The polarized optical reflectance spectra of the quasi-two-dimensional organic correlated electron system κ (BEDT TTF)2 Cu[N(CN)2]Y, Y = Br and Cl are measured in the infrared region. The former shows the superconductivity at Tc≃11.6K and the latter does the antiferromagnetic insulator transition at TN≃28K. 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*≃38K of κ (BEDT TTF)2Cu[N(CN)2]Br, although no indication of T* but the insulating behavior below Tins≃50−60K 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 Tins 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 Tins, 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 Tins is the Mott insulator.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 2004 Feb 20|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics