Magnetotransport studies of the Fermi surface in the organic superconductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br

Detailed measurements of the interlayer magnetotransport of the organic sup erconductor K-bis(ethylenedithio)terrathiafulvalene(2)Cu[N(CN)(2)]Br have b een carried out at ambient pressure as well as under quasihydrostatic press ure up to 12 kbar. Shubnikov-de Haas oscillations at frequencies approximat e to 150, 300, and 3900 T have been observed at various pressures. The rapi d oscillations correspond to 100% of the Brillouin zone cross-section area, revealing a magnetic breakdown orbit in accordance with the theoretically predicted Fermi surface. However, the lower two frequencies do not agree wi th the theoretical predictions based on the room-temperature crystal struct ure. In order to interpret the discrepancy, we propose that the Fermi surfa ce undergoes substantial topological changes at a superstructural transitio n below 200 K. The model appears to be fully consistent with the experiment al observations. [S0163-1829(99)00120-4].