Role Of Inhomogeneities Research Articles

Wave reflection dynamics in nonlocal viscoelastic media: The role of inhomogeneity and rotation

Wave reflection dynamics in nonlocal viscoelastic media: The role of inhomogeneity and rotation

Barrier Height, Ideality Factor and Role of Inhomogeneities at the AlGaN/GaN Interface in GaN Nanowire Wrap-Gate Transistor.

It is essential to understand the barrier height, ideality factor, and role of inhomogeneities at the metal/semiconductor interfaces in nanowires for the development of next generation nanoscale devices. Here, we investigate the drain current (Ids)-gate voltage (Vgs) characteristics of GaN nanowire wrap-gate transistors (WGTs) for various gate potentials in the wide temperature range of 130-310 K. An anomalous reduction in the experimental barrier height and rise in the ideality factor with reducing the temperature have been perceived. It is noteworthy that the variations in barrier height and ideality factor are attributed to the spatial barrier inhomogeneities at the AlGaN/GaN interface in the GaN nanowire WGTs by assuming a double Gaussian distribution of barrier heights at 310-190 K (distribution 1) and 190-130 K (distribution 2). The standard deviation for distribution 2 is lower than that of distribution 1, which suggests that distribution 2 reflects more homogeneity at the AlGaN/GaN interface in the transistor's source/drain regions than distribution 1.

Simulating superconducting properties of overdoped cuprates: The role of inhomogeneity

Theoretical studies of disordered $d$-wave superconductors have focused, with a few exceptions, on optimally doped models with strong scatterers. Addressing recent controversies about the nature of the overdoped cuprates, however, requires studies of the weaker scattering associated with dopant atoms. Here we study simple models of such systems in the self-consistent Bogoliubov-de Gennes (BdG) framework, and compare to disorder-averaged results using the self-consistent-T-matrix-approximation (SCTMA). Despite surprisingly linear in energy behavior of the low-energy density of states even for quite disordered systems, the superfluid density in such cases retains a quadratic low-temperature variation of the penetration depth, unlike other BdG results reported recently. We trace the discrepancy to smaller effective system size employed in that work. Overall, the SCTMA performs remarkably well, with the exception of highly disordered systems with strongly suppressed superfluid density. We explore this interesting region where gap inhomogeneity dominates measured superconducting properties, and compare with overdoped cuprates.

Role of inhomogeneities in resonance light absorption and luminescence of molecular aggregates

Within the linear response theory, we have calculated the resonance absorption of the light and resonance luminescence for the one-dimensional molecular aggregate. We have shown that the randomness of the distribution of signs of the interaction between dipoles of the aggregate dramatically changes the parameters of the absorption and luminescence. In the homogeneous case, the resonance response of the exciton band of the molecular aggregate produces narrow lines at edges of the band. On the other hand, inhomogeneities yield contributions to optical characteristics of all excitons of the band.

Cliques in rank-1 random graphs: The role of inhomogeneity

We study the asymptotic behavior of the clique number in rank-1 inhomogeneous random graphs, where edge probabilities between vertices are roughly proportional to the product of their vertex weights. We show that the clique number is concentrated on at most two consecutive integers, for which we provide an expression. Interestingly, the order of the clique number is primarily determined by the overall edge density, with the inhomogeneity only affecting multiplicative constants or adding at most a $\log\log(n)$ multiplicative factor. For sparse enough graphs the clique number is always bounded and the effect of inhomogeneity completely vanishes.

The Effect of the Frequency Inhomogeneity on the Synchronous States in Systems of Coupled Oscillators

Despite the ubiquity of systems with heterogeneous connectivity, the role of inhomogeneity on network synchronization has not been fully explored. We studied the effect of multiple inhomogeneities in coupling strengths and natural frequencies on the phase locking behaviors of coupled oscillators. In particular, we found that different Gaussian distributions in natural frequencies lead to diverse synchronization states of the coupled oscillator systems. With the stability analysis and numerical simulations, we analysed the transitions between synchronization states and its breakup in detail.

