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 ${{β}}^{{''}}{-}(BEDT{-}TTF{)}₂{SF}₅{CH}₂{CF}₂{SO}₃$ ${(T}c{≈}4.4$ K). At low temperatures the SdH oscillations persist clearly below the upper critical field ${B}c2(0){≈}3.6$ T 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.