Bandwidth Tuning Triggers Interplay of Charge Order and Superconductivity in Two-Dimensional Organic Materials

We observe charge-order fluctuations in the quasi-two-dimensional organic superconductor ${\beta }^{\prime \prime }-(\mathrm{BEDT}-\mathrm{TTF}{)}_2{\mathrm{SF}}_5{\mathrm{CH}}_2{\mathrm{CF}}_2{\mathrm{SO}}_3$, both by means of vibrational spectroscopy, locally probing the fluctuating charge order, and by investigating the in-plane dynamical response by infrared reflectance spectroscopy. The decrease of the effective electronic interaction in an isostructural metal suppresses both charge-order fluctuations and superconductivity, pointing to their interplay. We compare the results of our experiments with calculations on the extended Hubbard model.

Figure 1

(a) Phase diagram for the $1/4$-filled organic conductors calculated by the extended Hubbard model. The dashed line is for superconductivity, schematically based on Ref. 8. The solid lines show the approximate positions of the studied compounds in the phase diagram. (b) A schematic view of a 3D structure of the layered BEDT-TTF-based crystals, with the rectangles marking the BEDT-TTF molecules.

Figure 2

Temperature-dependent dc resistivity in the conducting plane of (a) ${\beta }^{\prime \prime }$-SC and (c) ${\beta }^{\prime \prime }$-M. Optical conductivity perpendicular to the conducting layers in the region of $B_{1u}({\nu }_{27})$ BEDT-TTF vibration is shown: (b) ${\beta }^{\prime \prime }$-SC, a band of $B_{1u}({\nu }_{27})$ splits into two components at 1448 and $1470{\mathrm{cm}}^{-1}$ as the temperature decreases; (c) ${\beta }^{\prime \prime }$-M, $B_{1u}({\nu }_{27})$ vibration of BEDT-TTF is a single wide band at all temperatures, showing the frequency of $1452{\mathrm{cm}}^{-1}$ at 10 K.

Figure 3

In-plane conductivity ($E\parallel b$) of (a) ${\beta }^{\prime \prime }$-SC and (b) ${\beta }^{\prime \prime }$-M at different temperatures. The insets show conductivity spectra at 10 K with the results of the Drude-Lorentz fit and marked electronic bands, as discussed in the text. The spectra in the perpendicular direction show the same electronic features.

Figure 4

(a) The conductivity in $E\perp b$ of ${\beta }^{\prime \prime }$-SC shows a narrow Drude contribution and the lattice mode at about $40{\mathrm{cm}}^{-1}$ coupled to charge carriers. (b) The conductivity in the $E\parallel b$ direction. The inset shows the opening of the superconducting gap at the ratio $R(1.75\mathrm{K})/R(10\mathrm{K})$. (c) Temperature dependence of Drude plasma frequency. Note the reentrant behavior. (d) The effective mass at 10 K calculated by the extended Drude analysis. The inset shows $\overline{\Pi }(\omega )$ at the lowest frequencies.