Test for Interlayer Coherence in a Quasi-Two-Dimensional Superconductor

Peaks in the magnetoresistivity of the layered superconductor κ-(BEDT-TTF)₂Cu(NCS)₂, measured in fields ≤45T applied within the layers, show that the Fermi surface is extended in the interlayer direction and enable the interlayer transfer integral (t_⊥≈0.04meV) to be deduced. However, the quasiparticle scattering rate τ^-1 is such that /τ~6t_⊥, implying that κ-(BEDT-TTF)₂Cu(NCS)₂ meets the criterion used to identify interlayer incoherence. The applicability of this criterion to anisotropic materials is thus shown to be questionable.