Evidence for exchange interaction between donor and acceptor layers in β'-(BEDT-TTF)(TCNQ)

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We assessed the infrared-absorption spectra and 13C-NMR measurements in a layered organic salt, β'-(BEDT-TTF)(TCNQ), which exhibits antiferromagnetic transitions at 20 and 3 K. The former originates from the spin in the bis-(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) layers, while the latter originates from the localized spin in the tetracyanoquinodimethane (TCNQ) layers. Using infrared-absorption spectroscopy, we estimated the degree of charge transfer, ρ, between BEDT-TTF and TCNQ as 0.5. Using 13C-NMR spectroscopy, we observed an exchange field at the BEDT-TTF site, which is produced by the localized spins of TCNQ dimers. Using the obtained value of ρ and the molecular arrangement of β'-(BEDT-TTF)(TCNQ), which is similar to that of the highest Tc organic superconductor, β'-(BEDT-TTF)_[2]ICl_[2], we concluded that the absence of the pressure-induced superconductivity in β'-(BEDT-TTF)(TCNQ) results from the presence of this exchange field. The exchange interaction, J, and the exchange field, H_[ex], were estimated as -12 K and -19 T/μB on the TCNQ dimer unit, respectively. These findings suggest that superconductivity may arise in β'-(BEDT-TTF)(TCNQ) by the application of an external field of 19 T under high pressure.
2009-11-01