High-resolution photoemission study of metallic, insulating, and superconducting BEDT-TTF salts

We have made a high-resolution photoemission study of quasi-two-dimensional organic conductors BEDT-TTF salts which have various ground states, namely, antiferromagnetic insulator kappa-(ET-d(8))(2)Cu[N(CN)(2)[Cl, paramagnetic metal alpha-(ET)(2)KHg(SCN)(4), and superconductors kappa-(ET)(2)Cu(NCS)(2) and kappa-(ET)(2)Cu[N(CN)(2)]Br, where ET denotes BEDT-TTF or bisethylenedithio-tetrathiafulvalene. In all the spectra, the photoemission intensity is suppressed near the Fermi level (E-F), especially for the K-phase salts, although we have clearly observed a finite E-F intensity for alpha-(ET)(2)KHg(SCN)(4). Hartree-Fock band-structure calculations for the paramagnetic metallic and antiferromagnetic insulating states qualitatively explain the spectra of alpha-(ET)(2)KHg(SCN)(4) and kappa-(ET-d(8))(2)Cu[N(CN)(2)]Cl, respectively, while those of kappa-(ET)(2)Cu(NCS)(2) and kappa-(ET)(2)Cu[N(CN)(2)]Br strongly deviate from the calculations for the corresponding ground states. These observations imply that, among the metallic compounds, electrons are more strongly correlated in the kappa-phase salts than in alpha-(ET)(2)KHg(SCN)(4). It is shown that long-range electron correlation is quite important to explain the strong deviation of the spectra from the band-structure calculation. The spectra of the kappa-phase salts exhibit a remarkable similarity between the metallic (superconducting) and insulating phases across the chemical pressure-induced phase transition. We have also observed unusual temperature dependence of the spectra for kappa-(ET)(2)Cu(NCS)(2), alpha-(ET)(2)KHg(SCN)(4), and kappa-(ET-d(8))(2)Cu[N(CN)(2)]Cl.