Behavior of excitonic levels in symmetric and asymmetric double quantum wells in a magnetic field

We studied theoretically the excitonic energy levels and the optical absorption spectra for double quantum wells, both symmetric and asymmetric, in the presece of an homogeneous magnetic field. Within the effective mass approach, we expanded the excitonic wave function, in an orthogonal basis formed by products of electron and hole wave functions in the growth direction z, and one particle solutions of the magnetic Hamiltonian in the x-y plane. We applied our method to the case of Al_xGa_{1-x}As, for which we showed how the exciton wave function vary, and how the basis functions are mixed in a non trivial way by the effect of the Coulomb potential. By taking into account all the mixing between the elements in our base, we get anti-crossing between excited excitonic states not reported previously.Comment: 6 pages, 6 figures, revtex, submitted to Phys Rev

Direct and indirect exciton mixing in a slightly asymmetric double quantum well

We studied, theoretically, the optical absorption spectra for a slightly asymmetric double quantum well (DQW), in the presence of electric and magnetic fields. Recent experimental results for a 10.18/3.82/9.61 nm GaAs Al(_{.33} )Ga(_{.67})As DQW show clearly the different behavior in the luminescence peaks for the indirect exciton (IX) and left direct exciton (DX) as a function of the external electric field. We show that the presence of a peak near the (DX) peak, attributed to an impurity bound left (DX) in the experimental results, could be a consequence of the non-trivial mixing between excitonic states.Comment: 8 pages and 8 figure

Extracting quasi-steady Lagrangian transport patterns from the ocean circulation: An application to the Gulf of Mexico

We construct a climatology of Lagrangian coherent structures (LCSs), the concealed skeleton that shapes transport, with a twelve-year-long data-assimilative simulation of the sea-surface circulation in the Gulf of Mexico (GoM). Computed as time-mean Cauchy-Green strain tensorlines of the climatological velocity, the climatological LCSs (cLCSs) unveil recurrent Lagrangian circulation patterns. cLCSs strongly constrain the ensemble-mean Lagrangian circulation of the instantaneous model velocity, thus we show that a climatological velocity may preserve meaningful transport information. Also, the climatological transport patterns we report agree well with GoM kinematics and dynamics, as described in several previous observational and numerical studies. For example, cLCSs identify regions of persistent isolation, and suggest that coastal regions previously identified as high-risk for pollution impact, are regions of maximal attraction. Also, we show examples where cLCSs are remarkably similar to transport patterns observed during the Deepwater Horizon and Ixtoc oil spills, and during the Grand LAgrangian Deployment (GLAD) experiment. Thus, it is shown that cLCSs are an efficient way of synthesizing vast amounts of Lagrangian information. The cLCS method confirms previous GoM studies, and contributes to our understanding by revealing the persistent nature of the dynamics and kinematics treated therein.Comment: To be submitte

Inertia-induced accumulation of flotsam in the subtropical gyres

Recent surveys of marine plastic debris density have revealed high levels in the center of the subtropical gyres. Earlier studies have argued that the formation of great garbage patches is due to Ekman convergence in such regions. In this work we report a tendency so far overlooked of drogued and undrogued drifters to accumulate distinctly over the subtropical gyres, with undrogued drifters accumulating in the same areas where plastic debris accumulate. We show that the observed accumulation is too fast for Ekman convergence to explain it. We demonstrate that the accumulation is controlled by finite-size and buoyancy (i.e., inertial) effects on undrogued drifter motion subjected to ocean current and wind drags. We infer that the motion of flotsam in general is constrained by similar effects. This is done by using a newly proposed Maxey--Riley equation which models the submerged (surfaced) drifter portion as a sphere of the fractional volume that is submerged (surfaced).Comment: Submitted to Geophys. Res. Letter

Near-optimal loop tiling by means of cache miss equations and genetic algorithms

The effectiveness of the memory hierarchy is critical for the performance of current processors. The performance of the memory hierarchy can be improved by means of program transformations such as loop tiling, which is a code transformation targeted to reduce capacity misses. This paper presents a novel systematic approach to perform near-optimal loop tiling based on an accurate data locality analysis (cache miss equations) and a powerful technique to search the solution space that is based on a genetic algorithm. The results show that this approach can remove practically all capacity misses for all considered benchmarks. The reduction of replacement misses results in a decrease of the miss ratio that can be as significant as a factor of 7 for the matrix multiply kernel.Peer ReviewedPostprint (published version

Propuesta de enseñanza del tema de polígonos y circunferencias mediante actividades de geometría dinámica, en el nivel bachillerato

Se presenta una propuesta de actividades para la enseñanza de los temas de Polígonos y Circunferencias a nivel bachillerato, que consiste en una secuencia didáctica elaborada en base a la teoría de la epistemología genética. Se incluyen prácticas en computadora con el uso de un programa de geometría dinámica, seguidas de una serie de preguntas dirigidas a centrar la atención del alumno en un objetivo específico y propiciar la reflexión sobre los contenidos matemáticos involucrados