Optical Modulation of Effective On-Site Coulomb Energy for the Mott Transition in an Organic Dimer Insulator

We report an optical modulation of the effective on-site Coulomb energy $U$ on a dimer ($U_{\mathrm{dimer}}$) for achieving the Mott insulator-to-metal transition in $\kappa \mathrm{\text{−}}(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_2\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_2]\mathrm{Br}$, as investigated by pump-probe spectroscopy. A reduction of $U_{\mathrm{dimer}}$ is optically induced by molecule displacement in the dimer under intradimer excitation. The mechanism of this metallization differs greatly from the photodoping-type mechanism reported previously. In contrast, a faster transition via the photodoping mechanism is detected for interdimer excitation. A metallic-domain-wall oscillation originating from the modulation of $U_{\mathrm{dimer}}$ was also observed near the critical end point of the Mott transition line.

Figure 1

Optical conductivity $\sigma (\omega )$ spectra of $d\mathrm{\text{−}}\mathrm{Br}$ [light gray (red)] and $h\mathrm{\text{−}}\mathrm{Br}$ [dark gray (blue)] at 10 K, together with the molecular arrangement of the BEDT-TTF layer ($ac$ plane). The phase diagram of $\kappa \mathrm{\text{−}}(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_{2}X$ as functions of external pressure and $U_{\mathrm{dimer}}/t$ is drawn by changing the deuteration of BEDT-TTF and substitutions of $X$. (AF denotes antiferromagnetic.)

Figure 2

(a, b) $\Delta R/R$ spectra at $t_d=0.1\mathrm{ps}$ and 2 ps for $d\mathrm{\text{−}}\mathrm{Br}$ (a) and $d\mathrm{\text{−}}\mathrm{Cl}$ (b) under excitation of 0.89 eV. Insets of (a) and (b) present differential spectra between $h\mathrm{\text{−}}\mathrm{Br}$ and $d\mathrm{\text{−}}\mathrm{Br}$ [(a) $\Delta R_{h\mathrm{\text{−}}d}/R_d$; light gray (red) curve] at 10 K and between 80 and 10 K [(b) $\Delta R_{\mathrm{temp}}/R_{10}$; dark gray (blue) curve] of $d\mathrm{\text{−}}\mathrm{Br}$.

Figure 3

(a) Time evolutions of $\Delta R/R$ for $d\mathrm{\text{−}}\mathrm{Br}$ detected at 0.26 eV under excitation at 0.89 eV (red circles with green fitted curve), 0.64 eV (solid curve), and 0.32 eV (dashed curve). (b) Time profile of the oscillating component of $\Delta R/R$ for $d\mathrm{\text{−}}\mathrm{Br}$ (0.89 eV excitation). (c, d) The $27{\mathrm{cm}}^{-1}$ (b) and $45{\mathrm{cm}}^{-1}$ (c) cosinelike oscillations, which were obtained through analysis (see the text). (e) Fourier transformed spectra of the oscillation for $d\mathrm{\text{−}}\mathrm{Br}$ and $d\mathrm{\text{−}}\mathrm{Cl}$. (f) Classical trajectories representing coherent phonon dynamics in the potential surface. A and B, respectively, denote the excited Mott insulator state and the photoinduced metallic state.

Figure 4

(a) Time evolutions of $\Delta R/R$ (inset) and the oscillating component detected at 0.31 eV for the slow time domain, together with a schematic illustration of the oscillating photoinduced metallic domain. (b) Amplitude spectrum of the photoinduced metallic state is shown by the red circles. The solid curve represents $\Delta R_{h\mathrm{\text{−}}d}/R_d$. (c) Oscillating amplitude as a function of temperature.