Electric-Field Control of Solitons in a Ferroelectric Organic Charge-Transfer Salt

The role of solitons in transport, dielectric, and magnetic properties has been revealed for the quasi-one-dimensional organic charge-transfer salt, $\mathrm{TTF}\mathrm{\text{−}}{\mathrm{QBrCl}}_3$ [tetrathiafulvalene (TTF)-2-bromo-3,5,6-trichloro-$p$-benzoquinone (${\mathrm{QBrCl}}_3$)]. The material was found to be ferroelectric and hence the solitons should be located at the boundary of the segments with opposite electric polarization. This feature enabled the electric-field control of soliton density and hence the clear-cut detection of soliton contributions. The gigantic dielectric response in the ferroelectric phase is ascribed to the dynamical bound and creeping motions of spinless solitons.

Figure 1

Schematic illustrations of mixed stack composed of donor ($D$) and acceptor ($A$) molecules. (a) and (b) Neutral and ionic stacks with charge transfer ${\rho }_N$ ($\sim 0.3$) and ${\rho }_I$ ($\sim 0.6$), respectively. (c) A pair of spin soliton and spin antisoliton. (d) A pair of spinless soliton and spinless antisoliton. The small and large arrows and underlines represent spin-$1/2$, electric polarization $P$, and a dimer singlet state, respectively. For simplicity, the chemical structures of solitons are drawn as an abrupt change in the $P$ direction in the strong-dimerization limit. In (c) and (d), fractional charge carried by each soliton is also presented in this strong coupling limit.

Figure 2

(a),(b) Temperature dependence of spontaneous polarization (a) and dielectric constant at various frequencies (b) along the mixed stack (the $a$ axis). The polarization was obtained from the pyroelectric current. (c),(d) Dielectric spectra of ${ϵ}_1$ (c) and ${ϵ}_2$ (d) from 1 kHz to 800 MHz. Here, ${ϵ}_1$ was measured by using a LCR meter below 2 MHz and an impedance analyzer above that.

Figure 3

(a) Dielectric dispersion ($\parallel a$) before and after the poling procedure of $\sim 0.95\mathrm{kV}/\mathrm{cm}$. (b) Soliton contributions $\Delta {ϵ}_1$ at 1 kHz obtained from (a). (c) Dielectric spectra of solitons under various magnitudes of $E_{\mathrm{ac}}$ at 67.4 K. For the ${ϵ}_1$ below 1 kHz, a Solartron 1260 with a 1296 dielectric interface was used. (d) Transport properties ($\parallel a$) measured under $50\mathrm{V}/\mathrm{cm}$. The broken lines in (c) and (d) stand for ${\omega }^{-0.6}$ dependence and Arrhenius law, $\propto \exp (-1100/k_{B}T)$, respectively. The data in (a) and (d) were taken with increasing temperature.

Figure 4

ESR signal arising from the spin solitons. (a) Temperature-dependent and poling-procedure-dependent variations in the ESR spectra of $\mathrm{TTF}\mathrm{\text{−}}{\mathrm{QBrCl}}_3$. (b) Temperature dependence of ESR signal intensity. An electric field of $\sim 1\mathrm{kV}/\mathrm{cm}$ was used for the poling. The broken lines in (b) are the guide for the eyes, representing the Curie law.