Molecular dynamics simulations on devitrification: the PbF2 case

In this work molecular dynamics simulations were performed to reproduce the kinetic and thermodynamic transformations occurring during melt crystallization, vitrification, and glass crystallization (devitrification) of PbF2. Two potential parameters were analyzed in order to access the possibility of modeling these properties. These interionic potentials are models developed to describe specific characteristic of PbF2, and thermodynamic properties were well reproduced by one of them, while the other proved well adapted to simulate the crystalline structure of this fluoride. By a modeled nonisothermal heat treatment of the glass, it was shown that the devitrification of a cubic structure in which the Pb-Pb distances are in good agreement with theory and experiment. (C) 2002 American Institute of Physics.

Bethe ansatz solutions for Temperley-Lieb quantum spin chains

We solve the spectrum of quantum spin chains based on representations of the Temperley-Lieb algebra associated with the quantum groups U-q(X-n) for X-n = A(1), B-n, C-n and D-n. The tool is a modified version of the coordinate Bethe ansatz through a suitable choice of the Bethe states which give to all models the same status relative to their diagonalization. All these models have equivalent spectra up to degeneracies and the spectra of the lower-dimensional representations are contained in the higher-dimensional ones. Periodic boundary conditions, free boundary conditions and closed nonlocal boundary conditions are considered. Periodic boundary conditions, unlike free boundary conditions, bleak quantum group invariance. For closed nonlocal cases the models are quantum group invariant as well as periodic in a certain sense.

Effect of temperature on the optical properties of GaAsSbN/GaAs single quantum wells grown by molecular-beam epitaxy

GaAsSbN/GaAs strained-layer single quantum wells grown on a GaAs substrate by molecular-beam epitaxy with different N concentrations were studied using the photoluminescence (PL) technique in the temperature range from 9 to 296 K. A strong redshift in optical transition energies induced by a small increase in N concentration has been observed in the PL spectra. This effect can be explained by the interaction between a narrow resonant band formed by the N-localized states and the conduction band of the host semiconductor. Excitonic transitions in the quantum wells show a successive red/blue/redshift with increasing temperature in the 2-100 K range. The activation energies of nonradiative channels responsible for a strong thermal quenching are deduced from an Arrhenius plot of the integrated PL intensity. (C) 2003 American Institute of Physics.

Comparison of SRIM, MCNPX and GEANT simulations with experimental data for thick Al absorbers

Proton computerized tomography deals with relatively thick targets like the human head or trunk. In this case precise analytical calculation of the proton final energy is a rather complicated task, thus the Monte Carlo simulation stands out as a solution. We used the GEANT4.8.2 code to calculate the proton final energy spectra after passing a thick Al absorber and compared it with the same conditions of the experimental data. The ICRU49, Ziegler85 and Ziegler2000 models from the low energy extension pack were used. The results were also compared with the SRIM2008 and MCNPX2.4 simulations, and with solutions of the Boltzmann transport equation in the Fokker-Planck approximation. (C) 2009 Elsevier Ltd. All rights reserved.

Pre-resonant Raman effect of CrO2- 4 in a metasilicate glass

Pre-resonant Raman effect of chromate ion, CrO2- 4, was observed in a metasilicate glass with molar composition 2Na2O · 1CaO · 3SiO2 containing 1.0 wt% of Cr2O3. Raman spectra were measured by the conventional 90° scattering geometry and by the microprobe Raman spectroscopic techniques. The presence of chromate ions in the glass is favoured by the glass composition and oxidizing conditions during the glass melting, and they are responsible for optical absorption bands at 370 and 250 nm. Raman spectrum of the undoped glass presents bands at 625, 860 and 980 cm-1, and the presence of chromate ions gives rise to additional bands at 365, 850 and a shoulder at 890 cm-1. An enhancement of the 850 cm-1 Raman band is observed with decreasing laser exciting wavelength. The exciting frequency dependence of the intensity of this band is discussed in terms of theoretical models given in the literature.

Carriers escape mechanisms in shallow InGaAs/GaAs quantum wells grown on vicinal (001) GaAs substrates

We used photoluminescence spectroscopy in order to investigate the carriers escape mechanisms in In0.15Ga0.85As/GaAs quantum wells grown on top of nominal (001) and 2°-, 4°-and 6°-off (001) towards (111)A GaAs substrates. We described the escape processes using two models that fit the Arrhenius plot of the integrated PL intensity as a function of the inverse of the sample temperature. In the first model, we considered equal escape probability for electrons and holes. In the second one, we assumed that a single type of carrier can escape from the well. At high temperature, the first model fits the experimental data well, whereas, between 50 K and 100 K, the second model has to be taken into account to describe the data. We observed that the escape activation energy depends on the misorientation angle. An unusual behavior was noted when the full width at half maximum of the photoluminescence main emission was plotted as a function of the sample temperature. We showed that the escape process of the less-confined carriers drives this behavior. © 1999 Academic Press.

Confinement, solitons and the equivalence between the sine-Gordon and massive Thirring models

We consider a two-dimensional integrable and conformally invariant field theory possessing two Dirac spinors and three scalar fields. The interaction couples bilinear terms in the spinors to exponentials of the scalars. Its integrability properties are based on the sl(2) affine Kac-Moody algebra, and it is a simple example of the so-called conformal affine Toda theories coupled to matter fields. We show, using bosonization techniques, that the classical equivalence between a U(1) Noether current and the topological current holds true at the quantum level, and then leads to a bag model like mechanism for the confinement of the spinor fields inside the solitons. By bosonizing the spinors we show that the theory decouples into a sine-Gordon model and free scalars. We construct the two-soliton solutions and show that their interactions lead to the same time delays as those for the sine-Gordon solitons. The model provides a good laboratory to test duality ideas in the context of the equivalence between the sine-Gordon and Thirring theories. © 2000 Elsevier Science B.V. All rights reserved.

