Schedule Nov 21, 2002
Ultrasonic investigation of the pressure-temperature phase diagram of k-(BEDT-TTF)2X quasi-two-dimensional organic conductors.
Mario Poirier, Université de Sherbrooke
Depending on the exact pressure conditions (hydrostatic or chemical by anion X substitution), the normal phase of the organic k-(BEDT-TTF)2X compounds is either a Mott insulator and/or an unconventional metal. Among them, k-(BEDT-TTF)2Cu[N(CN)2]Cl stands as the prototype material, since it shows the complete sequence of states, namely, the Mott-insulating, antiferromagnetic, metallic, and superconducting states, within a pressure interval of a few hundreds bars. Although ultrasonic waves constitute excellent tools to probe such electronic states, experiments on organic conductors are rather scarce due to their very small dimensions. We present the first ultrasonic study of a few k-(BEDT-TTF)2X organic compounds under hydrostatic pressure conditions. Different phase transitions were identified trough several elastic anomalies characterized from isobaric and isothermal sweeps. Beyond the confirmation of the P-T diagram of a typical Mott material, our ultrasonic data on the k-(BEDT-TTF)2Cu[N(CN)2]Cl conductor allows the first detailed study of the critical point region. The very large softening of the velocity at the critical point corresponds to the predicted compressibility divergence of the electronic degrees of freedom and validates then, the DMFT picture of the Mott transition. Most importantly, our data reveal that the high pressure crossover line coincides with the pseudogap features previously observed in magnetic, transport and elastic properties of organic compounds of the same family having a higher chemical pressure.

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