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 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.

Source-reported materials — not catalogue approval

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