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 conti nuously tunable highly non-parabolic system, the cyclotron masses of t he two Fermi surface orbits change in opposite directions with B-paral lel to. This causes the two corresponding ladders of Landau levels for med at finite B-perpendicular to, to exhibit multiple crossings. We al so observe a third set of Landau levels, independent of B-parallel to, which arise from magnetic breakdown of the Fermi surface. Both semicl assical and full-quantum mechanical calculations show good agreement w ith the data. (C) 1998 Elsevier Science B.V. All rights reserved.