Emergence of charge degrees of freedom under high pressure in the organic dimer-Mott insulator ${\beta }^{\prime }\text{−}{\left(\mathbf{BEDT}\mathbf{\text{−}}\mathbf{TTF}\right)}_{\mathbf{2}}{\mathbf{ICl}}_{\mathbf{2}}$

To elucidate the pressure evolution of the electronic structure in an antiferromagnetic dimer-Mott (DM) insulator ${\beta }^{\prime }\text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{ICl}}_2$, which exhibits superconductivity at 14.2 K under 8 GPa, we measured the polarized infrared (IR) optical spectra under high pressure. At ambient pressure, two characteristic bands due to intra- and interdimer charge transfers have been observed in the IR spectra, supporting that this salt is a typical half-filled DM insulator at ambient pressure. With increasing pressure, however, the intradimer charge transfer excitation shifts to much lower energies, indicating that the effective electronic state changes from half-filled to 3/4-filled as a result of weakening of dimerization. This implies that the system approaches a charge-ordered state under high pressure, in which charge degrees of freedom emerge as an important factor. The present results suggest that charge fluctuation inside of dimers plays an important role in the high-temperature superconductivity.

Figure 1

(a) Crystal structure of ${\beta }^{\prime }\text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{ICl}}_2$. (b) Arrangement of BEDT-TTF molecules within the $bc$ plane viewed along the long axis of the BEDT-TTF molecule. $U_0$ is the on-molecule Coulomb interaction, $V_0$ is the intradimer Coulomb interaction, and $U_d$ is the effective on-dimer Coulomb interaction. The transfer integrals $t_1\sim t_5$ are also shown. The ovals represent dimers. In the strong dimerization limit, $t_d\approx t_1$. (c) Schematic of intra- and interdimer charge transfer excitations in a DM insulator. ${\mathrm{\Delta }}_d$ denotes the dimerization gap. (d) Optical reflectivity and (e) conductivity spectra of ${\beta }^{\prime }\text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{ICl}}_2$ for $\mathit{E}\parallel {\mathit{b}}^{*}$ and $\mathit{E}\parallel \mathit{c}$ at various temperatures at ambient pressure. Solid lines in (e) indicate the Hubbard and dimer bands. The spectra in (e) are offset for clarity.

Figure 2

(a) Reflectivity spectra expected in a DAC at several temperatures for $\mathit{E}\parallel {\mathit{b}}^{*}$, which were calculated from the reflectivity spectra in Fig. 1 measured in vacuum. (b) Experimental setup for the reflectivity measurements under high pressure using a DAC. (c)–(f) Reflectivity spectra measured in the DAC for $\mathit{E}\parallel {\mathit{b}}^{*}$ at 0.54, 2.9, 5.9, and 8.5 GPa, respectively. Solid lines indicate the fitting results. The spectra around 2000 and $3000{\mathrm{cm}}^{-1}$ are blank owing to strong absorption by diamond and glycerin, respectively. (g)–(j) The corresponding optical conductivity spectra for $\mathit{E}\parallel {\mathit{b}}^{*}$. The gray lines in (g) show the ${\sigma }_1\left(\omega \right)$ spectra measured at ambient pressure. The Hubbard and dimer bands are shown. The spectra are offset for clarity.

Figure 3

(a) Reflectivity spectra measured at various pressures for $\mathit{E}\parallel \mathit{c}$. Solid lines indicate the fitting results. (b) Optical conductivity spectra obtained from the fitting analysis for $\mathit{E}\parallel \mathit{c}$. The spectra are offset for clarity.

Figure 4

(a)–(c) Pressure dependences of the central frequencies, SWs, and total SWs of the Hubbard and dimer bands for $\mathit{E}\parallel {\mathit{b}}^{*}$ and $\mathit{E}\parallel \mathit{c}$ at the lowest temperature, respectively. (d) The excitation energies of the Hubbard and dimer bands in the strong Coulomb interaction limit are shown as a function of degree of dimerization. The schematic phase diagram expected in the extended Hubbard model with the molecular dimer degree of freedom and the effective model Hamiltonian derived from it is also shown on the bottom plane as functions of degree of dimerization and bandwidth under the condition where the Coulomb interaction parameters are fixed. Here, the on-molecular Coulomb interaction $U_0$, the intradimer Coulomb interaction $V_0$, the interdimer Coulomb interaction $V$, the intradimer transfer integral $t_d$, and the interdimer transfer integral $t$ are taken into account. The Hartree-Fock-type approximation is adopted in the analyses of the extended Hubbard model. Dimerization and bandwidth axes in the phase diagram correspond to the parameters $t_d/t$ and $t/U_0$, respectively. (e) The $p\text{−}T$ phase diagram of ${\beta }^{\prime }\text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{ICl}}_2.T_{\mathrm{MI}}$ (green circle) and $T_c$ (blue circle) were determined by the resistivity measurements [1]. $T_N$ (orange circle) was obtained from the ${}^{13}\mathrm{C}$ NMR measurements under pressure up to 3 GPa [12]. The magnetic properties above 3 GPa (the blank area) have yet to be determined. The arrows depict pressure values studied here.