High-pressure transport measurements of α'-BEDT-TTF salts

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Abstract
The authors report on transport and optical measurements of some alpha '-phase (BEDT-TTF)2X salts with X=AuBr2-, Ag(CN)2- and CuCl2-. These salts are shown to be Mott-Hubbard semiconductors. Transport measurements were made under hydrostatic pressures up to 20 kbar and temperatures down to 4 K. At room temperature all three systems have low conductivities, approximately=0.05 S cm-1 and small activation energies, approximately=300 meV. On cooling, the AuBr2- and Ag(CN)2- salts show a transition between two semiconducting phases with slightly different activation energies, and for both salts the transition temperature increases under pressure. For all three salts the activation energy for conductivity decreases under pressure. In the AuBr2- system the transition changes abruptly from second to first order at 8.5 kbar. This is not seen for Ag(CN)2-. The thermopower has been measured for all three salts at ambient pressure. There are differences, with the AuBr2- system showing hole conduction and the other showing electron conduction. The authors have measured the optical absorption spectrum of the Ag(CN)2- salt, and this shows a peak at 1.3 eV in the solid state which is missing in a dilute solution of BEDT-TTF. They attribute this to a Hubbard correlation. Finally, they suggest an explanation for the similarities of the differences in behaviour between these three systems and discuss the general properties in terms of a large-U model.