Exciton behavior in GaAs/AlGaAs coupled double quantum wells with interface disorder

In this work, we present a detailed study on the optical properties of two GaAs/Al(0.35)Ga(0.65)As coupled double quantum wells (CDQWs) with inter-well barriers of different thicknesses, by using photoluminescence (PL) spectroscopy. The two CDQWs were grown in a single sample, assuring very similar experimental conditions for measurements of both. The PL spectrum of each CDQW exhibits two recombination channels which can be accurately identified as the excitonic e(1)-hh(1) transitions originated from CDQWs of different effective dimensions. The PL spectra characteristics and the behavior of the emissions as a function of temperature and excitation power are interpreted in the scenario of the bimodal interface roughness model, taking into account the exciton migration between the two regions considered in this model and the difference in the potential fluctuation levels between those two regions. The details of the PL spectra behavior as a function of excitation power are explained in terms of the competition between the band gap renormalization (BGR) and the potential fluctuation effects. The results obtained for the two CDQWs, which have different degrees of potential fluctuation, are also compared and discussed. (C) 2009 Elsevier B.V. All rights reserved.

Shubnikov de Haas oscillations in double wells with opposite signs of the electronic g-factor

We report on the measurements of the Shubnikov de Haas oscillations (SdH) in symmetrically doped AlxGa1-xAs double wells with different Al compositions in wells, which lead to the opposite signs of the electronic g-factor in each layer. Surprisingly, the spin splitting appears and collapses several times with increase in the magnetic field, We attribute such behaviour to the oscillations of the exchange-correlation term with Landau filling factor. (C) 2007 Elsevier B.V. All rights reserved.

Microwave induced magnetoresistance oscillations and inelastic scattering time in double quantum wells

The interference of microwave-induced resistance oscillations and magneto-intersubband oscillations in double quantum wells exposed to a continuous microwave irradiation is under study. By comparing experimental and theoretical magnetoresistance traces at different temperatures, we confirm that the inelastic mechanism of photoresistance explains our observations up to T similar or equal to 4 K. For higher temperatures, our results suggest a deviation of the inelastic scattering time tau(in) from the predicted T(-2) dependence. (C) 2009 Elsevier B.V. All rights reserved.

QUANTUM HALL FERROMAGNET IN A DOUBLE WELL WITH VANISHING g-FACTOR.

We have studied the quantum Hall effect in Al(x)Ga(1-x)As-double well structure with vanishing g-factor. We determined the density-magnetic field n(s) - B diagrams for the longitudinal resistance R(xx). In spite of the fact that the n(s) - B diagram for conventional GaAs double wells shows a striking similarity with the theory, we observed the strong difference between these diagrams for double wells with vanishing g-factor. We argue that the electron-electron interaction is responsible for unusual behavior of the Landau levels in such a system.

MAGNETORESISTANCE OSCILLATIONS IN DOUBLE QUANTUM WELLS UNDER MICROWAVE IRRADIATION

We report in detail oscillatory magnetoresistance in double quantum wells under microwave irradiation. The experimental investigation contains measurements of frequency, power and temperature dependence. In theory, the observed interference oscillations are explained in terms of the influence of subband coupling on the frequency-dependent photoinduced part of the electron distribution function. Thus, the magnetoresistance shows the interference of magneto-intersubband and conventional microwave induced resistance oscillations.

Luminescence studies on nitride quaternary alloys double quantum wells

We present theoretical photoluminescence (PL) spectra of undoped and p-doped Al(x)In(1-xy)Ga(y)N/Al(X)In(1) (X) (Y)Ga(Y)N double quantum wells (DQWs). The calculations were performed within the k.p method by means of solving a full eight-band Kane Hamiltonian together with the Poisson equation in a plane wave representation, including exchange-correlation effects within the local density approximation. Strain effects due to the lattice mismatch are also taken into account. We show the calculated PL spectra, analyzing the blue and red-shifts in energy as one varies the spike and the well widths, as well as the acceptor doping concentration. We found a transition between a regime of isolated quantum wells and that of interacting DQWs. Since there are few studies of optical properties of quantum wells based on nitride quaternary alloys, the results reported here will provide guidelines for the interpretation of forthcoming experiments. (C) 2008 Elsevier B.V. All rights reserved.

