Magnetic field induced charge redistribution in artificially disordered quantum Hall superlattices

The photoluminescence from individual quantum wells of artificially disordered weakly coupled multi-layers embedded in wide AlGaAs parabolic wells was investigated in a strong magnetic field. We show that the response of the individual wells is very different from the average response of the multi-layers studied by transport measurements and that photoluminescence represents a local probe of the quantum Hall state formed in three-dimensional electron system. The observed magnetic field induced variations of the in-layer electron density demonstrate the formation of a new phase in the quasi-three-dimensional electron system. The sudden change in the local electron density found at the Landau filling factor nu = 1 by both the magneto-transport and the magneto-photoluminescence measurements was assigned to the quantum phase transition. Copyright (C) EPLA, 2012

Magnetic excitations in the spin-1 anisotropic antiferromagnet NiCl2-4SC(NH2)(2)

The spin-1 anisotropic antiferromagnet NiCl2-4SC(NH2)(2) exhibits a field-induced quantum phase transition that is formally analogous to Bose-Einstein condensation. Here we present results of systematic high-field electron spin resonance (ESR) experimental and theoretical studies of this compound with a special emphasis on single-ion two-magnon bound states. In order to clarify some remaining discrepancies between theory and experiment, the frequency-field dependence of magnetic excitations in this material is reanalyzed. In particular, a more comprehensive interpretation of the experimental signature of single-ion two-magnon bound states is shown to be fully consistent with theoretical results. We also clarify the structure of the ESR spectrum in the so-called intermediate phase.

Influence of Ba-substitution on the structural and ferroelectric properties of Pb1-xBaxZr0.40Ti0.60O3 ceramic materials

This work presents a comprehensive study about the influence of Ba-substitution on the structural and ferroelectric properties of Pb1-xBaxZr0.40Ti0.60O3 (PBZT) ceramic system. Pb1-xBaxZr0.40Ti0.60O3 ceramic samples were then prepared by solid state reaction method and characterized as a function of composition and temperature by X-ray diffraction (XRD) and impedance spectroscopy techniques. The dielectric measurements show that the substitution of Pb2+ for Ba2+ ions leads to a diffuse behavior of the dielectric permittivity curves for all samples and that only the x = 0.50 sample presents a typical relaxor behavior. In good agreement with dielectric measurements, the structural phase transition study showed a phase transition from a tetragonal structure with P4mm space group to a cubic structure with Pm-3m space group for all samples, except for the x = 0.50 sample were a cubic structure was observed in the complete temperature interval measured.

Influence of Molecular Dynamics on the Dielectric Properties of Poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) -Based Devices

This paper uses Nuclear Magnetic Resonance (NMR) and Differential Scanning Calorimetry (DSC) techniques to study the molecular relaxations and phase transitions in poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT), which has been extensively studied as the active thin film in organic devices. Besides the identification of the glass transition, beta relaxation and crystal-to-crystal phase transition, we correlate such phenomena with dielectric and transport mechanisms in diodes with F8BT as the active layer. The beta relaxation has been assigned to a transition at about 210 K measured by H-1 and C-13 solid state NMR, and can be attributed to local motions in the side chains. The glass transition has been detected by DSC and H-1 NMR. Dielectric spectroscopy (DS) carried out at low frequencies on diodes made from F8BT show two peaks which are coincident with the above transitions. This allowed us to correlate the electrical changes in the film with the onset of specific molecular motions. In addition, DS indicates a third peak related with a crystal-to-crystal phase transition. Finally, these transitions were correlated with changes in the carrier mobility recorded in thin films and published recently.

