P. A. Schulz

Optical probing of interface roughness in resonant tunneling structures

We report on photoluminescence (PL) and photoluminescence-excitation measurements in GaAs/In0.1Ga0.9As/AlAs double-barrier tunneling structures as a function of bias voltage and temperature. We have observed a splitting in the quantum well photoluminescence due to island formation in the quantum well. We have a good correlation between the variation of integrated PL intensity, linewidth, and tunnel current bias for both lines. The temperature dependence of photoluminescence spectra shows that transfer of carrier between islands can be tuned by the applied bias and that states in different islands are populated by electrons in the resonant tunneling process.

Transport and optical properties of resonant tunneling structures

Transport properties of a GaAs/AlAs superlattice-like double barrier diode are studied in this work as a function of the sample temperature. An activation energy of about 60meV obtained from the Arrhenius plot is in good agreement with the confined level in the central well. Numerical simulations also confirm the importance of bound levels in the Gand X bands for the resonant tunneling process. The enhancement of photoluminescence as the temperature is increased is also studied. This behavior is associated to the transport properties of holes in the collector contact, which control the supply of minority carriers, which tunnel into the well. The description of the observed results requires the modi-cation of simple known models to take into account the two contributions to the pair generation rate in the well, responsible of the photoluminescence at zero and finite bias.

Localization length enhancement in two-dimensional self-assembled systems

Disorder correlations lead to important effects on the localization of states in one-dimensional systems, as shown for a large number of correlation models in the context of polymers and macromolecules. The extension of the problem to two-dimensional systems has been less frequently discussed due to the lack, until recently, of realistic systems with properties that could be addressed to the effects of correlations in disorder. The advent of self-assembled quantum dots, which may show short-range ordering, has changed this scenario. In the present work we investigate the properties of a two-dimensional disordered lattice with short-range correlations in the disorder that reveals itself as a minimal model for self-assembled quantum dots. The short-range correlations in disorder may lead to significant enhancement of the localization length for wide energy windows.

Diffusive-like minibands in finite superlattices of disordered quantum wells

The existence of quantized levels, as a consequence of spatial confinement, does not necessarily imply in formation of superlattices minibands due to the coupling between these quantum wells. The present work discuss a framework to analyze the analoge to minibands in periodic superlattices made of disordered quantum wells. The inter quantum well coupling leads to diffusive-like minibands.

Ballistic transport in open quantum dots: scar wavefunctions and resonance line shapes

We present simulations of transport through highly transmitting open cavities within a lattice Greens function formalism. The qualitative relation between the line shape of the conductance and the symmetries of the corresponding scar wavefunctions is discussed. This system presents similar scar wavefunctions at different conductance plateaus. At the high plateau index limit the scars associated with bouncing ball classic orbits suffer a transition from accessible to quasi-unaccessible from outside the square billiard.

Delocalized states in damaged DNA

Recent studies suggest that base pairing is an efficient electronic delocalization mechanism. However, defects may break down such effect. In the present work we show how a simple model of defects suppresses the delocalization, which survives only for low defect concentrations.

Dynamical localization of the hofstadter spectra

Recent results on magnetoresistance in a two dimensional electron gas (2DEG) under crossed magnetic and microwave fields show a new class of oscillations, suggesting a new kind of zero-resistance states. We consider the problem from the point of view of the electronic structure dressed by photons due to an in-plane linearly polarized ac field. In the strong modulation limit predictions on dressed Hofstadter spectra are discussed, which could be of interest since the bare spectra have been observed in the past few years.

Evolution of dynamic localization regimes in coupled minibands

We consider the behaviour of two coupled symmetric electronic minibands in the presence of an AC intense field. Symmetric coupled minibands are realized as dimerization procedures. The minibands are emulated by a tight-binding finite chain, whereas the interaction with an AC field is described by a time-independent Hamiltonian. We follow the evolution of dressed minibands, non perturbatively, in a wide frequency range. For one dimerization procedure, a transition between both limits can be systematically followed and a dynamic breakdown is unambigously identified. The most striking effect that may occur is a field-induced analog of an insulator-metal transition due to the suppression of a Peierls-like instability.