Low-energy nuclear reactions with double-solenoid based radioactive nuclear beam

The University of Notre Dame, USA (Becchetti et al, Nucl. Instrum. Metho ds Res. A505, 377 (2003)) and later the University of Sao Paulo, Brazil (Lichtenthaler et al, Eur. Phys. J. A25, S-01, 733 (2005)) adopted a system based on superconducting solenoids to produce low-energy radioactive nuclear beams. In these systems the solenoids act as thick lenses to collect, select, and focus the secondary beam into a scattering chamb er. Many experiments with radioactive light particle beams (RNB) such as (6)He, (7)Be, (8)Li, (8)B have been performed at these two facilities. These low-energy RNB have been used to investigate low-energy reactions such as elastic scattering, transfer and breakup, providing useful information on the structure of light nuclei near the drip line and on astrophysics. Total reaction cross-sections, derived from elastic scattering analysis, have also been investigated for light system as a function of energy and the role of breakup of weakly bound or exotic nuclei is discussed.

Limitation of double folding potentials to simulate the polarization in reactions involving halo nuclei

In this work, we investigate the limitation of the use of strength coefficients on double folding potentials to study the presence of the threshold anomaly in the elastic scattering of halo nuclei at near barrier energies. For this purpose, elastic angular distributions and reaction cross sections for the He-6 on Bi-209 are studied. (c) 2008 Elsevier B.V. All rights reserved.

The double Caldeira-Leggett model: Derivation and solutions of the master equations, reservoir-induced interactions and decoherence

In this paper we analyze the double Caldeira-Leggett model: the path integral approach to two interacting dissipative harmonic oscillators. Assuming a general form of the interaction between the oscillators, we consider two different situations: (i) when each oscillator is coupled to its own reservoir, and (ii) when both oscillators are coupled to a common reservoir. After deriving and solving the master equation for each case, we analyze the decoherence process of particular entanglements in the positional space of both oscillators. To analyze the decoherence mechanism we have derived a general decay function, for the off-diagonal peaks of the density matrix, which applies both to common and separate reservoirs. We have also identified the expected interaction between the two dissipative oscillators induced by their common reservoir. Such a reservoir-induced interaction, which gives rise to interesting collective damping effects, such as the emergence of relaxation- and decoherence-free subspaces, is shown to be blurred by the high-temperature regime considered in this study. However, we find that different interactions between the dissipative oscillators, described by rotating or counter-rotating terms, result in different decay rates for the interference terms of the density matrix. (C) 2010 Elsevier B.V. All rights reserved.

Structural role of fluoride in the ion-conducting glass system B(2)O(3)-PbO-LiF studied by single- and double-resonance NMR

The local structure of an ion-conducting glass with nominal composition 50B(2)O(3)-10PbO-40LiF has been investigated by complementary (7)Li, (11)B, (19)F, and (207)Pb single- and double-resonance experiments. The results give insight into the structural role of the lithium fluoride additive in borate glasses: (1) LiF is seen to actively participate in the network transformation process contributing to the conversion of three- into four-coordinate boron units, as shown by (11)B single-resonance as well as by (11)B{(19)F} and (19)F{(11)B} double-resonance experiments. (2) (19)F signal quantification experiments suggest substantial fluoride loss, presumably caused by formation of volatile BF(3). A part of the fluoride remains in the dopant role, possibly in the form of small LiF-like cluster domains, which serve as a mobile ion supply. (3) The extent of lithium-fluorine and lead-fluorine interactions has been characterized by (7)Li{(19)F} and (207)Pb{(19)F} REDOR and SEDOR experiments. On the basis of these results, a quantitative structural description of this system has been developed.

