Nonuniform Barriers Research Articles

Uniform and nonuniform thermal switching of magnetic particles

The pulse-noise approach to systems of classical spins weakly interacting with the bath has been applied to study thermally-activated escape of magnetic nanoparticles over the uniform and nonuniform energy barriers at intermediate and low damping. The validity of approximating a single-domain particle by a single spin is investigated. Barriers for a non-uniform escape of elongated particles for the uniaxial model with transverse and longitudinal field have been worked out. Pulse-noise computations have been done for finite magnetic chains. The linear stability of the uniform barrier state has been investigated. The crossover between uniform and nonuniform barrier states has been studied with the help of the variational approach.

Quantum focusing and filtering of electrons propagating coherently through non-uniform potential barriers

The possibility of engineering the coherent propagation of electrons through a confined medium with potential energy barriers of various spatial shapes is investigated. Using basic quantum mechanical principles, a semi-analytical method is developed to compute the current density of the particle's localization probability in a nanoscale strip, across regions with different values of potential energy and with various geometries. The results show uneven spatial distribution of the probability current density, with possible extreme contrast between electron flows through neighbouring areas. Thus, interactions of the confined electronic wave with customized shapes of potential energy barriers are shown to lead to either focusing or filtering effects of the electron propagation. It is further demonstrated that the method can be adapted for studying the probability flow scattering by insular potential energy obstacles. Though the present method is illustrated on a narrow two-dimensional strip, it can be straightforwardly generalized for three-dimensional cases.

Tunneling through one-dimensional piecewise-constant potential barriers

In this paper, we examine transmission through one-dimensional potential barriers that are piecewise constant. The transfer matrix approach is adopted, and a new formula is derived for multiplying long matrix sequences that not only leads to an elegant representation of the wave function but also results in much faster computation than earlier methods. The proposed method covers a broad spectrum of potentials, of which multi-barrier systems are special cases. The procedure is illustrated with a finite lattice of nonuniform rectangular barriers—non-uniformity being a novel feature, as the uniform case has been solved exactly by Griffiths and Steinke. For the nonuniform multi-barrier problem, the intervening wells strongly influence the transmission probability. Surprisingly, we find that the wells act “individually,” i.e., their influence is a function only of their width and is independent of their exact locations in a multi-barrier system. This finding leads to an observation that we have termed the “alias effect.” The exact solutions are supplemented with asymptotic formulas.

Effects of grain-boundary potential barrier height and its fluctuation on conductivity of polycrystalline semiconductors in the ionized-impurity-scattering dominated case

We propose a new conduction model based on an energy filtering model for thermionic emission over non-uniform grain-boundary barriers in polycrystalline semiconductors of arbitrary degeneracy in which the relaxation time of carriers is dominated by ionized-impurity scattering. It is shown that the non-linear curves of Arrhenius plots of electrical conductivity in several polycrystalline samples of different semiconductors (FeS2, WO3, SnO2, ZnO) in literature are well fitted using the model with the mean value and the standard deviation of the grain-boundary potential barrier height and the concentration of ionized impurities as the three fitting parameters.

How Kinetics within the Unfolded State Affects Protein Folding: An Analysis Based on Markov State Models and an Ultra-Long MD Trajectory

