Electronic correlation in the infrared optical properties of the quasi two dimensional $\kappa$-type BEDT-TTF dimer system

The polarized optical reflectance spectra of the quasi two dimensional organic correlated electron system $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]$Y$, $Y=$ Br and Cl are measured in the infrared region. The former shows the superconductivity at $T_{\mathrm{c}}\simeq$ 11.6 K and the latter does the antiferromagnetic insulator transition at $T_{\mathrm{N}}\simeq$ 28 K. Both the specific molecular vibration mode ${\nu }_3(a_g)$ of the BEDT-TTF molecule and the optical conductivity hump in the mid-infrared region change correlatively at $T^{\ast }\simeq$ 38 K of $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br, although no indication of $T^{\ast }$ but the insulating behaviour below $T_{\rm ins} \simeq$50-60Karefoundin$\kappa$-(BEDT-TTF)${2}$Cu[N(CN)${2}$]Cl. The results suggest that the electron-molecular vibration coupling on the ${\nu }_3(a_g)$ mode becomes weak due to the enhancement of the itinerant nature of the carriers on the dimer of the BEDT-TTF molecules below $T^{\ast }$, while it does strong below $T_{\mathrm{i}\mathrm{n}\mathrm{s}}$ because of the localized carriers on the dimer. These changes are in agreement with the reduction and the enhancement of the mid-infrared conductivity hump below $T^{\ast }$ and $T_{\rm ins}$, respectively, which originates from the transitions between the upper and lower Mott-Hubbard bands. The present observations demonstrate that two different metallic states of $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br are regarded as {\it a correlated good metal} below $T^{\ast }$ including the superconducting state and {\it a half filling bad metal} above $T^{\ast }$. In contrast the insulating state of $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Cl below $T_{\mathrm{i}\mathrm{n}\mathrm{s}}$ is the Mott insulator.