F. Agulló-Rueda

Temperature dependence of the electronic coherence of GaAs‐GaAlAs superlattices

We have shown that the coherence length of electrons in a 55‐A‐period GaAs‐GaAlAs superlattice does not depend strongly on temperature in the range 5–292 K, varying from 17 periods at 5 K to a minimum of nine periods at room temperature. The quantum coherence was determined by photocurrent spectroscopy experiments that exploit the formation of Stark ladders in superlattices under electric fields.

Stark ladder excitonic transitions

Abstract We have calculated the binding energy and the transition energies of excitons for a superlattice under an external electric field applied along the growth direction. At low fields the binding energy shifts strongly showing the localization of the superlattice states into the Stark ladder states. For higher fields the energy levels observe a shift associated to the single quantum well Stark effect. We also discuss the contribution of superlattice fluctuations to the carrier confinement.

Coherence and localization in superlattices under electric fields

Using optical techniques, we have demonstrated that the quantum coherence of electron states in GaAsGaAlAs superlattices under electric fields increases drastically as the period, D, and the electric field, ∄, decrease, reaching at least ten periods at zero field when D = 60 A. The electric field localizes the states, breaking the superlattice minibands into Stark ladders, and producing a transition from three- to quasi-two-dimensional excitons. These effects are manifested, respectively, by the appearance of oscillations in the absorption coefficient, which are periodic both in energy and in ∄−1, and by a sharp increase in the intrawell exciton binding energy.

Observation of miniband formation in the CdTe/Cd1−xMnxTe quantum well system

We report the effects of perpendicular electric fields on the eigenenergies of the CdTe/Cd1−xMnxTe quantum well system. The samples are probed at 6 K using photoluminescence. In an electric field, the photoluminescence spectra of wide‐barrier quantum well samples red shifted, consistent with the quantum‐confined Stark effect. The spectra of narrow‐barrier superlattice samples blue shifted, indicative of miniband reduction to localized states. A linear blue shift was observed up to a field of 50 kV/cm after which saturation occurred. Additionally, we have observed luminescence which appeared to arise from transitions between adjacent wells. No striking effects were observed at the paramagnetic‐spin glass transition temperature.

Growth and novel properties of magnetic heterostructures by molecular beam epitaxy

Abstract Diluted magnetic semiconductor Cd1−xMnxTe-Cd1−yMnyTe heterostructures of various layer thickness, where 0 ≤ x, y ≤ 0.4, were grown by molecular beam epitaxy for optical and magnetic studies of reduced-dimensional systems. X-ray diffraction patterns, low-temperature photoluminescence spectra (PL), and AC magnetic susceptibility measurements were used to verify the integrity of these structures. The carrier quantization in the quantum wells were revealed by the magnetic response in a magneto-optic microsusceptometer as well as by the results of PL measurement. These SLs were used to study the dimensional cross-over of the spin-glass phase. In addition, a magnetic-field-induced transition from type I to type II superlattice was observed.