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 ch aracteristics of several GaAs/AlAs ''double-barrier'' heterostructures pressurized just beyond the type-II transition, and in the presence o f longitudinal magnetic fields of up to 15 T. Towards he upper field l imit, clear periodic structure is observed in the second derivative cu rrent-voltage characteristic of the resonance attributed to the proces s X-t(1)-->X-t(1) + TOAlAs, where X-t(1) indicates the lowest quasicon fined subband associated with the transverse X minima in AlAs, and TOA lAs is a zone center transverse optical phonon. The periodic structure is interpreted as a series of transitions to collector states of incr easing 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 Lan dau-level separation, and thus also a value for the two-dimensional ge ometric effective mass of the transverse X minima in AlAs.