Intersubband Magnetophonon Resonances in Quantum Cascade Structures

We report on magnetotransport measurements of GaAs/GaAlAs quantum cascade structures in magnetic fields up to 62 T parallel to the current. We observe novel quantum oscillations series in tunneling current that are periodic in reciprocal magnetic field and have field positions independent of the applied bias. These oscillations are explained as intersubband magnetophonon resonance due to electron relaxation by emission of optical or acoustic phonons.