Hydrostatic pressure effect on photoinduced insulator-to-metal transition in the layered organic salt $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$

The effect of high pressure on the photoinduced insulator-to-metal transition in charge-ordered [bis(ethylenedithio)]tetrathiafulvalene (BEDT-TTF) salt $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ was investigated using femtosecond spectroscopy under hydrostatic pressure. By applying pressure $\left(P\right)$, the lifetime of the initially produced microscopic metallic domain becomes shorter, reflecting that condensation to the semimacroscopic metallic state is disturbed for $P<0.5\mathrm{GPa}$. However, condensation is promoted for $P=0.5–1\mathrm{GPa}$, thereby enhancing the nonlinearity of the photoinduced insulator-to-metal transition. Such nonmonotonous pressure dependence suggests competition between the increase of electronic bandwidth and photoinduced anisotropic lattice strain under pressure.

Figure 1

(Color online) (a) $T_c$ (transition temperature from metal to CO) is plotted as a closed circle; it is a function of pressure (Ref. 18) and the molecular arrangement of $\alpha \text{−}{\left(\mathrm{ET}\right)}_2{\mathrm{I}}_3$. (b) Steady-state (upper panel) and transient (lower panel) reflectivity spectra for $\parallel b$ at $20\mathrm{K}$, $t_d=0.1\mathrm{ps}$. (c) Time evolutions of $\Delta R∕R$ for various $P$ measured at $0.65\mathrm{eV}$ for $I_{\mathrm{ex}}=0.1\mathrm{mJ}∕{\mathrm{cm}}^2$, $20\mathrm{K}$. The thick gray curve shows the profile measured at $P=2\mathrm{GPa}$, $20\mathrm{K}$.

Figure 2

(Color online) (a) The time constants ${\tau }_{\text{fast}}$ and ${\tau }_{\text{slow}}$ and (b) their relative fractions as functions of $P$. In (a), the open circle, triangle, square, and cross, respectively, represent ${\tau }_{\text{fast}}$ for $I_{\mathrm{ex}}=0.1$, 0.03, 0.01, and $0.003\mathrm{mJ}∕{\mathrm{cm}}^2$. The closed circle and triangle, respectively, indicate ${\tau }_{\text{slow}}$ for $I_{\mathrm{ex}}=0.1$ and $0.03\mathrm{mJ}∕{\mathrm{cm}}^2$. Time evolutions of $\Delta R∕R$ for (c) $0.3\mathrm{GPa}$ and (d) $0.58\mathrm{GPa}$ at various temperatures and $I_{\mathrm{ex}}$. ($I_0$ denotes $1\mathrm{mJ}∕{\mathrm{cm}}^2$). Red, blue, and green curves represent the fitted curves ($0.3\mathrm{GPa}$ [red (gray solid), $20\mathrm{K}$; blue (gray dashed), $71\mathrm{K}$; green (gray dotted), $130\mathrm{K}$], $0.59\mathrm{GPa}$ [red (gray solid), $20\mathrm{K}$; blue (gray dashed), $78\mathrm{K}$; green (gray dotted) $102\mathrm{K}$]).

Figure 3

(Color online) [(a)–(c)] Fourier power spectra for the oscillating component observed for $P=0$, 0.3, and $0.58\mathrm{GPa}$, respectively. The dashed curve in (a) shows the Fourier spectrum at $I_{\mathrm{ex}}=0.001$ $I_0$. Arrows 1–4, respectively, indicate the phonon peaks at 48, 17, 29, and $38{\mathrm{cm}}^{-1}$. [(d)–(f)] ${\tau }_{\text{fast}}$ and ${\tau }_{\text{slow}}$ are plotted, respectively, as functions of reduced temperature $\mid T∕T_c-1\mid$ for $P=0$, 0.3, and $1\mathrm{GPa}$. In the figure, $I_0$ represents $1\mathrm{mJ}∕{\mathrm{cm}}^2$.

Figure 4

(Color online) Schematic illustration of the adiabatic potential energy surfaces of CO, photoinduced microscopic metallic domain, and semimacroscopic metallic domain. (a) $P=0\mathrm{GPa}$, (b) $P<0.5\mathrm{GPa}$, and (c) $P>0.5\mathrm{GPa}$.