Carrier hopping in InAs/AlyGa1−yAs quantum dot heterostructures: effects on optical and laser properties

The optical properties of InAs/Al_yGa_1−yAs self-assembled quantum dots are studied as a function of temperature from 10 K to room temperature. The temperature dependence of carrier hopping between dots is discussed in terms of the depth of the dot confinement potential and the dispersion in dot size and composition. We show that carrier hopping between dots influences both the electrical and optical properties of laser devices having dots as active medium.     

Effect of inter-miniband tunneling on current resonances due to the formation of stochastic conduction networks in superlattices

We study the effects of inter-miniband electron tunneling and electric field domains on the current–voltage and conductance–voltage curves of biased semiconductor superlattices under the action of a magnetic field that is tilted relative to the plane of the layers. For this geometry, electrons in the superlattice minibands exhibit a unique type of stochastic semiclassical motion. At certain critical values of the electric field within the superlattice layers, the stochastic trajectories change abruptly from fully localized to completely unbounded, and map out an intricate web-like mesh of conduction channels in phase space. Delocalization of the electron paths produces a series of strong resonant peaks in the electron drift velocity versus electric field curves. We use these drift velocity characteristics to make self-consistent drift-diffusion calculations of the current–voltage and differential conductance–voltage curves of the superlattices, which reveal strong resonant features originating from the sudden delocalization of the stochastic single-electron paths. We show that this delocalization has a pronounced effect on the distribution of space charge and electric field domains within the superlattices. Inter-miniband tunneling greatly reduces the amount of space-charge buildup, thus enhancing the domain structure and both the strength and number of the current resonances.     

Deforming subdivisions

Subdivisions are continuously deformed subject to three local conditions and shown to retain the global property of being a subdivision. Cell maps, which are introduced, enable elementary arguments.This research is supported in part by the National Science Foundation Grant DMS-8902662.     

Homotopies for computation of fixed points on unbounded regions

Givenf: R ⁿ R ^n* with some conditions, our aim is to compute a fixed pointx f(x) off; hereR ⁿ isn-dimensional Euclidean space andR ^n* is the collection of nonempty subsets ofR ⁿ . A typical application of the algorithm can be motivated as follows: Beginning with the constant mapf ₀:R ⁿ {0} R ⁿ and its fixed pointx ₀ = 0, we deformf _t ast tof f and follow the pathx _t of fixed points off _t . Cluster points of thex _t 's ast are fixed points off. This research was supported in part by Army Research Office-Durham Contract DAHC-04-71-C-0041 and by National Science Foundation Grant GK-5695.     

Piecewise optimal linearization method for nonlinear stochastic differential equations

A method for estimating the dynamical statistical properties of the solutions of nonlinear Langevin-type stochastic differential equations is presented. The non-linear equation is linearized within a small interval of the independent variable and statistical properties are expressed analytically within the interval. The linearization procedure is optimal in the sense of the Chebyshev inequality. Long-term behavior of the solution process is obtained by appropriately matching the approximate solutions at the boundaries between intervals. The method is applied to a model nonlinear equation for which the exact time-dependent moments can be obtained by numerical methods. The calculations demonstrate that the method represents a significant improvement over the method of statistical linearization in time regimes far from equilibrium.Supported in part by the National Science Foundation under Grants CHE77-16307 and PHY76-04761.     

Microscopic analysis of the valence band and impurity band theories of (Ga,Mn)As

We analyze microscopically the valence and impurity band models of ferromagnetic (Ga,Mn)As. We find that the tight-binding Anderson approach with conventional parametrization and the full potential local-density approximation+U calculations give a very similar band structure whose microscopic spectral character is consistent with the physical premise of the k·p kinetic-exchange model. On the other hand, the various models with a band structure comprising an impurity band detached from the valence band assume mutually incompatible microscopic spectral character. By adapting the tight-binding Anderson calculations individually to each of the impurity band pictures in the single Mn impurity limit and then by exploring the entire doping range, we find that a detached impurity band does not persist in any of these models in ferromagnetic (Ga,Mn)As.     

High-temperature single-site ethylene polymerization behavior of titanate complexes supported by 1,3-bis(3,5-dialkylpyrazol-1-yl)propan-2-olate ligation

Titanate(1-) complexes Na[(THF)(kappa1-O-bdbpzp)TiCl4] (1) and Na[(THF)(kappa1-O-bdmpzp)TiCl4] (2) and titanate(2-) complexes [Na(THF)]2[(kappa1-O-bdbpzp)2TiCl4] (4) and [Na(THF)]2[(kappa1-O-bdmpzp)2TiCl4] (5) were obtained in good yield from reaction of Na[bdbpzp] or Na[bdmpzp] (sodium salt of 1,3-bis(3,5-di-tert-butylpyrazol-1yl)propan-2-ol or 1,3-bis(3,5-dimethylpyrazol-1yl)propan-2-ol) with TiCl4 (in the appropriate molar ratio) at 0-25 degrees C. Protonolysis of TiCl4 with 1 equiv of bdmpzpH furnished related zwitterionic titanate(1-) complex 3 that possessed a kappa2-N,O-coordinated pyrazolyl-alkoxide with pendant pyrazolium group. Methylalumoxane (MAO) activation of 1-5 under high-temperature solution polymerization conditions produced active single-site ethylene polymerization catalysts that exhibit considerably higher thermal stability (especially 2/MAO, 3/MAO, and 5/MAO) than previously reported for Cp2TiCl2/MAO or Ti catalysts supported by related heteroscorpionate or scorpionate ligation.     

Self-assembling of In(Ga)As/GaAs quantum dots on (N11) substrates: the (311)A case

We have investigated the In(Ga)As island formation, in the Stranski-Krastanov growth mode, on (311)A GaAs substrates. The surface topography of InAs and InGaAs strained epilayers was studied by contact microscopies. The different substrate affects the overgrown island shape. In(Ga)As grown on (311)A gives rise to quantum wire-like islands. Quantum dots (QDs), but with highly anisotropic shapes, are the outcomes of InAs deposition. QD samples were also characterized by photoluminescence (PL) measurements. Correlation between optical and morphological properties was observed.     

Laser location and manipulation of a single quantum tunneling channel in an InAs quantum dot

We use a femtowatt focused laser beam to locate and manipulate a single quantum tunneling channel associated with an individual InAs quantum dot within an ensemble of dots. The intensity of the directed laser beam tunes the tunneling current through the targeted dot with an effective optical gain of 10(7) and modifies the curvature of the dot's confining potential and the spatial extent of its ground state electron eigenfunction. These observations are explained by the effect of photocreated hole charges which become bound close to the targeted dot, thus acting as an optically induced gate electrode.