Terahertz responses of the high-temperature metallic phase and photoinduced metallic state in the ferroelectric charge-ordered organic salt $\alpha \text{−}{\left(\text{ET}\right)}_2{\text{I}}_3$

The thermal and photoinduced phase transitions between the ferroelectric charge-ordered (CO) insulator state and the metallic state in the layered organic salt $\alpha \text{−}{\left(\text{ET}\right)}_2{\text{I}}_3$ (ET: [bis(ethylenedithio)]-tetrathiafulvalene) were investigated using terahertz time-domain spectroscopy. Characteristic increases were observed in the optical conductivity and the dielectric constant upon increasing the temperature to just below the insulator-to-metal transition temperature $\left(T_{\text{CO}}\right)$, which reflects the partial or precursory collapse of the ferroelectric state and charge ordering. The excitation of this partially melted CO state near $T_{\text{CO}}$ led to the formation of a photoinduced macroscopic metallic state, whose conductivity appeared to be greater than that of a high-temperature metallic state. At $T⪡T_{\text{CO}}$, a microscopic metallic state was formed within the rigid CO state.

Figure 1

(Color online) Temperature dependence of resistivity (Ref. 15) and molecular arrangement of $\alpha \text{−}{\left(\text{ET}\right)}_2{\text{I}}_3$. In the CO state, electrons are localized as illustrated by the red circles. However, they are extended as shown by the orange circles in the $A\text{−}{A}^{\prime }$ stack in the HT metallic state (but they are not so extended in the $B\text{−}C$ stack) (Ref. 12). The inset shows the optical conductivity spectra at 300 K (metallic) and 60 K (CO) (Refs. 18, 19).

Figure 2

(Color online) (a) THz profiles in time domain at 7 and 150 K $\left(\times 20\right)$ for $E_{\text{TH}}\parallel b$. [(b) and (c)] ${\sigma }_1$ and ${\epsilon }_1$ at various temperatures. The red (gray) curves indicate the values calculated using the equation given in the text. [(d) and (e)] Temperature dependence of ${\sigma }_1$ and ${\epsilon }_1$ at 5 meV [solid red (gray) circles], 11 meV [solid blue (gray) triangles], and 15 meV [green (gray) squares]. (f) Temperature dependence of $\eta$ $\left(T<T_{\text{CO}}\right)$ (solid black circles and solid curve). For $T>T_{\text{CO}}$, $\eta$ is indicated by the open circles and the dashed curve because the ${\sigma }_1\left(\omega \right)$ spectra cannot be well reproduced.

Figure 3

(Color online) [(a) and (c)] Absorption (OD) spectra of CO and HT phases for $E_{\text{TH}}\parallel b$ at 25 K [CO, blue (gray)], 125 K [CO, blue (gray)], and 150 K [HT metallic state, red (gray)]. Dashed curves in (a) and (c) indicate the differential spectra for [OD(150 K)-OD(25 K)] and [OD(150 K)-OD(125 K)], respectively. [(b) and (d)] Transient absorption $\left(\Delta \text{OD}\right)$ spectra under excitation (polarization of the pump light $E\parallel b$) at 0.89 eV at 20 and 124 K, respectively, for various excitation intensities $I_{\text{ex}}$. The blue dashed curves indicate the spectra determined using the multilayer model (see the text). The inset of (b) shows photoinduced changes in $\sigma$ $\left(\Delta \sigma \right)$, which are calculated using the multilayer model. The density of the photoinduced conducting state $\left(f\right)$ is shown schematically as a function of $d$ in the inset of (d).

Figure 4

(Color online) [(a) and (b)] Time evolution of $\Delta \text{OD}$ at 4 and 8 meV (open circles and squares) at 20 and 124 K, respectively. The red dashed curves indicate the time evolution of the normalized THz amplitude $E^{″}\left(t\right)=A\left(1\text{−}{E}_{\text{TH}}\left(t\right)/E_0\right)$.

Figure 5

(Color online) Decay time constants ${\tau }_{\text{fast}}$ and ${\tau }_{\text{slow}}$ [red (gray) diamonds] as functions of reduced temperature. The decay time constants obtained in a mid-IR pump-probe study are indicated by open triangles $\left(I_{\text{ex}}=I_0\right)$ (Ref. 21). The inset shows $E\left(t\right)$ at various temperatures $T<T_{\text{CO}}$.