Magnetic breakdown and Landau-level spectra of a tunable double-quantum-well Fermi surface

By measuring longitudinal resistance, we map the Landau-level spectra of double quantum wells as a function of both parallel ((Formula presented)) and perpendicular ((Formula presented)) magnetic fields. In this continuously tunable highly nonparabolic system, both the cyclotron mass and Fermi energy of the two Fermi surface orbits change in opposite directions with (Formula presented). The two corresponding ladders of Landau levels formed at finite (Formula presented) thus exhibit multiple crossings. A third set of Landau levels, independent of (Formula presented), arises from magnetic breakdown of the Fermi surface. Semiclassical calculations show good agreement with the data.