Coherent atomic oscillations and resonances between coupled Bose-Einstein condensates with time-dependent trapping potential

The atomic tunneling between two tunnel-coupled Bose-Einstein condensates (BECs) in a double-well time-dependent trap was studied. For the slowly varying trap, synchronization of oscillations of the trap with oscillations of the relative population was predicted. Using the Melnikov approach, the appearance of the chaotic oscillations in the tunneling phenomena between the condensates was confirmed.

Testing TeV scale quantum gravity using dijet production at the Tevatron

Dijet production at the Tevatron including effects of virtual exchanges of spin-2 Kaluza-Klein modes in theories with large extra dimensions is considered. The experimental dijet mass and angular distribution are exploited to obtain stringent limits (> 1.2TeV) on the effective string scale M s.

Stability of trapped Bose-Einstein condensates

In three-dimensional trapped Bose-Einstein condensate (BEC), described by the time-dependent Gross-Pitaevskii-Ginzburg equation, we study the effect of initial conditions on stability using a Gaussian variational approach and exact numerical simulations. We also discuss the validity of the criterion for stability suggested by Vakhitov and Kolokolov. The maximum initial chirp (initial focusing defocusing of cloud) that can lead a stable condensate to collapse even before the number of atoms reaches its critical limit is obtained for several specific cases. When we consider two- and three-body nonlinear terms, with negative cubic and positive quintic terms, we have the conditions for the existence of two phases in the condensate. In this case, the magnitude of the oscillations between the two phases are studied considering sufficient large initial chirps. The occurrence of collapse in a BEC with repulsive two-body interaction is also shown to be possible.

N-methyl-D-aspartate channel and consciousness: From signal coincidence detection to quantum computing

Research on Blindsight, Neglect/Extinction and Phantom limb syndromes, as well as electrical measurements of mammalian brain activity, have suggested the dependence of vivid perception on both incoming sensory information at primary sensory cortex and reentrant information from associative cortex. Coherence between incoming and reentrant signals seems to be a necessary condition for (conscious) perception. General reticular activating system and local electrical synchronization are some of the tools used by the brain to establish coarse coherence at the sensory cortex, upon which biochemical processes are coordinated. Besides electrical synchrony and chemical modulation at the synapse, a central mechanism supporting such a coherence is the N-methyl-D-aspartate channel, working as a 'coincidence detector' for an incoming signal causing the depolarization necessary to remove Mg 2+, and reentrant information releasing the glutamate that finally prompts Ca 2+ entry. We propose that a signal transduction pathway activated by Ca 2+ entry into cortical neurons is in charge of triggering a quantum computational process that accelerates inter-neuronal communication, thus solving systemic conflict and supporting the unity of consciousness. © 2001 Elsevier Science Ltd.

Soliton propagation in a medium with Kerr nonlinearity and resonant impurities: A variational approach

The soliton propagation in a medium with Kerr nonlinearity and resonant impurities was studied by a variational approach. The existence of a solitary wave was shown within the framework of a combined nonintegrable system composed of one nonlinear Schrödinger and a pair of Bloch equations. The analytical solution which was obtained, was tested through numerical simulations confirming its solitary wave nature.

Array of Bose-Einstein condensates under time-periodic Feshbach-resonance management

The investigation of the dynamics of a discrete soliton in an array of Bose-Einstein condensates under the action of a periodically time-modulated atomic scattering length [Feshbach-resonance management (FRM)] was discussed. The slow and rapid modulations, in comparison with the tunneling frequency were considered. An averaged equation, which was a generalized discrete nonlinear Schrödinger equation, including higher-order effective nonlinearities and intersite nonlinear interactions was derived in the case of the rapid modulation. It was demonstrated that the modulations of sufficient strength results in splitting of the soliton by direct simulations.

Quantum topology change and large-N gauge theories

We study a model for dynamical localization of topology using ideas from non-commutative geometry and topology in quantum mechanics. We consider a collection X of N one-dimensional manifolds and the corresponding set of boundary conditions (self-adjoint extensions) of the Dirac operator D. The set of boundary conditions encodes the topology and is parameterized by unitary matrices g. A particular geometry is described by a spectral triple x(g) = (A X, script H sign X, D(g)). We define a partition function for the sum over all g. In this model topology fluctuates but the dimension is kept fixed. We use the spectral principle to obtain an action for the set of boundary conditions. Together with invariance principles the procedure fixes the partition function for fluctuating topologies. The model has one free-parameter β and it is equivalent to a one plaquette gauge theory. We argue that topology becomes localized at β = ∞ for any value of N. Moreover, the system undergoes a third-order phase transition at β = 1 for large-N. We give a topological interpretation of the phase transition by looking how it affects the topology. © SISSA/ISAS 2004.

Classical versus quantum equivalence between sine-Gordon, Liouville and other solitons

In this work we present a mapping between the classical solutions of the sine-Gordon, Liouville, λφ4 and other kinks in 1+1 dimensions. This is done by using an invariant quantity which relates the models. It is easily shown that this procedure is equivalent to that used to get the so called deformed solitons, as proposed recently by Bazeia et al. [Phys. Rev. D. 66 (2002) 101701(R)]. The classical equivalence is explored in order to relate the solutions of the corresponding models and, as a consequence, try to get new information about them. We discuss also the difficulties and consequences which appear when one tries to extend the deformation in order to take into account the quantum version of the models.