Experimental verification of charge soliton excitations in the ionic Mott-Peierls ferroelectric TTF-CA

Strong coupling of charge, spin, and lattice in solids brings about emergent elementary excitations with their intertwining and, in one dimension, solitons are known as such. The charge-transferred organic ferroelectric, TTF-CA, has been argued to host charge solitons; however, the existence of the charge solitons remains unverified. Here, we demonstrate that the charge-transport gap in the ionic Mott-Peierls insulating phase of TTF-CA is an order of magnitude smaller than expected from quasiparticle excitations, however, being entirely consistent with the charge soliton excitations. We further suggest that charge and spin solitons move with similar diffusion coefficients in accordance with their coexistence. These results provide a basis for the thermal excitations of the emergent solitons.

Figure 1

Phase diagram of TTF-CA and schematic illustrations of the N, $I_{\mathrm{para}}$, and $I_{\mathrm{ferro}}$ phases with the charge transfer indicated by $\rho$ and the polarization of dimers pointed out by bold green arrows. The phase diagram is reproduced from Ref. [12]. Mobile charge and spin solitons appear in the $I_{\mathrm{para}}$ phase. In the crossover between the N and $I_{\mathrm{para}}$ phases, NI domain walls are excited.

Figure 2

(a) Comparison between the pressure dependence of the resistivities of TTF-CA along the $a$ axis and TTF-BA along the $b$ axis at room temperature (data for pressures below 20 kbar in TTF-CA are reproduced from Ref. [12]). (b) Arrhenius plot of the resistivity of TTF-BA along the $b$ axis at several pressures. The conductivity totally decreases with increasing pressure up to 10–15 kbar but increases with further pressure increase. The inset shows the pressure dependence of activation energy of TTF-BA estimated from the Arrhenius plot. (c) Arrhenius plot of the resistivity of TTF-CA along the $a$ axis at several pressures. Part of the data in Fig. 2 is reproduced from Ref. [12]. The arrows indicate the phase transition points, ${\mathit{T}}_c$, between the $I_{\mathrm{para}}$ and $I_{\mathrm{ferro}}$ phases. The inset shows the Arrhenius plot of the normalized conductivity $\sigma /{\sigma }_c$ against $1000(1/\mathit{T}-1/{\mathit{T}}_c)$, where ${\sigma }_c$ is the conductivity at $T_c$. The red and green regions in the inset correspond to the $I_{\mathrm{para}}$ and $I_{\mathrm{ferro}}$ phases, respectively.

Figure 3

(a) Pressure dependence of the activation energy of TTF-CA in the high-temperature region above the ferroelectric transition temperature, $T_c$. The results of four samples measured are distinguished by the different symbols. The activation energies are obtained from the plots in Fig. 2. The activation energies estimated along parallel to the inclined NI crossover line in the previous measurements [12] are also displayed. The pressure dependence of activation energy that appears to make sense is indicated by the guided curve. (b) Phase diagram of TTF-CA with the contour plot of electrical conductivity, $\sigma$, along the $a$ axis extracted from Ref. [12]. (c) Pressure dependence of activation energy of TTF-CA in the $I_{\mathrm{ferro}}$ phase.