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 κ-(ET-d₈)₂Cu[N(CN)₂]Cl, paramagnetic metal α-(ET)₂KHg(SCN)₄, and superconductors κ-(ET)₂Cu(NCS)₂ and κ -(ET)₂Cu[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 (EF), especially for the κ-phase salts, although we have clearly observed a finite EF intensity for α-(ET)₂KHg(SCN)₄. Hartree-Fock band-structure calculations for the paramagnetic metallic and antiferromagnetic insulating states qualitatively explain the spectra of α-(ET)₂KHg(SCN)₄ and κ-(ET-d₈)₂Cu[N(CN)₂]Cl, respectively, while those of κ-(ET)₂Cu(NCS)₂ and κ-(ET)₂Cu[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 κ-phase salts than in α-(ET)₂KHg(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 κ-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 κ-(ET)₂Cu(NCS)₂, α-(ET)₂KHg(SCN)₄, and κ-(ET-d₈)₂Cu[N(CN)₂]Cl.