Influence of interface inhomogeneities in thin-film Schottky diodes

The scalability of thin-film transistors has been well documented, but there have been very few investigations into the effects of device scalability in Schottky diodes. Indium-gallium-zinc-oxide (IGZO) Schottky diodes were fabricated with IGZO thicknesses of 50, 150, and 250 nm. Despite the same IGZO-Pt interface and Schottky barrier being formed in all devices, reducing the IGZO thickness caused a dramatic deterioration of the current-voltage characteristics, most notably increasing the reverse current by nearly five orders of magnitude. Furthermore, the forward characteristics display an increase in the ideality factor and a reduction in the barrier height. The origins of this phenomenon have been elucidated using device simulations. First, when the semiconductor layer is fully depleted, the electric field increases with the reducing thickness, leading to an increased diffusion current. However, the effects of diffusion only offer a small contribution to the huge variations in reverse current seen in the experiments. To fully explain this effect, the role of inhomogeneities in the Schottky barrier height has been considered. Contributions from lower barrier regions (LBRs) are found to dominate the reverse current. The conduction band minimum below these LBRs is strongly dependent upon thickness and bias, leading to reverse current variations as large as several orders of magnitude. Finally, it is demonstrated that the thickness dependence of the reverse current is exacerbated as the magnitude of the inhomogeneities is increased and alleviated in the limit where the LBRs are large enough not to be influenced by the adjacent higher barrier regions.

Magnetic order and the role of inhomogeneities in Ge1−x−yPbxCryTe bulk nanocomposite crystals

We present studies of the influence of the chemical composition changes on the type of magnetic order present in bulk nanocomposite Ge1−x−yPbxCryTe crystals with the average Pb-content close to x ≈ 0.20 and Cr-content, y, varying from y = 0.017 up to 0.043. Detailed magnetometric studies revealed the existence of two ordered magnetic states present in all our samples. All our crystals show a paramagnet to spin-glass-like state transition at temperatures from 135 K to 145 K, related to the presence of Cr5Te8 clusters in the host lattice. The second transition type depends on the Cr-content, y. In the case of samples with y equal to 0.038 and 0.043 the ferromagnetic phase is observed with TC slightly below 70 K, while in samples with lower chromium concentration the presence of spin-glass-like state slightly below 50 K is observed. The presence of clusters in our samples has significant impact on the carrier transport and magnetotransport in our samples. The presence of critical scattering on the magnetic impurities is observed in resistivity and magnetoresistance data. Nearly-linear magnetoresistance is also observed in our samples indicating a presence of strong scattering on the grain boundaries in our samples.

Stabilizing synchrony with heterogeneity

Correlation and synchrony between spike trains of neurons can arise from shared input they receive from other neurons, or from direct connections between the neurons. Physiological heterogeneity can destabilize both coupling-induced and shared input-induced synchronization [1]. In the classical models of synchronization, collective state of a system of coupled oscillators is determined by outcome of rivalry between synchronizing effect of connections and desynchronizing effect of inhomogeneity [2]. Yet there are examples of the systems in which inhomogeneity enhances synchrony [3,4]. In this study we have developed a general framework for the correlation of coupled neuronal oscillators with a given phase sensitivity and we have shown that in some cases, for identical neurons, synchronized state is an unstable attractor and arbitrarily weak noise can destroy synchrony. Inhomogeneity in such systems could stabilize synchrony by providing an asymmetric basin of attraction around the stable phase-locked state. This in turn results in a sharper PDF for the time difference between spikes of the two neurons in presence of noise (see Figure ​Figure1A).1A). The analytic results are obtained by solving Fokker-Planck equations for phase oscillators and it is also shown that in presence of stochastic inputs, the most probable phase difference between spike times of the two neurons does not coincide with the stable point of the deterministic equations. Numerical tests on LIF neurons confirm the positive role of inhomogeneity in stabilizing synchrony and increasing correlation of spike trains. The only difference is that for the model neurons with biologically realistic phase response curve (PRC), the time difference between the spikes of two neurons in the stable state increases with inhomogeneity, in the case of LIF neurons, they lock in almost zero phase lag for sufficiently small values of inhomogeneity (see Figure ​Figure1B1B). Figure 1 (A) PDF for the phase difference of two type1 phase oscillators in the presence of noise for three levels of frequency mismatch. Theoretical predictions (solid lines) are in accordance with numerical results shown as histograms. Dash lines show the phase-locked ...

