Step Barrier Research Articles

Experimental Observation of Acoustic Weyl Points and Topological Surface States

Weyl points emerge as topological monopoles of Berry flux in the three-dimensional (3D) momentum space and have been extensively studied in topological semimetals. As the underlying topological principles apply to any type of waves under periodic boundary conditions, Weyl points can also be realized in classical wave systems, which are easier to engineer compared to condensed matter materials. Here, we made an acoustic Weyl phononic crystal by breaking space inversion (P) symmetry using a combination of slanted acoustic waveguides. We conducted angle-resolved transmission measurements to characterize the acoustic Weyl points. We also experimentally confirmed the existence of acoustic "Fermi arcs" and demonstrated robust one-way acoustic transport, where the surface waves can overcome a step barrier without reflection. This work lays a solid foundation for the basic research in 3D topological acoustic effects.

Donor impurity binding energies of coaxial [formula omitted] cylindrical quantum wires in a parallel applied magnetic field

Theoretical investigations are carried out of binding energies of a donor charge assumed to be located exactly at the center of symmetry of two concentric cylindrical quantum wires. The intrinsic confinement potential in the region of the inner cylinder is modeled in any one of the three profiles: simple parabolic, shifted parabolic or the polynomial potential. The potential inside the shell is taken to be a potential step or potential barrier of a finite height. Additional confinement of the charge carriers is due to the vector potential of the axial applied magnetic field. It is found that the binding energies attain maxima in their variations with the radius of the inner cylinder irrespective of the particular intrinsic confinement of the inner cylinder. As the radius of the inner cylinder is increased further, the binding energies corresponding to either the parabolic or the polynomial potentials attain minima at some critical core-radius. Finally, as anticipated, the binding energies increase with the increase of the parallel applied magnetic field. This behaviour of the binding energies is irrespective of the particular electric potential of the nanostructure or its specific dimensions.

Studies on spatial nonlocal optical absorption properties in AlGaAs/GaAs three-step quantum well

To investigate the nonlocal linear intersubband optical absorption properties in AlGaAs/GaAs three-step quantum well (3SQW) nanostructures for a p-polarized states, we deduced the expression of the optical absorption coefficient in the two-level 3SQW system by using Green’s function method and the nonlocal relation between electron and photoelectric field. For the given 3SQW nanostructure, we performed numerical calculations of the optical absorption spectra for different parameters such as angles of incidence, the well width, and the height of the step barrier potential. The numerical results show that spatial nonlocal linear optical response of electrons to light field can induce the radiation shift. It is also demonstrated that the radiation shift and optical absorbance can be controlled by the structure of the 3SQW. These results may bring about a helpful impact in designing nanomaterials with multifarious nonlocality and observing the spatial nonlocal effects in experiment.

Tsu-Esaki modeling of tunneling currents in ferroelectric tunnel junctions

We model tunneling currents through step barrier structures representative of ferroelectric tunnel junctions wherein one of the electrodes contributes to the barrier potential profile or an extra layer is grown between the ferroelectric barrier and one of the electrodes. We study current density–voltage (J–V) and tunneling electroresistance (TER) curves using the Tsu-Esaki formula with numerically calculated transmission. This method is computationally robust, and the same results cannot be obtained with the standard methods usually applied for interpreting experimental I–V curves. Our results predict that the effect of resonant tunneling produces asymmetry in the J–V curves and negative differential resistance characteristics. We show that the asymmetry of the J–V curves can be tuned by adjusting the barrier heights and widths and that changing the extra barrier width affects the asymmetry the most. The barrier widths and the main barrier height affect the magnitude of the tunneling current the most. The change in the tilt of the main barrier is suggested to produce a significant TER of the order of 102. Our numerical method provides a systematic way to study trends in tunneling currents through step barrier structures across a wide range of barrier parameters and bias voltages, in contrast to the necessity of employing several different approximations when using analytical formulae. Therefore, our method provides the means for interpreting existing and future experiments and can be used as a tool for designing new devices with desired functionalities.

First-principles mechanistic study of ring hydrogenation and deoxygenation reactions of eugenol over Ru(0001) catalysts

