Observation of inter-Landau-level transitions in resonant tunneling between transverse X states in GaAs/AlAs double-barrier structures under hydrostatic pressure

We have carried out sensitive measurement of the vertical transport characteristics of several GaAs/AlAs ``double-barrier'' heterostructures pressurized just beyond the type-II transition, and in the presence of longitudinal magnetic fields of up to 15 T. Towards the upper field limit, clear periodic structure is observed in the second derivative current-voltage characteristic of the resonance attributed to the process Xₜ(1)→Xₜ(1)+TOAlAs, where Xₜ(1) indicates the lowest quasiconfined subband associated with the transverse X minima in AlAs, and TOAlAs is a zone center transverse optical phonon. The periodic structure is interpreted as a series of transitions to collector states of increasing Landau index, with the requirement for conservation of in-plane momentum being satisfied for any interlevel transition by the phonon emission. Quantitative analysis of the data yields a value for the Landau-level separation, and thus also a value for the two-dimensional geometric effective mass of the transverse X minima in AlAs.