Elastic study of antiferromagnetic fluctuations in the layered organic superconductors kappa-(BEDT-TTF)(2)X

In an ultrasonic experiment, we have investigated the temperature profile o f the velocity of longitudinal elastic waves propagating along a direction perpendicular to the layers in the organic superconductors kappa(BEDT-TTF)( 2)X, X = Cu[N(CN)(2)]Br and Cu(SCN)(2). Although a small decrease of the ve locity is observed at the superconducting transition, the most anomalous be havior is obtained in the normal metallic state where an important softenin g is identified around 40-50 K. In order to characterize the origin of this anomaly, we have studied its behavior under the application of hydrostatic pressure. The observed behavior is found to mimic those of the transport a nd magnetic properties of these materials which have been attributed to the magnetic fluctuations. Following the example of one-dimensional insulating systems where coupling between longitudinal acoustic waves and magnetic fl uctuations is known to occur, our results suggest that the pseudogap regime of these two-dimensional organic superconductors is dominated by a similar mechanism.