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

Abstract

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{sub 8}){sub 2}Cu[N(CN){sub 2}]Clthinsp, paramagnetic metal {alpha}-(ET){sub 2}KHg(SCN){sub 4}, and superconductors {kappa}-(ET){sub 2}Cu(NCS){sub 2} and {kappa} -(ET){sub 2}Cu[N(CN){sub 2}]Br, where ET denotes BEDT-TTF or bisethylenedithio-tetrathiafulvalene. In all the spectra, the photoemission intensity is suppressed near the Fermi level (E{sub F}), especially for the {kappa}-phase salts, although we have clearly observed a finite E{sub F} intensity for {alpha}-(ET){sub 2}KHg(SCN){sub 4}. Hartree-Fock band-structure calculations for the paramagnetic metallic and antiferromagnetic insulating states qualitatively explain the spectra of {alpha}-(ET){sub 2}KHg(SCN){sub 4} and {kappa}-(ET-d{sub 8}){sub 2}Cu[N(CN){sub 2}]Cl, respectively, while those of {kappa}-(ET){sub 2}Cu(NCS){sub 2} and {kappa}-(ET){sub 2}Cu[N(CN){sub 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){sub 2}KHg(SCN){sub 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 ofmore » the spectra for {kappa}-(ET){sub 2}Cu(NCS){sub 2}, {alpha}-(ET){sub 2}KHg(SCN){sub 4}, and {kappa}-(ET-d{sub 8}){sub 2}Cu[N(CN){sub 2}]Cl. {copyright} {ital 1997} {ital The American Physical Society}« less