First-principles calculations of 2x2 reconstructions of GaN(0001) surfaces involving N, Al, Ga, In, and as atoms

The ab initio studies presented here employed a pseudopotential-plane-wave method in order to obtain the minimum-energy configurations of various 22 GaN0001 surfaces involving N, Al, Ga, In, and As atoms. Comparison of the various possible reconstructions allows predictions to be made regarding the most energetically favorable configurations. Such comparisons depend on the value of the effective chemical potential of each atomic species, which can be related directly to experimental growth conditions. The most stable structure as a function of chemical potentials is determined. Based on these results we have characterized the effect of N in the adlayer surface and the stability dependence with number of substitutions as a function of the model employed and the possible surfactant character of some of the added atoms. Surface phase diagrams as a function of the chemical potential have been calculated to show the phase transition between the different reconstructions

Enhanced THz transmission apertures through sub-wavelength annular apertures

We report on the development of a surface micromachined process for the fabrication of coaxial apertures surrounded by periodic grooves. The process uses a combination of copper electroforming and the negative epoxy based resist, SU8, as a thin flexible substrate. The device dimensions are suitable for the implementation of filters at THz frequencies, and measurements show a pass band centred around 1.5 THz. These devices could form the basis of the next generation of THz biosensors

Crystal structure and magnetic modulation in β−Ce2O2FeSe2

We report a combination of X-ray and neutron diffraction studies, Mossbauer spectroscopy and muon spin relaxation (muSR) measurements to probe the structure and magnetic properties of the semiconducting beta-Ce2O2FeSe2 oxychalcogenide. We report a new structural description in space group Pna21 which is consistent with diffraction data and second harmonic generation measurements and reveal an order-disorder transition on one Fe site at TOD ~ 330 K. Susceptibility measurements, Mossbauer and muSR reveal antiferromagnetic ordering below TN = 86 K and more complex short range order above this temperature. 12 K neutron diffraction data reveal a modulated magnetic structure with q = 0.444 bN*

Convection in colloidal suspensions with particle-concentration-dependent viscosity

The onset of thermal convection in a horizontal layer of a colloidal suspension is investigated in terms of a continuum model for binary-fluid mixtures where the viscosity depends on the local concentration of colloidal particles. With an increasing difference between the viscosity at the warmer and the colder boundary the threshold of convection is reduced in the range of positive values of the separation ratio psi with the onset of stationary convection as well as in the range of negative values of psi with an oscillatory Hopf bifurcation. Additionally the convection rolls are shifted downwards with respect to the center of the horizontal layer for stationary convection (psi>0) and upwards for the Hopf bifurcation (psi<0).Comment: 8 pages, 6 figures, submitted to European Physical Journal

Atrial fibrillation fingerprinting; spotting bio-electrical markers to early recognize atrial fibrillation by the use of a bottom-up approach (AFFIP): Rationale and design

Background: The exact pathophysiology of atrial fibrillation (AF) remains incompletely understood and treatment of AF is associated with high recurrence rates. Persistence of AF is rooted in the presence of electropathology, defined as complex electrical conduction disorders caused by structural damage of atrial tissue. The atrial fibrillation fingerprinting (AFFIP) study aims to characterize electropathology, enabling development of a novel diagnostic instrument to predict AF onset and early progression. Hypotheses: History of AF, development of post-operative AF, age, gender, underlying heart disease, and other clinical characteristics impact the degree of electropathology. Methods: This study is a prospective observational study with a planned duration of 48 months. Three study groups are defined: (1) patients with (longstanding) persistent AF, (2) patients with paroxysmal AF, and (3) patients without a history of AF, all undergoing open-chest cardiac surgery. Intra-operative high-resolution epicardial mapping is performed to identify the patient-specific electrical profile, whereas the patient-specific biological profile is assessed by evaluating proteostasis markers in blood samples and atrial appendage tissue samples. Post-operative continuous rhythm monitoring is perfo

The Impact of Filter Settings on Morphology of Unipolar Fibrillation Potentials

Using unipolar atrial electrogram morphology as guidance for ablative therapy is regaining interest. Although standardly used in clinical practice during ablative therapy, the impact of filter settings on morphology of unipolar AF potentials is unknown. Thirty different filters were applied to 2,557,045 high-resolution epicardial AF potentials recorded from ten patients. Deflections with slope ≤ − 0.05 mV/ms and amplitude ≥ 0.3 mV were marked. High-pass filtering decreased the number of detected potentials, deflection amplitude, and percentage of fractionated potentials (≥ 2 deflections) as well as fractionation delay time (FDT) and increased percentage of single potentials. Low-pass filtering decreased the number of potentials, percentage of fractionated potentials, whereas deflection amplitude, percentage of single potentials, and FDT increased. Notch filtering (50 Hz) decreased the number of potentials and deflection amplitude, whereas the percentage of complex fractionated potentials (≥ 3 deflections) increased. Filtering significantly impacted morphology of unipolar fibrillation potentials, becoming a potential source of error in identification of ablative targets.

Noise sensitivity of sub- and supercritically bifurcating patterns with group velocities close to the convective-absolute instability

The influence of small additive noise on structure formation near a forwards and near an inverted bifurcation as described by a cubic and quintic Ginzburg Landau amplitude equation, respectively, is studied numerically for group velocities in the vicinity of the convective-absolute instability where the deterministic front dynamics would empty the system.Comment: 16 pages, 7 Postscript figure

Modulation of Localized States in Electroconvection

We report on the effects of temporal modulation of the driving force on a particular class of localized states, known as worms, that have been observed in electroconvection in nematic liquid crystals. The worms consist of the superposition of traveling waves and have been observed to have unique, small widths, but to vary in length. The transition from the pure conduction state to worms occurs via a backward bifurcation. A possible explanation of the formation of the worms has been given in terms of coupled amplitude equations. Because the worms consist of the superposition of traveling waves, temporal modulation of the control parameter is a useful probe of the dynamics of the system. We observe that temporal modulation increases the average length of the worms and stabilizes worms below the transition point in the absence of modulation.Comment: 4 pages, 4 figure

Grain boundary pinning and glassy dynamics in stripe phases

We study numerically and analytically the coarsening of stripe phases in two spatial dimensions, and show that transient configurations do not achieve long ranged orientational order but rather evolve into glassy configurations with very slow dynamics. In the absence of thermal fluctuations, defects such as grain boundaries become pinned in an effective periodic potential that is induced by the underlying periodicity of the stripe pattern itself. Pinning arises without quenched disorder from the non-adiabatic coupling between the slowly varying envelope of the order parameter around a defect, and its fast variation over the stripe wavelength. The characteristic size of ordered domains asymptotes to a finite value $R_g \sim \lambda_0\ \epsilon^{-1/2}\exp(|a|/\sqrt{\epsilon})$, where$\epsilon\ll 1$is the dimensionless distance away from threshold,$\lambda_0$the stripe wavelength, and$a$ a constant of order unity. Random fluctuations allow defect motion to resume until a new characteristic scale is reached, function of the intensity of the fluctuations. We finally discuss the relationship between defect pinning and the coarsening laws obtained in the intermediate time regime.Comment: 17 pages, 8 figures. Corrected version with one new figur