Mean-Field critical behavior and ergodicity break in a nonequilibrium one-dimensional RSOS growth model

We investigate the nonequilibrium roughening transition of a one-dimensional restricted solid-on-solid model by directly sampling the stationary probability density of a suitable order parameter as the surface adsorption rate varies. The shapes of the probability density histograms suggest a typical Ginzburg-Landau scenario for the phase transition of the model, and estimates of the "magnetic" exponent seem to confirm its mean-field critical behavior. We also found that the flipping times between the metastable phases of the model scale exponentially with the system size, signaling the breaking of ergodicity in the thermodynamic limit. Incidentally, we discovered that a closely related model not considered before also displays a phase transition with the same critical behavior as the original model. Our results support the usefulness of off-critical histogram techniques in the investigation of nonequilibrium phase transitions. We also briefly discuss in the appendix a good and simple pseudo-random number generator used in our simulations.

Statistical physics approach to dendritic computation: The excitable-wave mean-field approximation

We analytically study the input-output properties of a neuron whose active dendritic tree, modeled as a Cayley tree of excitable elements, is subjected to Poisson stimulus. Both single-site and two-site mean-field approximations incorrectly predict a nonequilibrium phase transition which is not allowed in the model. We propose an excitable-wave mean-field approximation which shows good agreement with previously published simulation results [Gollo et al., PLoS Comput. Biol. 5, e1000402 (2009)] and accounts for finite-size effects. We also discuss the relevance of our results to experiments in neuroscience, emphasizing the role of active dendrites in the enhancement of dynamic range and in gain control modulation.

Electrical characterization of poly(amide-imide) for application in organic field effect devices

The admittance spectra and current-voltage (I-V) characteristics are reported of metal-insulator-metal (MIM) and metal-insulator-semiconductor (MIS) capacitors employing cross-linked poly(amide-imide) (c-PAI) as the insulator and poly(3-hexylthiophene) (P3HT) as the active semiconductor. The capacitance of the MIM devices are constant in the frequency range from 10 Hz to 100 kHz, with tan delta values as low as 7 x 10(-3) over most of the range. Except at the lowest voltages, the I-V characteristics are well-described by the Schottky equation for thermal emission of electrons from the electrodes into the insulator. The admittance spectra of the MIS devices displayed a classic Maxwell-Wagner frequency response from which the transverse bulk hole mobility was estimated to be similar to 2 x 10(-5) cm(2) V(-1)s(-1) or similar to 5 x 10(-8) cm(2) V(-1)s(-1) depending on whether or not the surface of the insulator had been treated with hexamethyldisilazane (HMDS) prior to deposition of the P3HT. From the maximum loss observed in admittance-voltage plots, the interface trap density was estimated to be similar to 5 x 10(10) cm(-2) eV(-1) or similar to 9 x 10(10) cm(-2) eV(-1) again depending whether or not the insulator was treated with HMDS. We conclude, therefore, that HMDS plays a useful role in promoting order in the P3HT film as well as reducing the density of interface trap states. Although interposing the P3HT layer between the insulator and the gold electrode degrades the insulating properties of the c-PAI, nevertheless, they remain sufficiently good for use in organic electronic devices. (c) 2012 Elsevier B.V. All rights reserved.

Size-induced diffuse behavior in Pb0.89La0.11Zr0.40Ti0.60O3 nanocrystalline ferroelectric ceramics

The Pb1-xLaxZryTi1-yO3 system is a perovskite ABO(3) structured material which presents ferroelectric properties and has been used as capacitors, actuators, transducers and electro-optic devices. In this paper, we describe the synthesis and the characterization of Pb0.89La0.11Zr0.40Ti0.60O3 (PLZT11) nanostructured material. The precursor polymeric method and the spark plasma sintering technique were respectively used to prepare ceramic samples. In order to compare the effect of grain size, microcrystalline PLZT11 ceramic samples were also prepared. PLZT11 samples were characterized by X-ray diffraction technique which results show a reduction on the degree of tetragonality as the average grain size decreases. Moreover, the grain size decrease to a nanometer range induces a diffuse behavior on the dielectric permittivity curves as a function of the temperature and a reduction on the dielectric permittivity magnitude. Furthermore, the large number of grain boundaries due to the nanometer size gives rise to a dielectric anomaly. (C) 2012 Elsevier Masson SAS. All rights reserved.

