De HAAS-VAN alphen effect in the vortex state of the organic superconductor /spl kappa/-(BEDT-TTF)/sub 2/Cu(NCS)/sub 2/

Summary form only given. We report the unambiguous observation of the de Haas-van Alphen (dHvA) effect deep into the vortex state of /spl kappa/-(BEDT-TTF)/sub 2/Cu(NCS)/sub 2/. On passing into the vortex state the measured area of the closed section of Fermi surface and the corresponding carrier mass does not change. An additional attenuation of the dHvA signal is observed relative to an extrapolation of the Lifshitz-Kosevich formula calculated using the normal state parameters. To fit the data an additional field dependent reduction factor was introduced. We parameterise the additional reduction in amplitude below Hc/sub 2/ using an effective scattering rate which may be related to the direct influence of the (orbitally averaged) order parameter in the superconductor using recent theoretical models. Experiments have been carried out for a wide range of orientations of the sample with respect to the magnetic field, at temperatures down to 20 mK, in order to probe the symmetry of the order parameter over the Fermi surface.