mu SR studies of layered organic superconductors: vortex phases, penetration depth and anomalous superfluid properties

Muon-spin rotation (mu SR) measurements have been used to study the superconducting vortex properties of layered organic superconductors based on molecular donors such as BEDT-TTF. The mu SR is particularly sensitive to the degree of local ordering of pancake vortices and can detect when the pancake layers become decoupled by intrinsic or defect-driven decoupling mechanisms, or by thermally driven motion. Further novel features of the vortex system occur when the field is tilted away from a crystal axis. Knowledge of the vortex phase behaviour allows appropriate parameter regions to be selected for reliable determination of the superconducting penetration depth lambda and studies of the temperature dependence of lambda have shown a T-linear term at low fields that is suppressed with increasing field. Systematic studies of lambda across the range of organic superconductors have revealed a strong correlation between lambda and T-c. In contrast to the linear scaling T-c proportional to lambda(-2) seen in high T-c cuprates, the organics show an overall correlation better described as T-c proportional to lambda(-3). One interpretation is that the superconducting carriers are only a small fraction of the total carrier concentration in these low-T-c superconductors. Understanding this result may give us some important clues about the nature of the superconductivity in the organics.