De Haas–van Alphen and Shubnikov–de Haas oscillations in the organic superconductor $\kappa -(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F}{)}_2\mathrm{Cu}[\mathrm{N}\left(\mathrm{CN}{)}_2\right]\mathrm{Br}$ have been studied at different orientations of the magnetic field with respect to the conducting layers. In addition to the previously reported magnetic breakdown frequency $F_{\beta }(\theta =0\mathrm{}°)\simeq 3790$ T, a frequency of $\simeq 530$ T has been detected. This frequency is attributed to the classical $\alpha$ orbit enclosing $\simeq 14\%$ of the Brillouin zone cross-section area, confirming earlier extended Hückel model band structure calculations. The angular dependence of the $\beta$ oscillation amplitude exhibits pronounced spin-splitting zeros. At certain field orientations, distinct beating of the $\beta$ orbit dHvA oscillations is found, originating most likely from a very slight “warping” of the Fermi surface in the least conducting direction. In contrast, SdH oscillations, detected simultaneously at the same conditions, show no sign of beating.

• Received 27 August 1999

DOI:https://doi.org/10.1103/PhysRevB.60.R16259