Understanding how kinetics in the unfolded state affects protein folding is a fundamentally important yet less well-understood issue. Here we employ three different models to analyze the unfolded landscape and folding kinetics of the miniprotein Trp-cage. The first is a 208 μs explicit solvent molecular dynamics (MD) simulation from D. E. Shaw Research containing tens of folding events. The second is a Markov state model (MSM-MD) constructed from the same ultralong MD simulation; MSM-MD can be used to generate thousands of folding events. The third is a Markov state model built from temperature replica exchange MD simulations in implicit solvent (MSM-REMD). All the models exhibit multiple folding pathways, and there is a good correspondence between the folding pathways from direct MD and those computed from the MSMs. The unfolded populations interconvert rapidly between extended and collapsed conformations on time scales ≤40 ns, compared with the folding time of ∼5 μs. The folding rates are independent of where the folding is initiated from within the unfolded ensemble. About 90% of the unfolded states are sampled within the first 40 μs of the ultralong MD trajectory, which on average explores ∼27% of the unfolded state ensemble between consecutive folding events. We clustered the folding pathways according to structural similarity into "tubes", and kinetically partitioned the unfolded state into populations that fold along different tubes. From our analysis of the simulations and a simple kinetic model, we find that, when the mixing within the unfolded state is comparable to or faster than folding, the folding waiting times for all the folding tubes are similar and the folding kinetics is essentially single exponential despite the presence of heterogeneous folding paths with nonuniform barriers. When the mixing is much slower than folding, different unfolded populations fold independently, leading to nonexponential kinetics. A kinetic partition of the Trp-cage unfolded state is constructed which reveals that different unfolded populations have almost the same probability to fold along any of the multiple folding paths. We are investigating whether the results for the kinetics in the unfolded state of the 20-residue Trp-cage is representative of larger single domain proteins.

ChemInform Abstract: Topological Effects in Nanomagnetism: From Superparamagnetism to Chiral Quantum Solitons

AbstractReview: 367 refs.

Control of the tunnel current flowing though nonuniform heterogeneous barriers with spherically symmetric inclusions

The effect of spherical quantum objects (scatterers) embedded into semiconductor barriers on the tunnel current flowing through them has been studied. For this purpose, the problem of the scattering of incident and reflected wave functions (damping if their energy is less than the barrier potential) of the electron by the stepwise spherically symmetric scattering potential has been solved.

Ballistic transport properties across nonuniform strain barriers in graphene

We study the effect of uniaxial strain on the transmission and the conductivity across a strain-induced barrier in graphene. At variance with conventional studies, which consider sharp barriers, we consider a more realistic, smooth barrier, characterized by a nonuniform, continuous strain profile. Our results are instrumental towards a better understanding of the transport properties in corrugated graphene.

Topological effects in nanomagnetism: from superparamagnetism to chiral quantum solitons

Micromagnetics has been the method of choice to interpret experimental data in the area of microscopic magnetism for several decades. In this article, we show how progress has been made to extend this formalism to include thermal and quantum fluctuations in order to describe recent experimental developments in nanoscale magnetism. For experimental systems with constrained dimensions such as nanodots, atomic chains, nanowires, and thin films, topological defects such as solitons, vortices, skyrmions, and monopoles start to play an increasingly important role, all forming novel types of quasiparticles in patterned low-dimensional magnetic systems. We discuss in detail how soliton–antisoliton pairs of opposite chirality form non-uniform energy barriers against thermal fluctuations in nanowires or pillars. As a consequence of their low barrier energy compared to uniform reversal, they limit the thermal stability of perpendicular recording media. For sufficiently short samples, the non-uniform energy barrier continuously merges into the conventional uniform Néel–Brown barrier. Partial formation of chiral domain walls also determines the magnetic properties of granular nanostructured magnets and exchange spring systems. For a long time, the reconciliation between micromagnetics and quantum mechanics has remained an unresolved challenge. Here it is demonstrated how inclusion of Berry's phase in a micromagnetic action allows for a semiclassical quantization of spin systems, a method that is demonstrated by the simple example of an easy-plane spin. This powerful method allows for a description of quantum dynamics of solitons and breathers which in the latter case agrees with the anisotropic spin-½ XYZ-model. The domain wall or soliton chirality plays an important role as it is coupled to the wavevector of the quasiparticle dispersion. We show how this quantum soliton chirality is detected by polarized neutron scattering in one-dimensional quantum antiferromagnets.

Transport properties of graphene across strain-induced nonuniform velocity profiles

We consider the effect of uniaxial strain on ballistic transport in graphene, across single and multiple tunneling barriers. Specifically, we show that applied strain not only shifts the position of the Dirac points in reciprocal space, but also induces a deformation of the Dirac cones, and that both effects are of the same order on the applied strain intensity. We therefore study the deviations thereby induced on the angular dependence of the tunneling transmission across a single barrier, as well as on the conductivity and Fano factor across a single barrier and a superstructure of several, periodically repeated, such sharp barriers. Our model is generalized to the case of nonuniform barriers, where either the strain or the gate potential profiles may depend continuously on position. This should afford a more accurate description of realistic ``origami'' nanodevices based on graphene, where ``foldings'' are expected to involve several lattice spacings.

