Relaxor ferroelectricity induced by electron correlations in a molecular dimer Mott insulator

We have investigated the dielectric response in an antiferromagnetic dimer-Mott insulator $\beta$’-(BEDT-TTF)${}_2$ICl${}_2$ with square dimer lattice, compared to a spin liquid candidate $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$. Temperature dependence of the dielectric constant shows a peak structure obeying Curie-Weiss law with a strong frequency dependence. We found an anisotropic glassy ferroelectricity by pyrocurrent measurements, which suggests the charge disproportionation resulting in an electric dipole in a dimer. Each ferroelectric and antiferromagnetic transition temperature is closely related to the antiferromagnetic interaction energy and a freezing temperature of dipole dynamics in a dimer, respectively. These correspondences suggest the possible charge-spin coupled degrees of freedom in the system.

Figure 1

Crystal structure of $\beta$’-(BEDT-TTF)${}_2$ICl${}_2$. (a) BEDT-TTF molecule arrangement viewed along the long axis of BEDT-TTF molecules and (b) the crystal structure seen from the $c$ axis. The blue ovals represent dimerized molecules symmetric with respect to the inversion operation at the point represented by open circles. The unit cell is represented by blue lines in (b). Hydrogen atoms are not drawn for clarity.

Figure 2

Temperature dependence of the dielectric constant $\varepsilon$ in (a) $E\parallel a^{*}$ ($\perp bc$ plane) at frequencies from 500 Hz to 100 kHz and (b) $E\parallel b$ from 20 Hz to 200 kHz for $\beta$’-(BEDT-TTF)${}_2$ICl${}_2$. The data in the high temperature region was omitted for clarity. The inset shows the Vogel-Fulcher (VF) relation using the dielectric constant at frequencies from 20 Hz to 7 kHz indicated as open triangles in (b). The black solid line in the inset is a best fit to the VF relation. $T_{\max }$ and $T_{\mathrm{VF}}=23$ K represent the temperatures at which $\varepsilon$ takes a maximum value at each frequency and the charge freezing temperature obtained by the VF fitting, respectively. Solid triangles in (b) represent the expected $T_{\max }$ by the relation indicated by the filled circles in the inset.

Figure 3

(a) Temperature dependence of inverse dielectric constant at each frequency in $\beta$’-(BEDT-TTF)${}_2$ICl${}_2$, where ${\varepsilon }_{\mathrm{const}.}/{\varepsilon }_0=13.8$ The black straight line represents the best fit result to the Curie-Weiss law. Temperature dependence of (b) polarization with $E\parallel b$ and a poling electric field from $-$1.211 to $+$4.844 kV/cm obtained by pyrocurrent measurements, and (c) magnetic susceptibility with $H\parallel a^{*}$, $b$ and $c$ for $\beta$’-(BEDT-TTF)${}_2$ICl${}_2$. The polarization at 7 K for each poling field in (b) is plotted in the upper inset, which does not correspond to the standard $P-E$ curve at 7 K. A typical pyrocurrent for $E_{\mathrm{pol}}=2.422$ kV/cm is shown in the lower inset.

Figure 4

Schematic pictures of (a) dimer-Mott insulator, interdimer Coulomb correlation ($V$) with (b) ferroelectric and (c) antiferroelectric dimers, and (d) antiferromagnetic interaction ($J_{\mathrm{AF}}$) with antiferroelectric dimers within the $bc$ layer, where a dimer is represented by an open circle. Solid circles and arrows represent charge and spin, respectively. (e) Schematic electronic phase diagram for $\beta$’-(BEDT-TTF)${}_2$ICl${}_2$ and $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$, where the dielectric constant starts to increase at around 120 K and 60 K, respectively. $T_{\mathrm{N}}$ and $T_{\mathrm{FE}}$ correspond to antiferromagnetic and ferroelectric transition temperatures, respectively. The change in parameters of the Curie-Weiss ($T_{\mathrm{C}}$) and the Vogel-Fulcher ($T_{\mathrm{VF}}$) temperatures are also shown.