Measurement of Fermi-surface distortion in double quantum wells from in-plane magnetic fields

In double quantum wells with interwell tunneling, we find that a small in-plane magnetic field substantially alters the beating observed in Shubnikov--de Haas oscillations. This phenomenon is attributed to distortion of the Fermi surface due to mixing of the symmetric and antisymmetric double-well states. The measured magnitude of the distortion is in good quantitative agreement with a calculation of the symmetric and antisymmetric mixing from the in-plane magnetic field.