Coherent versus incoherent interlayer transport in layered metals

The magnetic-field, temperature, and angular dependence of the interlayer magnetoresistance of two different quasi-two-dimensional (2D) organic superconductors is reported. For kappa-(BEDT-TTF)(2)I-3, where BEDT-TTF is bisethylenedithio-tetrathiafulvalene, we find a well-resolved peak in the angle-dependent magnetoresistance at Theta=90degrees (field parallel to the layers). This clear-cut proof for the coherent nature of the interlayer transport is absent for beta(')-(BEDT-TTF)(2)SF5CH2CF2SO3. This and the nonmetallic behavior of the magnetoresistance suggest an incoherent quasiparticle motion for the latter 2D metal.