^{13}\mathrm{C}$-enriched BEDT-TTF molecule. (b) Dimerized BEDT-TTF molecules.

Figure 2

Temperature dependence of spin susceptibility in $\kappa \text{−}Y$ ($Y$=Cl, Br, and I) salts. The $\kappa$-Cl and $\kappa$-Br data are from Ref. [23].

Figure 3

${}^{13}\mathrm{C}$ NMR spectra measured at several temperatures in ${\mu }_0\mathit{H}\left|\right|c$. The low- (high-) frequency peaks in each spectrum correspond to the inner (outer) site. The solid lines in (c) at 30 and 12 K are Lorentzian fitting curves.

Figure 4

Temperature dependence of NMR shift of $\kappa \text{−}Y$ ($Y$=Cl, Br, and I) salts in ${\mu }_0\mathit{H}\left|\right|b,c$. The solid and open symbols represent the inner and outer sites, respectively. The Knight shift, where the chemical shift was subtracted, is shown in the corresponding right axis.

Figure 5

Temperature dependence of linewidth $\mathrm{\Delta }\omega$ in $\kappa \text{−}Y$ ($Y$=Cl, Br, and I) salts in (a) ${\mu }_0\mathit{H}\left|\right|a$ and (b) ${\mu }_0\mathit{H}\left|\right|c$. Top (bottom) panels show the linewidth of the outer (inner) site. $\mathrm{\Delta }\omega$ of $\kappa$-Cl in ${\mu }_0\mathit{H}\left|\right|a,c$ and that of $\kappa$-Br in ${\mu }_0\mathit{H}\left|\right|a$ are cited from Refs. [15, 32].

Figure 6

Temperature dependence of $1/T_2$ of $\kappa \text{−}Y$ ($Y$=Cl, Br, and I) salts in ${\mu }_0\mathit{H}\left|\right|a$. Top (bottom) panel shows the $1/T_2$ of the outer (inner) site. The data of $\kappa$-Cl and $\kappa$-Br are cited from Refs. [29, 32]. The dashed lines are guides to the eye.

Figure 7

Temperature dependence of $1/T_{1}T$ of $\kappa \text{−}Y$ ($Y$=Cl, Br, and I) salts in ${\mu }_0\mathit{H}\left|\right|b$.

Figure 8

Temperature dependence of $(\mathrm{\Delta }{\omega }^i-\mathrm{\Delta }{\omega }_{\min }^i)/A^i$ [$i$= inner (I), outer (O)] for $\kappa \text{−}Y$ ($Y$=Cl, Br, and I) salts in ${\mu }_0\mathit{H}\left|\right|a$ (top panels) and ${\mu }_0\mathit{H}\left|\right|c$ (bottom panels). $A^{\mathrm{O}}$ in ${\mu }_0\mathit{H}\left|\right|a,c$ are 2.4 and 4.0 $\mathrm{kOe}/{\mu }_B$, and $A^{\mathrm{I}}$ in ${\mu }_0\mathit{H}\left|\right|a,c$ are 0.7 and 1.4 $\mathrm{kOe}/{\mu }_B$, respectively [30]. The inset in (c) shows $\mathrm{\Delta }{\omega }^{\mathrm{O}}-\mathrm{\Delta }{\omega }_{(T=40\mathrm{K})}^{\mathrm{O}}$ vs $\mathrm{\Delta }{\omega }^{\mathrm{I}}-\mathrm{\Delta }{\omega }_{(T=40\mathrm{K})}^{\mathrm{I}}$. Dashed lines represent the slope corresponding to $A^{\mathrm{O}}/A^{\mathrm{I}}$.