Observation of magnetic breakdown in coupled double quantum wells

A high mobility coupled double-quantum-well structure is used to study distortions in the Fermi surfaces as a function of in-plane field and perpendicular field. The longitudinal resistance is measured with a fixed perpendicular field component while both the subband carrier density and in-plane field are varied. The formation of Landau levels in each coupled subband enables the density variations of both subbands to be determined as an in-plane field is applied. For a range of small fixed perpendicular fields large variations are observed in the subband carrier densities with in-plane field. At higher perpendicular fields the Fermi surfaces are predominantly unaffected by the in-plane field. This is evidence for the occurrence of ``magnetic breakdown'' of the distorted Fermi surfaces.