We measured the field strength and angular dependence in the electrical resistance of the organic conductor $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Cl}}_4$ under a pressure of $6\mathrm{kbar}$. In addition to the slow Shubnikov–de Haas (SdH) oscillations similar to those observed at ambient pressure, much faster oscillations are detected and attributed to the magnetic breakdown (MB) orbit. The increase of the SdH frequency indicates an increase in the size of the closed Fermi surface under pressure. The effective masses as determined from the closed and MB orbit oscillations are $(2.9\pm 0.3)m_0$ and $(5.5\pm 0.3)m_0$, respectively. The effective mass becomes smaller under pressure most likely due to the broadening of the bandwidth. The estimated internal field increases from $32\text{to}52\mathrm{T}$ under a pressure of $6\mathrm{kbar}$. Angular-dependent magnetoresistance oscillations reveal a very small closed orbit which is not detected in the SdH oscillation measurements.

• Received 22 November 2005

DOI:https://doi.org/10.1103/PhysRevB.73.214532