Oscillatory magnetoresistance in the charge-transfer salt ${\beta }^{\prime \prime }$-${\mathrm{}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})\mathrm{}}_2$Au${\mathrm{Br}}_2$ in magnetic fields up to 60 T: Evidence for field-induced Fermi-surface reconstruction

Magnetoresistance measurements carried out in pulsed magnetic fields of up to 60 T and at temperatures down to 350 mK and angle-dependent magnetoresistance experiments performed in quasistatic fields have been used to establish that the charge-transfer salt ${\beta }^{\prime \prime }$-${\mathrm{}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})\mathrm{}}_2$Au${\mathrm{Br}}_2$ undergoes a change in electronic structure at ∼10 T. We propose that this arises due to a field-induced transition between two different spin-density-wave states. Furthermore, at the highest magnetic fields, both the background magnetoresistance and effective masses of the quasiparticles were found to increase, possibly as a result of an enhancement of the density of states. It is found that frequency mixing effects at very high magnetic fields, observed in the Fourier spectra of the magnetoresistance, cannot be explained by the Shoenberg magnetic interaction, but are instead probably caused by oscillations in the chemical potential which become important as the extreme quantum limit is approached.