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 (B-parallel to) and perpendicular (B-perpendicular to) 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 B-parallel to. The two corresponding ladders of Landau levels formed at finite B-perpendicular to thus exhibit multiple crossings. A third set of Landau levels, independent of B-parallel to, arises from magnetic breakdown of the Fermi surface. Semiclassical calculations show good agreement with the data.