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

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The polarized optical reflectance spectra of the quasi two dimensional organic correlated electron system κ-(BEDT-TTF)₂Cu[N(CN)₂]Y, Y = Br and Cl are measured in the infrared region. The former shows the superconductivity at T c ≃ 11.6 K and the latter does the antiferromagnetic insulator transition at T N ≃ 28 K. Both the specific molecular vibration mode ν₃(ag) of the BEDT-TTF molecule and the optical conductivity hump in the mid-infrared region change correlatively at T* ≃ 38 K of κ-(BEDT-TTF)₂Cu[N(CN)₂]Br, although no indication of T* but the insulating behaviour below T ins ≃ 50-60 K are found in κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl. The results suggest that the electron-molecular vibration coupling on the ν₃(ag) 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 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 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 κ-(BEDT-TTF)₂Cu[N(CN)₂]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 κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl below T ins is the Mott insulator.