Femtosecond Carrier Relaxation Dynamics and Photoinduced Phase Separation in $\kappa \mathrm{\text{−}}(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_2\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_2]X$ ($X=\mathrm{Br},\mathrm{Cl}$)

We investigate the relaxation dynamics of nonequilibrium carriers in organic conductors $\kappa \mathrm{\text{−}}(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_2\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_2]X$ ($X=\mathrm{Br}$ and Cl) using ultrafast time-resolved optical spectroscopy. The dynamics for both salts show similar temperature dependences, which is well characterized by the carrier relaxation across the pseudogap (PG) of the magnitude ${\Delta }_{\mathrm{PG}}\approx 16\mathrm{meV}$ for Br salt and 7.0 meV for Cl salt. On the other hand, only the Br salt shows an abrupt increase of the decay time at low temperature, indicating an additional decay component associated with the superconducting (SC) gap below $T_c$. The fluence dependent dynamics at low temperature evidences the superposition of the SC component onto the PG component. These results indicate a metallic-insulating phase separation in the Br salt triggered by photoexcited nonequilibrium carriers.

Figure 1

(a) The crystal structure of $\kappa \mathrm{\text{−}}(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_{2}X$. (b) A schematic phase diagram of $\kappa \mathrm{\text{−}}(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_{2}X$ as a function of Coulomb interaction $U/t$. The on-site Coulomb repulsion $U$ is nearly constant for various anions $X$. On the other hand, the substitution of $X$ changes the transfer integral $t$ between BEDT-TTF dimers. AFM denotes antiferromagnetic. (c),(d) Density plots of $\Delta R/R$ transients as a function of $T$ ($5–265\mathrm{K}$ with 5 K step) for Br salt ($\mathcal{F}=60\mu \mathrm{J}/{\mathrm{cm}}^2$) and Cl salt $\mathcal{F}=96\mu \mathrm{J}/{\mathrm{cm}}^2$), respectively. Insets show the data obtained at low-$T$ region with high resolution steps. (e),(f) $\Delta R/R$ transients at selected temperatures in (c) and (d), respectively. The vertical axis with a log scale represents a pump-excited reduction ($-\Delta R$) of reflectivity ($R$).

Figure 2

$T$ dependences of the $\Delta R/R$ amplitudes ($\mathcal{A}$) (a) and decay times ($\tau$) (b). Open and closed circles are the data of Br and Cl salts, respectively. The solid lines in (a) are fits using $T$-independent gap [3]. To emphasize the similarity of $T$-dependent $\mathcal{A}$ between the salts, the same data are displayed as a function of $\log T$ in the inset of (a), where the data for Br salt are reduced by 2.4. Inset of (b) shows the density plot for Br data at low $\mathcal{F}$ ($\mathcal{F}=17\mu \mathrm{J}/{\mathrm{cm}}^2$).

Figure 3

$\Delta R/R$ transients at low $T$ excited by various $\mathcal{F}$ for (a) Br ($T=5\mathrm{K}$) and (b) Cl ($T=10\mathrm{K}$) salts. (c),(d) $\mathcal{F}$ dependences of $\tau$ (main panels) and $\mathcal{A}$ (insets). The data were obtained by fits of single-exponential functions [see in (a) and (b)]. Triangles in the inset of (c) show $\mathcal{A}$ for the long-lived component.