Pion, Kaon, and Proton Production in Central Pb-Pb Collisions at root s(NN)=2.76 TeV

In this Letter we report the first results on pi(+/-), K-+/-, p, and (p) over bar production at midrapidity (vertical bar y vertical bar < 0.5) in central Pb-Pb collisions at root s(NN) = 2.76 TeV, measured by the ALICE experiment at the LHC. The p(T) distributions and yields are compared to previous results at root s(NN) = 200 GeV and expectations from hydrodynamic and thermal models. The spectral shapes indicate a strong increase of the radial flow velocity with root s(NN), which in hydrodynamic models is expected as a consequence of the increasing particle density. While the K/pi ratio is in line with predictions from the thermal model, the p/pi ratio is found to be lower by a factor of about 1.5. This deviation from thermal model expectations is still to be understood.

Local order and electronic structure of Pb1-xLaxZr0.40Ti0.60O3 materials and its relation with ferroelectric properties

The knowledge of electronic and local structures is a fundamental step towards understanding the properties of ferroelectric ceramics. X-ray absorption near-edge structure (XANES) of Pb1-xLaxZr0.40Ti0.60O3 ferroelectric samples was measured in order to know how the local order and electronic structure are related to their ferroelectric property, which was tailored by the substitution of lead by lanthanum atoms. The analysis of XANES spectra collected at Ti K- and L-edges XANES showed that the substitution of Pb by La leads to a decrement of local distortion around Ti atoms on the TiO6 octahedron. The analysis of O K-edge XANES spectra showed that the hybridization between O 2p and Pb 6sp states is related to the displacement of Ti atoms in the TiO6 octahedra. Based on these results, it is possible to determine that the degree of ferroelectricity in these samples and the manifestation of relaxor behavior are directly related to the weakening of O 2p and Pb 6sp hybridization. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4720472]

The Extracellular Matrix of the Lateral Pharyngeal Wall in Obstructive Sleep Apnea

Study Objectives: To compare the components of the extracellular matrix in the lateral pharyngeal muscular wall in patients with and without obstructive sleep apnea (OSA). This may help to explain the origin of the increased collapsibility of the pharynx in patients with OSA. Design: Specimens from the superior pharyngeal constrictor muscle, obtained during pharyngeal surgeries, were evaluated using histochemical and immunohistochemical analyses to determine the fractional area of collagen types I and II, elastic fibers, versican, fibronectin, and matrix metalloproteinases 1 and 2 in the endomysium. Setting: Academic tertiary center. Patiens: A total of 51 nonobese adult patients, divided into 38 patients with OSA and 13 nonsnoring control subjects without OSA. Interventions: Postintervention study performed on tissues from patients after elective surgery. Measurements and Results: Pharyngeal muscles of patients with OSA had significantly more collagen type I than pharyngeal muscles in control subjects. Collagen type I was correlated positively and independently with age. The other tested components of the extracellular matrix did not differ significantly between groups. In a logistic regression, an additive effect of both the increase of collagen type I and the increase in age with the presence of OSA was observed (odds ratio (OR), 2.06; 95% confidence interval (CI), 1.17-3.63), when compared with the effect of increased age alone (OR, 1.11; 95% CI, 1.03-1.20). Conclusion: Collagen type I in the superior pharyngeal constrictor muscle was more prevalent in patients with OSA and also increased with age. It was hypothesized that this increase could delay contractile-relaxant responses in the superior pharyngeal constrictor muscle at the expiratory-inspiratory phase transition, thus increasing pharyngeal collapsibility.