Transmission characteristics in double and triple non-uniform magnetic barriers based on graphene

We evaluate the transmission and the conductance through double and triple non-uniform magnetic barriers in graphene-based nanostructures. The results indicate that non-uniform magnetic modulation results in rich transport properties. Compared with uniform double barrier magnetic structures, both the transmission and the conductance are drastically reduced through non-uniform ones. In the non-uniform triple magnetic barriers, the transmission and the conductance exhibit complicated oscillations, which vary with the width or the height of magnetic barriers. The non-uniformity introduced by altering the height and width of magnetic barriers have different effects on the transmission. For the magnetic barriers with different heights, the transmission is suppressed greatly in both low and high incident wave vector regions, while for the magnetic barriers with different widths, the obvious suppression occurs in the low incident wave vector region.

Tunnelling transport in nonuniform barriers as a possible way for current control

This paper presents the results of computer simulation of the under barrier tunnelling transport through the nonuniform heterostructure barriers containing spherical or cylindrical nano objects. Embedding of nano objects in the barrier enables one to construct the semiconductor barriers with predefined properties. Application of external potential to these objects exponentially changes the current through the barrier and it can be used for current control and amplification of signals.

Internationalisation in the telecommunications services industry: Literature review and research agenda

A review of 356 publications addressing internationalisation aspects of the telecommunications services industry (TSI) shows a growing body of contributions, mainly covering research themes like settlement and regulation policy, strategic alliances and incumbents’ reactions to deregulation as well as opening of markets in-depth until recently. Wireline telephony TSI has been the dominant segment in most contributions, while the mobile or broadband Internet segments only featured in more recent work. Segment characteristics like non-uniform barriers to entry, different reasons for alliances and non-uniform levels of internationalisation differentiate these TSI segments pose a number of additional questions concerning telecom firms’ internationalisation strategy and success promising for future research.

Explanation of the linear correlation between barrier heights and ideality factors of real metal-semiconductor contacts by laterally nonuniform Schottky barriers

A new and simple-to-use method to obtain homogeneous Schottky barrier heights from effective barrier heights and ideality factors that are determined from current-voltage (I-V) characteristics of metal-semiconductor contacts is presented. This approach is justified by a theory of metal-semiconductor interfaces with laterally inhomogeneous distributions of barrier heights. Effective barrier heights and ideality factors were determined from I-V characteristics of Si and GaN Schottky contacts and a linear reduction of the effective barrier heights with increasing ideality factors was always observed. These findings are explained by numerical simulations of inhomogeneous Schottky contacts which are based on theoretical results by Tung [Phys. Rev. B 45, 13509 (1992)]. The homogeneous barrier heights of metal-semiconductor contacts are obtained by a linear extrapolation of the effective barrier heights to nif ≅ 1.01, the value of the ideality factor characteristic for image-force lowering of Schottky barriers only.

Kramers’s rate theory, broken symmetries, and magnetization reversal (invited)

The theory of thermally induced magnetization reversal in small particles is reviewed. The conventional Néel–Brown theory for uniform magnetization reversal and its derivation from Kramers’s rate theory are first discussed. For sufficiently elongated particles, however, a nonuniform energy barrier (‘‘nucleus’’) has lower energy than the uniform barrier and thus yields a lower coercivity. This coercivity reduction is shown to occur also for vanishing hard-axis anisotropy when the nucleus breaks the rotational symmetry around the easy axis. The prefactor of the Arrhenius factor is calculated for uniform and nonuniform barriers.

Dielectric breakdown induced by sodium in MOS structures

Scanning internal photoemission measurements of several MOS structures, which contained mobile sodium ions, displayed nonuniform contact barriers at the Si–SiO2 interface. A direct correspondence was found between the areas at which the contact barrier was a minimum and the point at which subsequent dielectric breakdown occurred. The direct relationship between barrier reduction and breakdown may result from enhanced electronic injection at the areas with a lower contact barrier.