Figure 1

(Color online) Structures of TTF-CA. (a) Chemical structure of TTF and CA. (b) Schematic of the ionic chain structure of TTF-CA. The DW is located between the rightward polarized domain (IA) and the leftward one (IB). The spontaneous polarization, $P_{\text{s}}$, is directed in opposite directions in the IA and IB domains.Reuse & Permissions

Figure 2

(Color online) Reflectivity and electroreflectance spectra of TTF-CA. (a) Reflectivity $\left(R\right)$ spectra of TTF-CA at 4 K (dashed-dotted line), 77 K (solid line), and 90 K (dashed line). (b) Difference spectra of reflectivity $\left(\Delta R/R\right)$ with a change in temperature from 77 to 4 K $\left[\left(R_{4\text{K}}-R_{77\text{K}}\right)/R_{77\text{K}}\right]$ and from 77 to 90 K $\left[\left(R_{90\text{K}}-R_{77\text{K}}\right)/R_{77\text{K}}\right]$. (c) $1f$ component of the ER spectrum of TTF-CA (77 K). (d) $2f$ component of the ER spectrum of TTF-CA (77 K). In the measurements of the $1f$ and $2f$ components, the polarization of light, $E$, is perpendicular to the one-dimensional axis $\mathit{a}$, and the applied electric field, $F$, is parallel to the $a$ axis. The reflectivity spectra for 4 and 77 K and ER spectra are presented in Ref. 30.Reuse & Permissions

Figure 3

(Color online) The mechanism of the origin of ER signals in TTF-CA. (a) The mechanism of the $1f$ signal. The lengths of IA and IB domains, $L_{\text{A}}$ and $L_{\text{B}}$, are not balanced. The direction of spontaneous polarizations, $P_{\text{s}}$, in IA and IB are opposite, while the electric-field-induced polarizations, $\Delta P$, in IA and IB are parallel. Consequently, the polarizations are enhanced and reduced in IA and IB, respectively. These changes in the polarization are accounted for by the changes in $\rho$. (b) The mechanism of the $2f$ signal. The rightward applied electric field [upper row in Fig. 3b] moves the DW rightward from the point at $F=0$ (middle). In the dominant domain (IA), $\rho$ is increased. The leftward field (lower) moves the DW leftward. In the dominant domain (IB), $\rho$ is increased again. In a single cycle of the applied electric field, $\rho$ is increased twice.Reuse & Permissions

Figure 4

(a) Electric-field dependence of ER signals (filled squares: $1f$, open squares: $2f$) measured at 3.35 eV. (b) Temperature dependence of $1f$ and $2f$ ER signals between 12 and 100 K with $F=1.56\text{kV}/\text{cm}$ $\left(1f\right)$ and 2.34 kV/cm $\left(2f\right)$.Reuse & Permissions

Figure 5

(Color online) (a) Typical $1f$ spectra at the measured points in the IA (upper panel) and IB (lower) domains. The measured photon energy in the mapping measurement is indicated by arrows. (b) The map of the $1f$ signals obtained by two-dimensional smoothing of the raw data. The measuring points of the spectra shown in Fig. 5a are shown in Fig. 5c with $+$ and $-$ symbols. [(c), (d), and (e)] The maps of the $2f$ signals measured at (c) 2 kV/cm, (d) 5 kV/cm, and (e) 10 kV/cm. The black and gray area in (e) indicates the domain boundary between IA and IB, obtained from the $1f$ measurements.Reuse & Permissions

Figure 6

(Color online) Infrared reflectivity spectra perpendicular and parallel to the one-dimensional axis, corresponding to the $b_{1\text{u}}$ $\text{C}=\text{O}$ stretching mode of CA (a) and $a_{\text{g}}$ C-Cl stretching modes of CA (d) measured at 79 K. In (b) and (e), the difference spectra of the reflectivity spectra with decreasing temperature are shown. In (c) and (f), the $2f$-component ER spectra $\left(\Delta R/R\right)$ are shown.Reuse & Permissions