U-Pb-Hf-trace element systematics and geochronology of zircon from a granulite-facies metamorphosed mafic-ultramafic layered complex in Central Brazil

Zircon recrystallization is a common process during high-grade metamorphism and promotes partial or complete resetting of the original isotopic and chemical characteristics of the mineral and thus complicates U-Pb geochronological interpretation. In Central Brazil, this complexity may be illustrated by three composite mafic-ultramafic intrusions metamorphosed under amphibolite-to-granulite conditions. Their ages of emplacement and metamorphic ages have been a matter of controversy for the last thirty years. The Serra da Malacacheta and Barro Alto complexes make up the southernmost of these layered bodies and four samples from distinct rock types were investigated in order to verify the consequences of metamorphic alteration of zircon for U-Pb dating. Cathodoluminescent imaging reveals internal features which are typical of concomitant dissolution-reprecipitation processes, such as convolute zoning and inward-moving recrystallization fronts, even in samples in which partially preserved igneous textures are observed. Due to this extensive alteration, LA-ICPMS U-Pb isotopic analysis yielded inconclusive data. However, in situ Hf isotopic and trace-element analyses help to clarify the real meaning of the geochronological data. Low Lu/Hf (<0.004) and homogeneous (176)Hf/(177)Hf(t) values imply that the zircon populations within individual samples have crystallized in a single episode, despite the observed variations in age values. Trace element signatures of zircon grains from garnet-bearing samples reveal that they were unreactive to the development of the peak metamorphism mineral assemblage and, thus, the main chemical feature in such grains is attributed to a coupled dissolution-reprecipitation process. However, in the Cafelandia amphibolite an additional alteration process is identified, probably related to the influx of late-stage fluids. Combined isotopic and geochemical investigation on zircon grains allowed the distinction of two magmatic events. The first corresponds to the crystallization of the Serra da Malacacheta Complex and characterizes a juvenile magmatism at similar to 1.3 Ga. The younger episode, recognized in the Barro Alto Complex, is dated at ca. 800 Ma and is represented by mafic and ultramafic rocks showing intense contamination with continental crust, implying that the emplacement took place, most likely, in a continental back-arc setting. Altered zircon domains as well as titanite grains date the metamorphic event at ca. 760-750 Ma. (C) 2011 Elsevier B.V. All rights reserved.

Proximal methods for nonlinear programming: double regularization and inexact subproblems

This paper describes the first phase of a project attempting to construct an efficient general-purpose nonlinear optimizer using an augmented Lagrangian outer loop with a relative error criterion, and an inner loop employing a state-of-the art conjugate gradient solver. The outer loop can also employ double regularized proximal kernels, a fairly recent theoretical development that leads to fully smooth subproblems. We first enhance the existing theory to show that our approach is globally convergent in both the primal and dual spaces when applied to convex problems. We then present an extensive computational evaluation using the CUTE test set, showing that some aspects of our approach are promising, but some are not. These conclusions in turn lead to additional computational experiments suggesting where to next focus our theoretical and computational efforts.

Intralamellar structural modifications related to the proton exchanging in K(4)Nb(6)O(17) layered phase

Basic structural aspects about the layered hexaniobate of K(4)Nb(6)O(17) composition and its proton-exchanged form were investigated mainly by spectroscopic techniques. Raman spectra of hydrous K(4)Nb(6)O(17) and H(2)K(2)Nb(6)O(17)center dot H(2)O show significant modifications in the 950-800 cm(-1) region (Nb-O stretching mode of highly distorted NbO(6) octahedra). The band at 900 cm(-1) shifts to 940 cm(-1) after the replacement of K(+) ion by proton. Raman spectra of the original materials and the related deuterated samples are similar suggesting that no isotopic effect occurs. Major modifications were observed when H(2)K(2)Nb(6)O(17) was dehydrated: the relative intensity of the band at 940 cm(-1) decreases and new bands seems to be present at about 860-890 cm(-1). The H(+) ions should be shielded by the hydration sphere what preclude the interaction with the layers. Removing the water molecules, H(+) ions can establish a strong interaction with oxygen atoms, decreasing the bond order of Nb-O linkage. X-ray absorption near edge structure studies performed at Nb K-edge indicate that the niobium coordination number and oxidation state remain identical after the replacement of potassium by proton. From the refinement of the fine structure, it appears that the Nb-Nb coordination shell is divided into two main contributions of about 0.33 and 0.39 nm, and interestingly the population, i.e., the number of backscattering atoms is inversed between the two hexaniobate materials. 2009 Elsevier Ltd. All rights reserved.

