Shubnikov--de Haas effect in the superconducting state of an organic superconductor

We report the observation of Shubnikov--de Haas (SdH) oscillations in the mixed state of the organic superconductor β''-(BEDT-TTF)2SF5CH2CF2SO3 (Tc∼4.4K). At low temperatures the SdH oscillations persist clearly below the upper critical field Bc2(0)∼3.6T almost down to the field where the resistivity vanishes. Rather unusually, no additional damping of the SdH-signal amplitude -- as well as of the de Haas--van Alphen amplitude -- is observed in the superconducting state. This indicates that the damping in the vortex state of this quasi-two-dimensional superconductor is different in character to that observed for most three-dimensional materials