Subband transitions in AlxGa1−xN /GaN/ AlxGa1−xN and AlxGa1−xN /InN/ AlxGa1−xN single quantum wells

We have calculated the spectral regime of subband transitions in Al_xGa_1−xN/GaN and Al_xGa_1−xN/InN single quantum wells. We used a simplified model to account for the internal electric fields, which modify the shape of the quantum well. Some of the parameters for these materials have not yet been firmly established. Therefore, we carried out the analysis for the extremes of the reported values of conduction band discontinuities and band gaps (in the case of InN). This analysis shows that the spectral regime of interband transitions for 1–4 nm thick wells has wavelengths above 0.5 μm for AlGaN/InN and above 0.8 μm for AlGaN/GaN and both heterostructures cover several μm wavelengths. The spectral variation with alloy composition is less pronounced in the Al_xGa_1−xN/InN single quantum wells due to the higher electric field present across the InN quantum well as compared to GaN. The results of these calculations are in good agreement with more rigorous theoretical approaches and available experimental values for Al_xGa_1−xN/GaN.     

Wedge-shaped potential and Airy-function electron localization in oxide superlattices

Oxide superlattices and microstructures hold the promise for creating a new class of devices with unprecedented functionalities. Density-functional studies of the recently fabricated, lattice-matched perovskite titanates (SrTiO3)n/(LaTiO3)m reveal a classic wedge-shaped potential well for the monolayer (m = 1) structure, originating from the Coulomb potential of a two-dimensional charged La sheet. The potential in turn confines the electrons in the Airy-function-localized states. Magnetism is suppressed for the monolayer structure, while in structures with a thicker LaTiO3 part, bulk antiferromagnetism is recovered, with a narrow transition region separating the magnetic LaTiO3 and the nonmagnetic SrTiO3.     

Dynamical electron localization and self-induced transparency in semiconductor superlattices

The mechanisms of the occurrence of self-induced and selective transparencies of semiconductor superlattices in a strong time-dependent electric field are investigated. The association of these mechanisms with Bloch oscillations, dynamical localization, and collapse of electron quasi-energy minibands is analyzed, and a comparison with the properties of Josephson junctions is made. It is shown that the self-induced transparency is due to the fact that the current-contributing component of the electron distribution function is destroyed by collisions at discrete values of the amplitude of the time-harmonic field, while the selective transparency is associated with the nonmonotonic dependence of the spectrum of nonlinear electron oscillations in the electric field on the amplitude of the field. The dynamical localization and collapse of quasi-energy minibands lead to the field energy dissipation and are favorable to destruction of the transparency states of the superlattice.