Elastic Study of Antiferromagnetic Fluctuations in the Layered Organic Superconductors kappa-(BEDT-TTF)_2X

TL;DR

This study investigates elastic wave velocity in layered organic superconductors, revealing anomalies linked to magnetic fluctuations and suggesting a pseudo-gap regime dominated by magnetic interactions.

Contribution

It provides the first ultrasonic measurements of elastic wave velocity in these materials and links elastic anomalies to magnetic fluctuations in the pseudo-gap regime.

Findings

Elastic wave velocity softens around 40-50 K in the normal state.

Anomalies under pressure mimic magnetic and transport property behaviors.

Results suggest magnetic fluctuations dominate the pseudo-gap regime.

Abstract

In an ultrasonic experiment, we have investigated the temperature profile of the velocity of longitudinal elastic waves propagating along a direction perpendicular to the layers in the organic superconductors kappa-(BEDT-TTF)_2X, X = Cu(SCN)_2 and Cu[N(CN)_2]Br. Although a small decrease of the velocity is observed at the superconducting transition, the most anomalous behavior is obtained in the normal metallic state where an important softening 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 and 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 fluctuations is known to occur, our results suggest that the pseudo-gap regime of these two-dimensional organic superconductors is dominated by a similar mechanism.