Abstract. Details of the Fermi-surface topology of deuterated κ-(BEDT-TTF) 2 Cu(NCS) 2 have been measured as a function of pressure, and compared with equivalent measurements of the undeuterated salt. We find that the superconducting transition temperature is much more dramatically suppressed by increasing pressure in the deuterated salt. It is suggested that this is linked to pressure-induced changes in the Fermi-surface topology, which occur more rapidly in the deuterated salt than in the undeuterated salt as the pressure is raised. Our data suggest that the negative isotope effect observed on deuteration is due to small differences in Fermi-surface topology caused by the isotopic substitution.

The syntheses, crystal structures, and physical properties of two new crystalline charge-transfer salts of BEDT-TTF, bis(ethylenedithio)tetrathiafulvalene, containing tris(oxalato)metallate(III) anions of 3d elements are reported. Electrochemical oxidation of BEDT-TTF in the presence of (NH 4 )The crystal structure of [1] has been solved at 120 K in the monoclinic space group C2/c, and that of [2] in the same space group at 298 and 120 K. For [1], a~10.273 A ˚, b~19.949 A ˚, c~35.030 A ˚, b~92.97u, V~7169.6(2) A ˚3, Z~8. For [2], at 298 K: a~10.304 A ˚, b~20.091 A ˚, c~35.251 A ˚, b~92.70u, V~7289.3(2) A ˚3, Z~8, and at 120 K a~10.283 A ˚, b~19.917 A ˚, c~34.939 A ˚, b~93.30u, V~7144.4(1) A ˚3, Z~8. The crystal structures of both compounds consist of alternating layers of BEDT-TTF cations and layers containing [M(C 2 O 4 ) 3 ] 32 , H 3 O z or NH 4 z , and PhNO 2 . The BEDT-TTF molecules are arranged in the b'' packing motif and the tris(oxalato)metallate(III) ions form the well-known honeycomb motif found in many molecular based magnets. SQUID magnetometry, Raman spectroscopy and electron paramagnetic resonance (EPR) measurements were performed on crystals of [1]. SQUID magnetometry, single-crystal four-probe conductivity measurements, Raman spectroscopy, EPR and polarised infrared reflectance were performed on crystals of [2]. Both compounds have metal to superconducting transitions with T c ~6.2 K for [1] and for [2], T c ~5.8 K.

Abstract.We compare magnetotransport measurements under pressure on the organic superconductor κ-(BEDT-TTF) 2 Cu(SCN) 2 with different pressure-media and discover that the results are pressure media dependent. This pressure-medium dependence is thought to originate from the difference in thermal contraction between the very soft and highly anisotropic sample and the isotropically contracting, but solid pressure medium, thus resulting in non-hydrostatic pressure on the sample. However, comparison of pressure measurements with different media reveals a pressure-medium independent correlation between the superconducting transition temperature, Tc, and the size of the quasi 2-dimensional Fermi surface pocket and thus the quasi 2-dimensional carrier density in κ-(BEDT-TTF) 2 Cu(SCN) 2 . The observed pressure-induced increase in the quasi 2-dimensional carrier density can be interpreted as a transfer of carriers from quasi 1-dimensional Fermi surface sections, reminiscent of a mechanism in cuprate superconductors, where pressure is known to transfer carriers from the insulating charge reservoir layers into the conducting cuprate sheets.

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