Angle-dependent magnetoresistance oscillations due to magnetic breakdown orbits

We present experimental evidence for a hitherto unconfirmed type of angle-dependent magnetoresistance oscillation caused by magnetic breakdown. The effect was observed in the organic superconductor $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{Cu}{(\mathrm{NCS})}_{2}$ using hydrostatic pressures of up to $9.8\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$ and magnetic fields of up to $33\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. In addition, we show that similar oscillations are revealed in ambient-pressure measurements, provided that the Shubnikov\char21{}de Haas oscillations are suppressed either by elevated temperatures or filtering of the data. These results provide a compelling validation of Pippard's semiclassical picture of magnetic breakdown [Philos. Trans. R. Soc. London, Ser. A 270, 1 (1962)].