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 (Formula presented)-(ET-(Formula presented), paramagnetic metal (Formula presented)-(ET)(Formula presented)KHg(SCN)(Formula presented), and superconductors (Formula presented)-(ET)(Formula presented)Cu(NCS)(Formula presented) and (Formula presented) -(ET)(Formula presented)Cu[N(CN)(Formula presented)]Br, where ET denotes BEDT-TTF or bisethylenedithio-tetrathiafulvalene. In all the spectra, the photoemission intensity is suppressed near the Fermi level (Formula presented), especially for the (Formula presented)-phase salts, although we have clearly observed a finite (Formula presented) intensity for (Formula presented)-(ET)(Formula presented)KHg(SCN)(Formula presented). Hartree-Fock band-structure calculations for the paramagnetic metallic and antiferromagnetic insulating states qualitatively explain the spectra of (Formula presented)-(ET)(Formula presented)KHg(SCN)(Formula presented) and (Formula presented)-(ET-(Formula presented)Cu[N(CN)(Formula presented)]Cl, respectively, while those of (Formula presented)-(ET)(Formula presented)Cu(NCS)(Formula presented) and (Formula presented)-(ET)(Formula presented)Cu[N(CN)(Formula presented)]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 (Formula presented)-phase salts than in (Formula presented)-(ET)(Formula presented)KHg(SCN)(Formula presented). 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 (Formula presented)-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 (Formula presented)-(ET)(Formula presented)Cu(NCS)(Formula presented), (Formula presented)-(ET)(Formula presented)KHg(SCN)(Formula presented), and (Formula presented)-(ET-(Formula presented)Cu[N(CN)(Formula presented)]Cl.