We assessed carbon-13 nuclear magnetic resonance ͑ 13 C NMR͒ measurements of the layered organic salt ␤Ј-͑BEDT-TTF͒ 2 ICl 2 , which exhibits antiferromagnetic transition at ambient pressure and 22 K and superconductive transition at 8.2 GPa and 14.2 K ͑the highest known superconductive transition temperature among organic superconductors͒. By analyzing the 13 C NMR spectrum with the tensor, we determined the antiferromagnetic moment of this salt to be B per dimer at ambient pressure, strongly indicating that this salt is a dimer Mott insulator. From NMR measurements under pressure, we found that the structure of the antiferromagnetic phase changed at 0.6 GPa. The moment of this antiferromagnetic phase was estimated to be 0.47 B per dimer at 0.6 GPa and 26 K. In addition, applying pressure rapidly decreased the spin susceptibility in the paramagnetic state, and the pressure dependence of T N showed anomalous behavior consistent with theoretical proposals, including dimensional crossover.

We measured the 13C-NMR spectrum and T1 of the quasi-two-dimensional organic superconductor kappa-(BEDT-TTF)2Cu(NCS)_{2} under pressure. This material was thought to show a relationship between T_{c} and the effective cyclotron mass m_{c};{*}, obtained from the Shubnikov-de Haas (SdH) effect. We found that kappa-(BEDT-TTF)2Cu(NCS)_{2} behaved as a Fermi liquid at low temperature under all pressures, and antiferromagnetic fluctuations were expected. The pressure dependence of the Korringa factor is similar to that of the effective cyclotron mass m_{c};{*}, suggesting that antiferromagnetic fluctuations contribute to the superconductivity of this material. We also found that, under pressure, T;{*} was shifted to 150 K, the temperature characteristic of the shift from bad metal to good metal.

An organic salt, -͑BEDT-TTF͒ 4 Hg 2.89 Br 8 exhibits superconductivity at 4.3 K under ambient pressure suggesting non-Fermi-liquid ͑NFL͒ behavior just above T c . Whereas most organic superconductors are controlled by the bandwidth in the half-filled electron system, this salt realizes a carrier doping away from the half-filled electron system as well as high-T c cuprates. In order to investigate the origin of NFL behavior, we assessed 13 C-NMR measurements in this salt and observed the antiferromagnetic fluctuation as same as in an organic antiferromagnet -͑BEDT-TTF͒ 2 Cu͓N͑CN͒ 2 ͔Cl with the gap structure. Application of pressure suppresses ͑T 1 T͒ −1 and shifts its maximum to lower temperatures with ͑T 1 T͒ −1 becoming constant above 2 GPa. These results suggest that applying pressure alters the electron system from NFL to FL state and that antiferromagnetic fluctuations contribute to the origin of NFL behavior. The physics of organic conductors involve a strongly correlated electron system, similar to that of high-T c cuprates and heavy-fermion systems.1 For example, bis͑ethylenedithio͒-tetrathiafulvalene ͑BEDT-TTF͒, together with inorganic ions, forms many conducting salts, which have various crystal structures. Although the conducting layers of all of these salts consist of the same molecule, these salts show a variety of behavior, from superconductivity to high-resistance insulator.2 These salts can also be classified by the arrangement of their BEDT-TTF molecules with the arrangement and the electronic properties being closely related. In -͑BEDT-TTF͒ 2 X, two BEDT-TTF molecules form a dimer and constitute a two-dimensional conducting sheet, with one electron per dimer.3 Hence -͑BEDT-TTF͒ 2 X is regarded as a two-dimensional half-filled electron system. Many -͑BEDT-TTF͒ 2 X salts act as superconductors with the superconducting and antiferromagnetic insulating phases being adjacent to or coexisting at low temperatures. 5,6 Therefore, these salts are ideal for investigating the relationship between superconductivity and antiferromagnetism.Carrier doping and applying pressure are complementary methods for research on the phase diagram. Whereas high-T c cuprates showed superconductivity after carrier doping, many organic superconductors showed superconductivity after applying pressure.7 Cuprates and -͑BEDT-TTF͒ 2 X show both similarities and dissimilarities. Despite differences in their ionic and molecular crystals, cuprates and -͑BEDT-TTF͒ 2 X have similar properties, in that their superconductive and antiferromagnetic phases are neighboring, and the order parameter of the superconductivity has d-wave symmetry.8-13 Just above T c , -type salts show Fermi-liquid ͑FL͒ behavior, as shown by their conductivity, spin susceptibility and ͑T 1 T͒ −1 , 5,14,15 whereas high-T c cuprates show non-Fermi-liquid ͑NFL͒ behavior just above T c . To investigate the origin of those similarities and dissimilarities, it is important to assess the material that connects cuprates and -͑BEDT-TTF͒ 2 X. Since carrier doping to organ...

We assessed the infrared-absorption spectra and 13 C-NMR measurements in a layered organic salt, ␤Ј-͑BEDT-TTF͒͑TCNQ͒, which exhibits antiferromagnetic transitions at 20 and 3 K. The former originates from the spin in the bis-͑ethylenedithio͒-tetrathiafulvalene ͑BEDT-TTF͒ layers, while the latter originates from the localized spin in the tetracyanoquinodimethane ͑TCNQ͒ layers. Using infrared-absorption spectroscopy, we estimated the degree of charge transfer, , between BEDT-TTF and TCNQ as 0.5. Using 13 C-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 and the molecular arrangement of ␤Ј-͑BEDT-TTF͒͑TCNQ͒, which is similar to that of the highest T c organic superconductor, ␤Ј-͑BEDT-TTF͒ 2 ICl 2 , we concluded that the absence of the pressure-induced superconductivity in ␤Ј-͑BEDT-TTF͒͑TCNQ͒ 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 / B on the TCNQ dimer unit, respectively. These findings suggest that superconductivity may arise in ␤Ј-͑BEDT-TTF͒͑TCNQ͒ by the application of an external field of 19 T under high pressure.

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