Catalytic hydrogenation and deoxygenation reactions of ligno-cellulosic (LC) biomass resources are the main valorisation routes of bio-based oxygenates in bio-refineries. Eugenol is a model compound for lignin, which is besides cellulose and hemicellulose a major constituent of wood. Hydrodeoxygenation of eugenol on a Ru(0001) catalyst and in bulk was investigated by plane wave density functional theory (DFT). The adsorption of fragments (benzene, propylene, methanol…) was studied, where several adsorption modes were found. A complex pathway framework, accounting for all elementary steps, was developed and analysed to obtain activation barriers and reaction energies. Additionally, the kinetics of the fragment recombination into methane and water were assessed. To investigate mobility of smaller species, surface diffusion was also modelled explicitly. We found out that homogenously, i.e. without ruthenium, only hydrogenation of allyl functional group proceeded under high hydrogen pressure. On the ruthenium catalyst, however, two predominant cascades were plausible; the hydrogen addition to the substituted benzene ring, followed by oxygen-containing substituent elimination was slightly more favourable in the experiments, while calculations demonstrated that the order of processes can also be reversed. Aromatic ring hydrogenation was found to be endothermic with the rate-determining step (RDS) barriers of approximately 1.0 eV. Methoxyl and hydroxyl were cleaved off differently, depending on aromaticity. With cyclohexane, methoxy, dihydroxy, and hydroxy intermediate moieties were formed, whereas on the aromatic ring (hydro)deoxygenation proceeds via dihydroxy and hydroxyl precursor components. Aryl CO bond cleavage requires 1.8 eV, while for alkyls scission proceeds more readily with activation energies as low as 0.5 eV. Ultimately, completely hydrogenated propyl-cyclohexane is produced. The carbon radical (CHx∗) combination into CH4 is fast enough to avoid coke deposition poisoning, while the formation of H2O is much slower.

Optimal detrended fluctuation analysis as a tool for the determination of the roughness exponent of the mounded surfaces

We present an optimal detrended fluctuation analysis (DFA) and apply it to evaluate the local roughness exponent in nonequilibrium surface growth models with mounded morphology. Our method consists in analyzing the height fluctuations computing the shortest distance of each point of the profile to a detrending curve that fits the surface within the investigated interval. We compare the optimal DFA (ODFA) with both the standard DFA and nondetrended analysis. We validate the ODFA method considering a one-dimensional model in the Kardar-Parisi-Zhang universality class starting from a mounded initial condition. We applied the methods to the Clarke-Vvedensky (CV) model in 2+1 dimensions with thermally activated surface diffusion and absence of step barriers. It is expected that this model belongs to the nonlinear molecular beam epitaxy (nMBE) universality class. However, an explicit observation of the roughness exponent in agreement with the nMBE class was still missing. The effective roughness exponent obtained with ODFA agrees with the value expected for the nMBE class, whereas using the other methods it does not agree. We also characterize the transient anomalous scaling of the CV model and obtained that the corresponding exponent is in agreement with the value reported for other nMBE models with weaker corrections to the scaling.

Direct decarbonylation of furfural to furan: A density functional theory study on Pt-graphene

The catalytic mechanism of direct decarbonylation of furfural to furan on Pt graphene surface has been investigated by means of density functional theory (DFT) calculations. The main catalytic mechanism proposed in this study has six steps such as furfural adsorption, dissociation of H from adsorbed furfural, dissociation of carbon monoxide (molecularly) from adsorbed complex, furan formation and desorption of products. It has been concluded that the rate determining step for direct decarbonylation of furfural to furan is furan formation step and global activation barrier for this catalytic system is 82kJ/mol consisting of zero point energy and thermal energy corrections. Pt-graphene structure has an important role on the catalytic decarbonylation of furfural to furan without any other reactants. This accordingly points out that Pt doped graphene structure might be an encouraging catalyst for direct decarbonylation of furfural to furan molecule as a valuable chemical material.

Generalization of Lundberg’s inequality for the case of stock insurance company

The ruin probability of an insurance company paying dividends according to a barrier strategy with a step barrier function is considered. Upper bounds for the probability of ruin are obtained within the framework of Sparre Andersen and Cramer–Lundberg risk models.

Reflection and transmission of an ultrashort electromagnetic pulse incident normal to a flat plasma layer

The process of reflection and transmission of ultrashort pulses incident normal to a flat plasma layer of finite thickness and on a step barrier is calculated within an exact mathematical approach for various parameters (barrier width and duration and carrier frequency of a corrected Gaussian pulse) of the problem.

Methanol decomposition on low index and stepped CeO2 surfaces from GGA+U

GGA+U calculations have been carried out to study the complete methanol decomposition on the more stable Ceria surfaces, i.e. (111), (221), (331) and (110). These results have shown that the methanol adsorption is exothermic on oxidized as well as on the partially reduced surfaces though the adsorption energy is greater for the latest. The first dehydrogenation step of methanol is highly probable for all the studied sites with activation barriers smaller than 0.2eV. The first dehydrogenation reaction could also occur by breaking the CH methyl bond, but we found that this reaction is very unlikely. Reaction and activation energies for the second dehydrogenation – from methoxy to formaldehyde, are very similar for perfect (111) and stepped surfaces but these activation barriers are not negligible, almost ten times as many the first step barriers. Next, the formaldehyde decomposition to formyl and CO species on perfect CeO2(111) have an important energetic cost, therefore these reactions could occur only on stepped surfaces.