Double-strand DNA cleavage induced by oxindole-Schiff base copper(II) complexes with potential antitumor activity

Some oxindole-Schiff base copper(II) complexes have already shown potential antitumor activity towards different cells, inducing apoptosis in a process modulated by the ligand, and having nuclei and mitochondria as main targets. Here, three novel copper(II) complexes with analogous ligands were isolated and characterized by spectroscopic techniques, having their reactivity compared to the so far most active complex in this class. Cytotoxicity experiments carried out toward human neuroblastoma SH-SY5Y cells confirmed its proapoptosis property. DNA cleavage studies were then performed in the presence of these complexes, in order to verify the influence of ligand structural features in its nuclease activity. All of them were able to cause double-strand DNA scissions, giving rise to nicked circular Form II and linear Form III species, in the presence of hydrogen peroxide. Additionally, DNA Form II was also detected in the absence of peroxide when the most active complex, [Cu(isaepy)(2)](2+) 1, was used. In an effort to better elucidate their interactions with DNA, solutions of the different complexes titrated with DNA had their absorption spectra monitored. An absorbance hyperchromism observed at 260 nm pointed to the intercalation of these complexes into the DNA structure. Further, investigations of 2-deoxy-D-ribose (DR) oxidation catalyzed by each of those complexes, using 2-thiobarbituric acid reactive species (TBARS) method, and detection of reactive oxygen species (ROS) formation by spin-trapping EPR, suggested that their mechanism of action in performing efficiently DNA cleavage occurs preferentially, but not only by oxidative pathways. (C) 2007 Elsevier Inc. All rights reserved.

Polyaniline/layered zirconium phosphate nanocomposites: Secondary-like doped polyaniline obtained by the layer-by-layer technique

In the present work, nanocomposites of polyaniline (PANI) and layered alpha-Zr(HPO4)(2).H2O (alpha-ZrP) were prepared using two different approaches: (i) the in situ aniline polymerization in the presence of the layered inorganic material and (ii) the layer-by-layer (LBL) assembly using an aqueous solution of the polycation emeraldine salt (ES-PANI) and a dispersion of exfoliated negative slabs of alpha-ZrP. These materials were characterized spectroscopically using mainly resonance Raman scattering at four exciting radiations and electronic absorption in the UV-VIS-NIR region. Structural and textural characterizations were carried out using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The polymer obtained by the in situ aniline polymerization is located primarily in the external surface of the inorganic material although aniline monomers were intercalated between alpha-ZrP interlayer regions before oxidative polymerization. Through resonance Raman spectroscopy, it was observed that the formed polymer has semiquinone units (ES-PANI) and also azo bonds (-N = N-), showing that this method results in a polymer with a different structure from the usual ""head-to-tail"" ES-PANI. The LBL assembly of pre-formed ES-PANI and exfoliated alpha-ZrP particles produces homogeneous films with reproducible deposition from layer to layer, up to 20 bilayers. Resonance Raman (lambda(0) = 632.8 nm) spectrum of PANI/ZrP LBL film shows an enhancement in the intensity of the polaronic band at 1333 cm(-1) (nu C-N center dot+) and the decrease of the band intensity at 1485 cm(-1) compared to bulk ES-PANI. Its UV-VIS-NIR spectrum presents an absorption tail in the NIR region assigned to delocalized free charge carrier. These spectroscopic features are characteristic of highly conductive secondary doped PANI suggesting that polymeric chains in PANI/ZrP LBL film have a more extended conformation than in bulk ES-PANI.