Mixed-state magnetoresistance in organic superconductors κ–(BEDT-TTF)2Cu(NCS)2

We report transport measurements with field and current parallel to the a axis (perpendicular to the conducting plane) in the organic superconductor κ–(BEDT-TTF${)}_2$Cu(NCS${)}_2$. The magnetoresistance displays a peak effect as a function of field and temperature with the peak fields increasing linearly with decreasing temperatures. The peak resistance is found to be greater than the normal-state value extrapolated from both high- and low-field measurements. The results are in sharp contrast to the conventional dissipation mechanisms in the mixed state for anisotropic superconductors, as in the case of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$. We propose a phenomenological model that the peak in the magnetoresistance is caused by a scattering mechanism due to a strong coupling to the underlying crystal lattice of fluctuating vortices (vortex polarons). The model can semiquantitatively fit the data. © 1996 The American Physical Society.