Condensed matter analogues of cosmology

Condensed matter analogues of cosmology

Electrical properties and the role of inhomogeneities at the polyvinyl alcohol/n‐inp schottky barrier interface

ABSTRACTIn this work, we have investigated the electrical properties of Au/n‐InP contacts with a thin layer of polyvinyl alcohol (PVA) as an interlayer. The current–voltage (I–V) and capacitance–voltage (C–V) measurements are carried out in the temperature range of 175–425 K. The Au/PVA/n‐InP Schottky structure show nonideal behaviors and indicates the presence of a nonuniform distribution of interface states. The temperature dependent interface states densities (NSS), ideality factor and barrier height are obtained. An abnormal decrease in zero‐bias barrier height (BH) and increase in the ideality factor ( ) with decreasing temperature have been explained on the basis of the thermionic emission theory with Gaussian distribution (GD) of the BHs due to the BH inhomogeneities. The experimental I–V characteristics of Au/PVA/n‐InP Schottky diode has revealed the existence of a double GD with mean BH values of ( ) of 1.246 and 0.899 eV and standard deviation ( ) of 0.176 and 0.137 V, respectively. Consequently, the modified conventional activation energy versus plot gives and Richardson constants ( ) and the values are 1.17 and 0.71 eV and 9.9 and 6.9 A/cm2 K2, respectively, without using the temperature coefficient of the BH. The effective Richardson constant value of 9.9 A/cm2 K2 is very close to the theoretical value of 9.4 A/cm2 K2 for n‐InP. The discrepancy between Schottky barrier heights estimated from I–V and C–V measurements is also discussed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39773.

Prediction of Fundamental Properties of Ionic Liquid Electrospray Thrusters using Molecular Dynamics

Molecular dynamics (MD) simulations are performed to model an electrospray thruster for the ionic liquid (IL) EMIM-BF4 using two coarse-grained (CG) potentials. Different equilibrium properties were obtained for the two potentials and then both were used to study the electrical extrusion of the IL for different electric field strengths and mass flow rates. The MD simulations provide the first insight into the atomistic modeling of a capillary-tip-extractor system, the basic elements of an electrospray thruster. One of the CG potentials was found to predict the formation of the Taylor cone, the cone-jet, and other extrusion modes for similar electric fields and mass flow rates observed in experiments of a IL fed capillary-tip-extractor system. Current distributions and anion and cation behavior were characterized and estimates of thrust and specific impulse are presented and compare reasonably well with measurements. Moreover, the role of inhomogeneities in the electric field as well as that of the IL space-charge most likely will improve agreement between modeling and experiment.

Determination of critical exponents of inhomogeneous Gd films

The role of inhomogeneity on the critical behavior is studied for non-epitaxial Gd films. For this purpose, the film inhomogeneity was varied experimentally by annealing otherwise identical samples at different temperatures TAN=200, 400, and 500°C. Vibrating sample magnetometry (VSM) was used for magnetization M vs. T measurements at different external fields H. A method based upon the linear superposition of different sample parts having different Curie temperatures TC was used to extract the critical exponents and the intrinsic distribution of Curie temperatures. We found that this method allows extracting reliable values of the critical exponents for all annealing temperatures, which enabled us to study the effects of disorder onto the universality class of Gd films.

Inhomogeneities as a possible factor responsible for the acceleration of the Universe: a (2+1)-gravity study

Here we present an inhomogeneous cosmological model, in 2+1 gravity, which satisfy all the energy conditions, although it generates non deaccelerated universes. We work in a self-similar 2+1 gravity scenario, in order to simplify the equations system and to allow us to find analytical and simple solutions. Our propose is basically to improve our understanding on the role of inhomogeneities on the acceleration of the Universe.

Microstrip photonic crystals used for measuring parameters of liquids

Computer simulation and experimental study of the frequency dependences of the transmission and reflection coefficients of photonic crystals based on a microstrip structure are carried out in the case when the periodicity of the structure is violated by a change in the permittivity of the substrate of one of the alternating segments of the microstrip line. A method is developed for measuring parameters of the material playing the role of inhomogeneity in the open microwave photonic structure. The complex permittivity is determined for a number of polar and nonpolar liquids.

