Barrier Width Research Articles

Anisotropic and valley-resolved beam-splitter based on a tilted Dirac system

We investigate theoretically valley-resolved lateral shift of electrons traversing an n–p–n junction bulit on a typical tilted Dirac system (8-Pmmn borophene). A gauge-invariant formula on Goos–Hänchen (GH) shift of transmitted beams is derived, which holds for any anisotropic isoenergy surface. The tilt term brings valley dependence of relative position between the isoenergy surface in n region and that in the p region. Consequently, valley double refraction can occur at the n–p interface. The exiting positions of two valley-polarized beams depend on the incident angle and energy of incident beam and barrier parameters. Their spatial distance D can be enhanced to be ten to a hundred times larger than the barrier width. Due to tilting-induced high anisotropy of the isoenergy surface, D depends strongly on the barrier orientation. It is always zero when the junction is along the tilt direction of Dirac cones. Thus GH effect of transmitted beams in tilted Dirac systems can be utilized to design anisotropic and valley-resolved beam-splitter.

Solvation Effects on Quantum Tunneling Reactions.

A decisive factor for obtaining high yields and selectivities in organic synthesis is the choice of the proper solvent. Solvent selection is often guided by the intuitive understanding of transition state-solvent interactions. However, quantum-mechanical tunneling can significantly contribute to chemical reactions, circumventing the transition state and thus depriving chemists of their intuitive handle on the reaction kinetics. In this Account, we aim to provide rationales for the effects of solvation on tunneling reactions derived from experiments performed in cryogenic matrices.The tunneling reactions analyzed here cover a broad range of prototypical organic transformations that are subject to strong solvation effects. Examples are the hydrogen tunneling probability for the cis-trans isomerization of formic acid which is strongly reduced upon formation of hydrogen-bonded complexes and the [1,2]H-shift in methylhydroxycarbene where a change in product selectivity is predicted upon interaction with hydrogen bond acceptors.Not only hydrogen but also heavy atom tunneling can exhibit strong solvent effects. The direction of the nearly degenerate valence tautomerization between benzene oxide and oxepin was found to reverse upon formation of a halogen or hydrogen bond with ICF3 or H2O. But even in the absence of strong noncovalent interactions such as hydrogen or halogen bonding, solvation can have a decisive effect on tunneling as evidenced by the Cope rearrangement of semibullvalenes via heavy-atom tunneling. Can quantum tunneling be catalyzed? The acceleration of the ring expansion of 1H-bicyclo[3.1.0.]-hexa-3,5-dien-2-one by complexation with Lewis acids provides a proof-of-concept for tunneling catalysis.Two concepts are central for the explanation and prediction of solvation effects on tunneling phenomena: a simple approach expands the Born-Oppenheimer approximation by separating nuclear degrees of freedom into intra- and intermolecular degrees. Intermolecular movements represent the slowest motions within molecular aggregates, thus effectively freezing the position of the solvent in relation to the reactant during the tunneling process. Another useful approach is to treat reactants and products by separate single-well potentials, where the intersection represents the transition state. Thus, stabilization of the reactants via solvation should result in an increase in barrier heights and widths which in turn lowers tunneling probabilities. These simple models can predict trends in tunneling kinetics and provide a rational basis for controlling tunneling reactions via solvation.

Influence of well position on the electroluminescence characteristics of InGaN/GaN single quantum well red light-emitting diodes

In this paper, the influence of the position of InGaN layer well on the electroluminescence (EL) properties of red InGaN/GaN single quantum well (SQW) light-emitting diodes (LEDs) is investigated numerically. It is found that as the InGaN well shifts from the P-region to the N-region, the LED's EL intensity decreases, and the spectrum red-shifts. By analyzing the EL spectra, energy band structures and carrier distribution, it is considered that, when the well position moves away from the P-region, the injection barrier width of holes increases, while that of electrons decreases. Thus, the injection efficiency of electrons and holes is enhanced and weakened, respectively. Compared with electrons, the injection efficiency of holes with larger effective mass can be significantly reduced by a thicker barrier. As a result, the hole concentration in InGaN QW is reduced more severely, leading to a reduction in the total amount of carriers. The decreased concentration of injected carriers may weaken the carrier screening effect to the polarization electrical field, thereby the polarization-induced quantum confined-Stark effect is enhanced. As a consequence, for the SQW red LED whose InGaN well is far away from the P-region, the EL intensity of the SQW red LED is reduced and the peak wavelength becomes longer.

Programmable Skyrmion Logic Gates Based on Skyrmion Tunneling

Magnetic skyrmions are promising candidates as elementary nanoscale bits in logic-in-memory devices, intrinsically merging high density memory and computing capabilities. Here we exploit the dynamics of skyrmions interacting with anisotropy energy barriers patterned by ion irradiation to design programmable logic gates. Using micromagnetic simulations with experimental parameters, we show that a fine tuning of the barrier height and width allows the selective tunneling of skyrmions between parallel nanotracks triggered by skyrmion-skyrmion interaction. This can be leveraged to design skyrmion De-multiplexer (DMux) logic gate which works solely using skyrmions as logic inputs. By cascading and connecting demultiplexer gates with a specific topology, we develop a fully programmable logic gate capable of producing any possible logic output as a sum of all minterms generated by a given set of inputs without requiring any complex additional electric/magnetic interconversion. The proposed design is fully conservative and cascadable and paves a new pathway for full skyrmionic-based logic-in-memory devices.

