Low-temperature Fermi surface of the organic conductor ${\beta }^{″}\text{−}\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)({\mathrm{TCNQ})}_{(1-x)}{\left(\mathrm{F}1\text{−}\mathrm{TCNQ}\right)}_x$ $(x=0,0.05)$ from magnetooptical measurements

Magnetooptical measurements have been performed on the organic conductor ${\beta }^{″}$-(BEDT-TTF)(TCNQ) ($x=0$ salt), and a related compound, ${\beta }^{″}\text{−}\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)\left(\mathrm{TCNQ}{)}_{0.95}\right(\mathrm{F}1\text{−}\mathrm{TCNQ}{)}_{0.05}$ ($x=0.05$ salt), to investigate their low-temperature Fermi surfaces (FSs). Although the room-temperature FS of both salts can be considered the same, our results indicate that the low-temperature FSs are completely different from each other. In the $x=0$ salt, the low-temperature FS consists of two very anisotropic quasi-two-dimensional (Q2D) FSs. Cross-sectional areas of anisotropic Q2D-FSs are very small, and correspond to $1–2\%$ of the first Brillouin zone at room temperature. On the other hand, in the $x=0.05$ salt, only a pair of quasi-one-dimensional FSs exist at low temperature. Compared with the band calculation of $x=0$ salt, this Q1D-FS originates from the BEDT-TTF conducting layers. This result indicates that the two hump-like anomalies in the temperature dependence of resistivity at 80 and $20\mathrm{K}$, which are observed only in the $x=0$ salt, are related to the nesting of BEDT-TTF Q1D-FS. Hence, it is considered that anisotropic Q2D-FS pockets observed in the $x=0$ salt are generated by the imperfect nesting of Q1D-FS, and the density wave state may be formed in the $x=0$ salt at low temperatures.

Figure 1

$\theta$ dependence of cavity transmission spectra of the $x=0$ salt at $1.6\mathrm{K}$ and $71.9\mathrm{GHz}$ for (a) $\varphi =0°$ and (b) $\varphi =-45°$. Dashed lines are guides for the eyes for several resonances.

Figure 2

Model calculation of $\theta$ dependence of $\nu ∕B_{\mathrm{res}}$ for $\varphi =0°$. (a) Q1D-FS parallel to the $b^{*}c$ plane is assumed. The bold solid line shows the $\theta$ dependence of the most fundamental resonance originating from the $(0,1)$ corrugation. (b) Q2D-FS with weakly warped cylindrical shape is assumed.

Figure 3

Observed $\theta$ dependence of $\nu ∕B_{\mathrm{res}}$ of Fig. 1 for $\varphi =0°$ in the $x=0$ salt. Dashed lines are fitted lines using Eq. (3).

Figure 4

The dependence of resonance intensities indicated by the bold dashed line (A) in Fig. 1 vs $\tan \theta$ for various $\varphi$. Dashed lines are guides for the eyes.

Figure 5

$\varphi$ dependence of transmission spectra when magnetic field is in the conducting plane of the $x=0$ salt. Solid and dashed lines show two different series of resonances.

Figure 6

Polar plot of $h\nu ∕B_{\mathrm{res}}e{b}^{*}$ corresponding to results in Fig. 5. The two kinds of symbols indicate the two series of resonances (A) and (B). Fine solid lines are fitted results using Eq. (4) and ellipses are the obtained FS cross sections in velocity space.

Figure 7

$\theta$ dependence of transmission spectra of the $x=0.05$ salt at $1.6\mathrm{K}$ and $72.1\mathrm{GHz}$ for $\varphi =0°$. Dashed lines are guides for the eyes. Sharp absorption lines appearing at about $2.5\mathrm{T}$ correspond to the electron spin resonance.

Figure 8

Plot of $\nu ∕B_{\mathrm{res}}$ vs $\theta$ for $\varphi =0°$ corresponding to result in Fig. 7. Dashed lines are fitted lines using Eqs. (1, 2).

Figure 9

$\theta$ dependence of transmission spectra of the $x=0.05$ salt at $1.6\mathrm{K}$ and $72.1\mathrm{GHz}$ for $\varphi =90°$. No clear absorption line is observed.

Figure 10

Schematic figures of FS cross sections of the $x=0$ salt in the $k$ space for the (a) $\alpha$ orbit and the (b) $\beta$ orbit. One would be an electron pocket and the other would be a hole pocket. Anisotropies in the conducting plane are estimated from our results. Cross-sectional areas obtained by the SdH measurement performed by Yasuzuka et al. (Ref. 26) are used. An example of the relative topology of $\alpha$- and $\beta$-orbits is shown in the (c). Dashed orbit shows the magnetic breakdown orbit $\alpha +\beta$.

Figure 11

Schematic diagram of FS for the $x=0.05$ salt at low temperatures. First Brillouin zone is described using lattice parameters of the $x=0$ salt at room temperature.