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

The polarized optical reflectance spectra of the quasi-two-dimensional organic correlated electron system $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]Y,$ $Y=\mathrm{Br}$ and Cl are measured in the infrared region. The former shows the superconductivity at ${T}_{\mathrm{c}}\ensuremath{\simeq}11.6\mathrm{K}$ and the latter does the antiferromagnetic insulator transition at ${T}_{\mathrm{N}}\ensuremath{\simeq}28\mathrm{K}.$ Both the specific molecular vibration mode ${\ensuremath{\nu}}_{3}{(a}_{g})$ of the BEDT-TTF molecule and the optical conductivity hump in the mid-infrared region change correlatively at ${T}^{*}\ensuremath{\simeq}38\mathrm{K}$ of $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br},$ although no indication of ${T}^{*}$ but the insulating behavior below ${T}_{\mathrm{ins}}\ensuremath{\simeq}50\char21{}60\mathrm{K}$ are found in $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Cl}.$ The results suggest that the electron-molecular vibration coupling on the ${\ensuremath{\nu}}_{3}{(a}_{g})$ mode becomes weak due to the enhancement of the itinerant nature of the carriers on the dimer of the BEDT-TTF molecules below ${T}^{*},$ while it does strong below ${T}_{\mathrm{ins}}$ 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 ${T}^{*}$ and ${T}_{\mathrm{ins}},$ respectively, which originates from the transitions between the upper and lower Mott-Hubbard bands. The present observations demonstrate that two different metallic states of $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br}$ are regarded as a correlated good metal below ${T}^{*}$ including the superconducting state and a half filling bad metal above ${T}^{*}.$ In contrast the insulating state of $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Cl}$ below ${T}_{\mathrm{ins}}$ is the Mott insulator.