Detailed measurements of the interlayer magnetotransport of the organic superconductor $\kappa -\mathrm{bis}\left(\mathrm{ethylenedithio}\right){\mathrm{tetrathiafulvalene}}_2\mathrm{Cu}\left[\mathrm{N}\right(\mathrm{CN}{)}_2]\mathrm{Br}$ have been carried out at ambient pressure as well as under quasihydrostatic pressure up to 12 kbar. Shubnikov–de Haas oscillations at frequencies $\approx 150,\hspace{0.5em}\hspace{0.5em}300$, and 3900 T have been observed at various pressures. The rapid 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 with the theoretical predictions based on the room-temperature crystal structure. In order to interpret the discrepancy, we propose that the Fermi surface undergoes substantial topological changes at a superstructural transition below 200 K. The model appears to be fully consistent with the experimental observations.

• Received 16 October 1998

DOI:https://doi.org/10.1103/PhysRevB.59.12370