Mapping the temperature-dependent quasiparticle scattering rate over the Fermi surface of an organic superconductor*

The interlayer magnetoresistance $\rho_{zz}$ of the organic metal $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ is studied in fields of up to 45~T and at temperatures $T$ from 0.5~K to 30~K. The peak in $\rho_{zz}$ seen in in-plane fields, a definitive signature of interlayer coherence, remains to $T$s exceeding the Anderson criterion for incoherent transport by a factor $\sim 30$. Angle-dependent magnetoresistance oscillations are modeled using an approach based on field-induced quasiparticle paths on a 3D Fermi surface, to yield the $T$ dependence of the scattering rate $\tau^{-1}$. The results suggest that $\tau^{-1}$ does not vary strongly over the Fermi surface, and that it has a $T^2$ dependence due to electron-electron scattering. These findings are contrasted with recent experiments on cuprates, and their implications for models of organic superconductivity (e.g. FLEX) are discussed.