Discounted dividends in a strategy with a step barrier function

A model of insurance company performance with dividend payment is studied. We investigate the dividend strategy according to which the level of the barrier can be changed after the receipt of claims. A function representing the value of expected discounted dividends paid until ruin is obtained.

A comparison study of oxygen reduction on the supported Pt, Pd, Au monolayer on WC(0001)

Using the first principles methods, the geometric and electronic structures of the metal monolayers supported on WC(0001) surfaces (MML/WC(0001) (M = Pt, Pd, and Au)) and their catalytic activity toward the oxygen reduction reaction (ORR) were comparatively studied. Both the kinetics and the density of states results demonstrated that the direct dissociation of O2 on all three MML/WC(0001) surfaces are almost impossible. Yet the barriers of the formation and dissociation of OOH on AuML/WC(0001) are much smaller than those on the PtML/WC(0001) and the PdML/WC(0001) surfaces, implying that the AuML/WC(0001) exhibits the highest catalytic activity for ORR via a combination of 2e− hydrogenation of O2 and 4e− dissociation of OOH. The rate-limiting step barrier of 0.83 eV for the hydrogenation of OH forming H2O is also comparable to that on the traditional Pt-based catalysts. The deactivation mechanism of PtML/WC(0001) and the performance of PdML/WC(0001) for ORR were identified. The present study is conductive to designing new efficient catalyst without using of the precious Pt for efficiently promoting ORR.

Stability diagrams for the surface patterns of GaN(0001¯) as a function of Schwoebel barrier height

Height and type of Schwoebel barriers (direct or inverse) decides about the character of the surface instability. Different surface morphologies are presented. Step bunches, double steps, meanders, mounds and irregular patterns emerge at the surface as a result of step (Schwoebel) barriers at some temperature or miscut values. The study was carried out on the two-component kinetic Monte Carlo (kMC) model of GaN(0001¯) surface grown in nitrogen rich conditions. Diffusion of gallium adatoms over N-polar surface is slow and nitrogen adatoms are almost immobile. We show that in such conditions surfaces remain smooth when gallium adatoms diffuse in the presence of low inverse Schwoebel barrier. It is illustrated by adequate stability diagrams for surface morphologies.

AlOx barrier growth in magnetic tunnel junctions for sensor applications

Magnetic tunnel junction (MTJ) research has been focused on MgO-based crystalline structures due to high tunnel magnetoresistance (TMR), despite requiring a more severe process control than previous generations of MTJ stacks based on amorphous barriers (e.g. AlOx). In this work, we study the electrical transport properties in AlOx barriers in MTJ sensors fabricated using Ion beam sputtering and remote plasma oxidation. Amorphous barriers were prepared from oxidation of thin Al films, deposited in single step barrier (SSB-Al 1nm/oxidation) or double step barrier (DSB-Al 0.5nm/oxidation/Al 0.5nm/oxidation) structures. We show tunable resistance-area products (RxA) ranging from ≈10Ωμm2 (suited for nano devices) up to ≈100kΩμm2 (suited for large area sensors) with TMR above 30%. For all geometries studied, the structures have a coercivity free linear response and require none or one annealing step. This makes them very competitive for all industrial applications where the TMR level is not the dominant specification to meet.

The Lambert-W step-potential – an exactly solvable confluent hypergeometric potential

We present an asymmetric step–barrier potential for which the one-dimensional stationary Schrödinger equation is exactly solved in terms of the confluent hypergeometric functions. The potential is given in terms of the Lambert W-function, which is an implicitly elementary function also known as the product logarithm. We present the general solution of the problem and consider the quantum reflection at transmission of a particle above this potential barrier. Compared with the abrupt-step and hyperbolic tangent potentials, which are reproduced by the Lambert potential in certain parameter and/or variable variation regions, the reflection coefficient is smaller because of the lesser steepness of the potential on the particle incidence side. Presenting the derivation of the Lambert potential we show that this is a four-parametric sub-potential of a more general five-parametric one also solvable in terms of the confluent hypergeometric functions. The latter potential, however, is a conditionally integrable one. Finally, we show that there exists one more potential the solution for which is written in terms of the derivative of a bi-confluent Heun function.

Mid-infrared Photoconductive Response in AlGaN/GaN Step Quantum Wells.

AlGaN/GaN quantum structure is an excellent candidate for high speed infrared detectors based on intersubband transitions. However, fabrication of AlGaN/GaN quantum well infrared detectors suffers from polarization-induced internal electric field, which greatly limits the carrier vertical transport. In this article, a step quantum well is proposed to attempt solving this problem, in which a novel spacer barrier layer is used to balance the internal electric field. As a result, a nearly flat band potential profile is obtained in the step barrier layers of the AlGaN/GaN step quantum wells and a bound-to-quasi-continuum (B-to-QC) type intersubband prototype device with detectable photocurrent at atmosphere window (3–5 μm) is achieved in such nitride semiconductors.