Platinum-Based Nanoalloys PtnTM55-n (TM=Co, Rh, Au): A Density Functional Theory Investigation

Structural and electronic properties of the PtnTM55-n (TM = Co, Rh, Au) nanoalloys are investigated using density functional theory within the generalized gradient approximation and employing the all-electron projected augmented wave method. For TM = Co and Rh, the excess energy, which measures the relative energy stability of the nanoalloys, is negative for all Pt compositions. We found that the excess energy has similar values for a wide range of Pt compositions, i.e., n = 20-42 and n = 28-42 for Co and Rh, respectively, with the core shell icosahedron-like configuration (n = 42) being slightly more stable for both Co and Rh systems because of the larger release of the strain energy due to the smaller atomic size of the Co and Rh atoms. For TM = Au, the excess energy is positive for all compositions, except for n = 13, which is energetically favorable due to the formation of the core-shell structure (Pt in the core and Au atoms at the surface). Thus, our calculations confirm that the formation of core-shell structures plays an important role to increase the stability of nanoalloys. The center of gravity of the occupied d-states changes almost linearly as a function of the Pt composition, and hence, based on the d-band model, the magnitude of the adsorption energy of an adsorbate can be tuned by changing the Pt composition. The magnetic moments of PtnCo55-n decrease almost linearly as a function of the Pt composition; however, the same does not hold for PtRh and PtAu. We found an enhancement of the magnetic moments of PtRh by a few times by increasing Pt composition, which we explain by the compression effects induced by the large size of the Pt atoms compared with the Rh atoms.

Effect of external fields in Axelrod's model of social dynamics

The study of the effects of spatially uniform fields on the steady-state properties of Axelrod's model has yielded plenty of counterintuitive results. Here, we reexamine the impact of this type of field for a selection of parameters such that the field-free steady state of the model is heterogeneous or multicultural. Analyses of both one- and two-dimensional versions of Axelrod's model indicate that the steady state remains heterogeneous regardless of the value of the field strength. Turning on the field leads to a discontinuous decrease on the number of cultural domains, which we argue is due to the instability of zero-field heterogeneous absorbing configurations. We find, however, that spatially nonuniform fields that implement a consensus rule among the neighborhood of the agents enforce homogenization. Although the overall effects of the fields are essentially the same irrespective of the dimensionality of the model, we argue that the dimensionality has a significant impact on the stability of the field-free homogeneous steady state.

Non-mean-field effects in systems with long-range forces in competition

We investigate the canonical equilibrium of systems with long-range forces in competition. These forces create a modulation in the interaction potential and modulated phases appear at the system scale. The structure of these phases differentiate this system from monotonic potentials, where only the mean-field and disordered phases exist. With increasing temperature, the system switches from one ordered phase to another through a first-order phase transition. Both mean-field and modulated phases may be stable, even at zero temperature, and the long-range nature of the interaction will lead to metastability characterized by extremely long time scales.

Interaction of gramicidin with DPPC/DODAB bilayer fragments

The interaction between the antimicrobial peptide gramicidin (Gr) and dipalmitoylphosphatidylcholine (DPPC)/dioctadecyldimethylammonium bromide (DODAB) 1:1 large unilamellar vesicles (LVs) or bilayer fragments (BFs) was evaluated by means of several techniques. The major methods were: 1) Gr intrinsic fluorescence and circular dichroism (CD) spectroscopy; 2) dynamic light scattering for sizing and zeta-potential analysis; 3) determination of the bilayer phase transition from extrinsic fluorescence of bilayer probes; 4) pictures of the dispersions for evaluation of coloidal stability over a range of time and NaCl concentration. For Gr in LVs, the Gr dimeric channel conformation is suggested from: 1) CD and intrinsic fluorescence spectra similar to those in trifluoroethanol (TFE); 2) KCl or glucose permeation through the LVs/Gr bilayer. For Gr in BFs, the intertwined dimeric, non-channel Gr conformation is evidenced by CD and intrinsic fluorescence spectra similar to those in ethanol. Both LVs and BFs shield Gr tryptophans against quenching by acrylamide but the Stern-Volmer quenching constant was slightly higher for Gr in BFs confirming that the peptide is more exposed to the water phase in BFs than in LVs. The DPPC/DODAB/Gr supramolecular assemblies may predict the behavior of other antimicrobial peptides in assemblies with lipids. (C) 2012 Elsevier B.V. All rights reserved.