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

Abstract

Muon-spin rotation (μSR) measurements have been used to study the superconducting vortex properties of layered organic superconductors based on molecular donors such as BEDT-TTF. The μSR 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 λ and studies of the temperature dependence of λ have shown a T-linear term at low fields that is suppressed with increasing field. Systematic studies of λ across the range of organic superconductors have revealed a strong correlation between λ and T c. In contrast to the linear scaling T c ∝ λ −2 seen in high T c cuprates, the organics show an overall correlation better described as T c ∝ λ −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.