Ising t–J model close to half filling: a Monte Carlo study

Within the recently proposed doped-carrier representation of theprojected lattice electron operators we derive a full Ising version of thet–J model. This model possesses the global discreteZ2 symmetry as a maximal spin symmetry of the Hamiltonian at any values of the coupling constants,t and J. In contrast, in the spin anisotropic limit of thet–J model, usuallyreferred to as the t–Jz model, the globalSU(2) invariance isfully restored at Jz = 0, so that only the spin–spin interaction has in this model the true Ising form.We discuss a relationship between these two models and the standard isotropict–J model. We show that the low-energy quasiparticles in all three modelsshare qualitatively similar properties at low doping and small values ofJ/t. The main advantageof the proposed Ising t–J modelover the t–Jz one is that the former allows for the unbiased Monte Carlo calculations on large clusters of up to103 sites. Within this model we discuss in detail the destruction of the antiferromagnetic (AF)order by doping as well as the interplay between the AF order and hole mobility. We alsodiscuss the effect of the exchange interaction and that of the next-nearest-neighbourhoppings on the destruction of the AF order at finite doping. We show that theshort-range AF order is observed in a wide range of temperatures and dopings,much beyond the boundaries of the AF phase. We explicitly demonstrate that thelocal no-double-occupancy constraint plays the dominant role in destroying themagnetic order at finite doping. Finally, a role of inhomogeneities is discussed.

Nonlinear spinor field in cosmology

Within the scope of Bianchi type VI (BVI) model the self-consistent system of nonlinear spinor and gravitational fields is considered. Exact self-consistent solutions to the spinor and gravitational field equations are obtained for some special choice of spatial inhomogeneity and nonlinear spinor term. The role of inhomogeneity in the evolution of spinor and gravitational field is studied. Oscillatory mode of expansion of the BVI universe is obtained for some special choice of spinor field nonlinearity.

Lagrangian turbulence in the Adriatic Sea as computed from drifter data: Effects of inhomogeneity and nonstationarity

The properties of mesoscale Lagrangian turbulence in the Adriatic Sea are studied from a drifter data set spanning 1990–1999, focusing on the role of inhomogeneity and nonstationarity. A preliminary study is performed on the dependence of the turbulent velocity statistics on bin averaging, and a preferential bin scale of 0.25° is chosen. Comparison with independent estimates obtained using an optimized spline technique confirms this choice. Three main regions are identified where the velocity statistics are approximately homogeneous: the two boundary currents, West (East) Adriatic Current, WAC (EAC), and the southern central gyre, CG. The CG region is found to be characterized by symmetric probability density function of velocity, approximately exponential autocorrelations, and well‐defined integral quantities such as diffusivity and timescale. The boundary regions, instead, are significantly asymmetric, with skewness indicating preferential events in the direction of the mean flow. The autocorrelation in the along mean flow direction is characterized by two timescales, with a secondary exponential with slow decay time of ≈11–12 days particularly evident in the EAC region. Seasonal partitioning of the data shows that this secondary scale is especially prominent in the summer‐fall season. Possible sampling issues as well as physical explanations for the secondary scale are discussed. Physical mechanisms include low‐frequency fluctuations of forcings and mean flow curvature inducing fluctuations in the particle trajectories. Consequences of the results for transport modeling in the Adriatic Sea are discussed.

Semi-insulating GaN and its evaluation for α particle detection

The application of semi-insulating GaN for detection of ionising particles, specifically α-particles, is presented. The electrical properties of GaN epitaxial layers have been investigated and the space charge limited and Ohmic currents were observed. A microwave method was used for temporal measurements of the photocurrent. Transient behaviour of the injection current and photo-response was observed, with a wide range of instantaneous time constants. The charge collection efficiency was measured to be approximately 100% and the role of inhomogeneities in the charge collection peculiarities was analysed.

Instabilities of inhomogeneous plasmas streaming relative to inhomogeneous dust distributions

Streaming instabilities are studied for an inhomogeneous distribution of dust particles. Such situations are characteristic of stellar neighbourhoods, where the star's plasma wind ploughs through a surrounding dust shell. This should result in the acceleration of dust particles at the expense of the plasma momentum, which may throw light on certain dynamical features of the dust cloud vis-à-vis the plasma dynamical features. Streaming instabilities are a first step towards such a study. Various cases of the relative magnitudes of the plasma and dust velocities are analysed, and the role of inhomogeneities is highlighted.