Disturbed Shear Stress Promotes Caveolae‐Mediated oxLDL Uptake Leading to Increased Endothelial Stiffness

IntroductionOxidized low density lipoprotein (oxLDL) is a major risk factor in the development of atherosclerotic plaques, which preferentially form at vascular bifurcations subject to bidirectional recirculating disturbed shear stress (DSS).MethodsA microfluidic channel with a step barrier was designed, tested and fabricated to generate atheroprotective unidirectional laminar shear stress (LSS) and atheroprone DSS onto an EC monolayer for 48 hours. EC stiffness was assessed using atomic force microscopy.ResultsOur results show a statistically significant increase in oxLDL uptake in DSS vs. LSS regions in three EC types: human aortic (55% increase), human microvascular (40% increase), and mouse microvascular (50% increase). OxLDL uptake was reduced in cells lacking Cav1, the main protein in caveolae (n=4, p<0.05). The mechanism of differential oxLDL uptake was further investigated using siRNA treated ECs for CD36 and Lox1, two oxLDL receptors. Our experiments revealed that CD36 (n=4, p<0.05), but not Lox1 (n=4, p<0.05), is required for increased uptake under DSS. Furthermore, the increased uptake under DSS results in a 60% increase in EC stiffness (n=3, p<0.05). This data is further supported by ex vivo stiffness measurements of vessels from WT mice which show a 50% increase in EC stiffness in the aortic arch (DSS) than the LSS region of the descending aorta (DA) (n=5, p<0.05). In Cav1 KO mice, EC stiffness was markedly reduced (n=5, p<0.05) and no differences observed between the DA and arch.ConclusionTaken together, this study suggests that DSS‐induced increase in oxLDL uptake is mediated by caveolae and plays a significant role in increasing localized EC stiffness.

Low driving voltage in polymer light-emitting diodes with CdS nanoparticles as an electron transport layer

Water-soluble cadmium sulfide (CdS) nanoparticles were synthesized through aqueous thermochemical method using thioglycolic acid as a capping agent. The resulting CdS nanoparticles were characterized using x-ray diffraction, field-emission scanning electron microscopy, ultraviolet–visible, and photoluminescence spectroscopy. The CdS prepared has been used as an electron transport layer (ETL) in a polymer light-emitting diode (PLED) with the structure of poly(ethylene terephthalate)/indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene/CdS/aluminium (PET/ITO/PEDOT:PSS/MEH-PPV/CdS/Al). The performance of the fabricated device containing CdS nanoparticles as an ETL was compared with the device with the structure of PET/ITO/PEDOT:PSS/MEH-PPV/Al. According to the obtained results, it was shown that the ETL yields a higher current and lower turn-on voltage. The improvement was ascribed to the step barrier theory and hole blocking effect of the CdS layer. These results show that the CdS layer could be very useful in the fabrication of efficient PLEDs.

Nonlinear refractive index change and optical rectification in a GaN-based step quantum wells with strong built-in electric field

Based on the compact density matrix approach, the linear and nonlinear refractive index change (RIC) and optical rectification (OR) coefficients in a GaN -based step QW with strong built-in electric field (BEF) have been theoretically deduced and investigated in detail. The analytical electronic state is derived by the two airy functions. And the band nonparabolicity is taken into account by using an energy dependence effective mass (EDEM) method. Numerical calculations on a four-layer AlN / GaN / Al x Ga 1-x N / AlN step QW are performed, and the curves for the geometric factors, the linear, the nonlinear, the total RICs and the OR coefficients as functions of the structural parameters of the step QW are discussed. The features for these curves were specified and reasons for the features were explained reasonably. It is found that the decreasing of well width Lw, and step barrier width Lb and the doped concentration x in step barrier will result in the significant enhancement of the RICs. With the decrease of Lw, Lb and x, the resonant photon energies of RIC and OR coefficients have obvious blue-shift. Moreover, the RIC and OR coefficients behave different dependence on the structural parameters of the GaN -based step QWs. The profound physical reasons are also analyzed.

Physical properties of Zener tunnelling nano‐devices in graphene

By considering the direction of charge carriers and the conservation of probablity current the transmission properties of graphene Zener tunnelling nano‐devices were obtained. The scattering properties were then used with an adaptation of the Landauer formalism to calculate an analytical expression for current and conductance. The numerical results of the IV characteristics were then briefly discussed for the graphene step and Zener barrier. A comparison between the theoretical model and experimental results shows the similarities of graphene nanoribbons and infinite sheet graphene.