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
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