Anomalous behaviour of the in-plane electrical conductivity of the layered superconductor κ-(BEDT-TTF)₂Cu(NCS)₂

The apparent quasiparticle scattering rates in high-quality crystals of the quasi-two-dimensional superconductor κ-(BEDT-TTF)₂Cu(NCS)₂ are studied using the Shubnikov–de Haas effect and megahertz penetration-depth experiments. The width of the superconducting transition observed in the megahertz experiments, taken in conjunction with the field dependence of the Shubnikov–de Haas oscillations, gives evidence that the broadening of the Landau levels is primarily caused by spatial inhomogeneities. This indicates a quasiparticle lifetime for the Landau states ≫ 3 ps. The megahertz data can also be used to derive an apparent scattering time (0.14– 0.56 ps) from the skin depth. This is much shorter than the Landau-state lifetime, in strong contrast to the expectations of Landau Fermi-liquid theory. The simplest explanation for the data is that only a fraction of the crystal contributes to the metallic conductivity, an observation which may be related to the recently observed 'glassy' transition in κ-(BEDT-TTF)₂Cu(NCS)₂.