Effects analogous to the Kekulé distortion induced by pseudospin polarization in graphene nanoribbons: confinement and coupling by breakdown of chiral correlation

We show here that potential barriers, applied to armchair nanoribbons, induce a hexagonal effective lattice, polarized in pseudospin on the sides of the barriers system, which has an effective unit cell greater than that of infinite graphene (pseudospin superstructure). This superstructure is better defined with the increase of the barrier potential, until a transport gap is generated. The superstructure, as well as the induced gap, are fingerprints of Kekulé distortion in graphene, so here we report an analogous effect in nanoribbons. These effects are associated with a breakdown of the chiral correlation. As a consequence, an effective zigzag edge is induced, which controls the electronic transport instead of the original armchair edge. With this, confinement effects (quasi-bound states) and couplings (splittings), both of chiral origin (decorrelation between chiral counterparts), are observed in the conductance as a function of the characteristics of the applied barriers and the number of barriers used. In general, the Dirac-like states in the nanoribbon can form quasi-bound states within potential barriers, which explains the Klein tunneling in armchair nanoribbons. On the other hand, for certain conditions of the barriers (width L and potential V) and the energy (E) of the quasi-particle, quasi-bound states between the barriers can be generated. These two types of confinement would be generating tunneling peaks, which are mixed in conductance. In this work we make a systematic study of conductance as a function of E, L and V for quantum dots systems in graphene nanoribbons, to determine fingerprints of chirality: line shapes and behaviors, associated with each of these two contributions. With these fingerprints of chirality we can detect tunneling through states within the barriers and differentiate these from tunneling through states formed between the barriers or quantum dot. With all this we propose a technique, from conductance, to determine the spatial region that the state occupies, associated with each tunneling peak.

Limiting Factors of Detectivity in Near-Infrared Colloidal Quantum Dot Photodetectors.

Lead sulfide colloidal quantum dots (PbS CQDs) have shown great potential in photodetectors owing to their promising optical properties, especially their strong and tunable absorption. However, the limitation of the specific detectivity (D*) in CQD near-infrared (NIR) photodetectors remains unknown due to the ambiguous noise analysis. Therefore, a clear understanding of the noise current is critically demanded. Here, we elucidate that the noise current is the predominant factor limiting D*, and the noise is highly dependent on the trap densities in halide-passivated PbS films and the carriers injected from the Schottky contact (EDT-passivated PbS films/metal). It is found that the thickness of CQDs is proportional to their interface trap density, while it is inversely proportional to their minimal bulk trap density. A balance point can be reached at a certain thickness (136 nm) to minimize the trap density, giving rise to the improvement of D*. Utilizing thicker PbS-EDT films broadens the width of the tunneling barrier and thereby reduces the carrier injection, contributing to a further enhancement of D*. The limiting factors of D* determined in this work not only explain the physical mechanism of the influence on detection sensitivity but also give guidance to the design of high-performance CQD photodetectors.

Latex-Based Paper Devices with Super Solvent Resistance for On-the-Spot Detection of Metanil Yellow in Food Samples

The following paper presents a construct for a paper-based device which utilizes latex as the hydrophobic material for the fabrication of its hydrophobic barrier, which was deposited onto the cellulose surface either by free-hand or stenciled drawing. This method demands the least amount of expertise and time from its use, enabling a simple and rapid fabrication experience. Several properties of the hydrophobic material were characterized, such as the hydro head and penetration rate, with the aim of assessing its robustness and stability. The presented hydrophobic barriers fabricated using this approach have a barrier width of 4 mm, a coating thickness of 208 µm, and a hydrophilic resolution of 446.5 µm. This fabrication modality boasts an excellent solvent resistance with regard to the hydrophobic barrier. These devices were employed for on-the-spot detection of Metanil Yellow, a banned food adulterant often used in curcumin and pigeon peas, within successful limits of detection (LOD) of 0.5% (w/w) and 0.25% (w/w), respectively. These results indicate the great potential this fabricated hydrophobic device has in numerous paper-based applications and other closely related domains, such as diagnostics and sensing, signalling its capacity to become commonplace in both industrial and domestic settings.

Core transport barriers induced by fast ions in global gyrokinetic GENE simulations

A novel type of internal transport barrier called F-ATB (fast ion-induced anomalous transport barrier) has been recently observed in state-of-the-art global gyrokinetic simulations on a properly optimized ASDEX Upgrade experiment and presented in Di Siena et al (2021 Phys. Rev. Lett. 127 025002). Unlike the transport barriers previously reported in the literature, the trigger mechanism for the F-ATB has been shown to be a wave-particle resonant interaction between supra-thermal particles—generated via ion cyclotron resonance heating—and ion scale plasma turbulence. This resonant mechanism strongly depends on the particular shape of the fast ion temperature and density profiles. Therefore, to further improve our theoretical understanding of this transport barrier, we present results exploring the parameter space and physical conditions for the F-ATB generation by performing a systematic study with global GENE simulations. Particular emphasis is given to the transport barrier width and its localization by scanning over different energetic particle temperature profiles. The latter are varied in amplitude, half-width, and radial localization of an ad-hoc Gaussian-like energetic particle logarithmic temperature gradient profile. For the reference parameters at hand, a threshold in the ratio between the fast ion and electron temperature and the amplitude of the fast ion logarithmic temperature gradient is identified to trigger the transport barrier effectively. The role of q = 1 rational surface to the transport barrier formation is investigated as well by retaining electromagnetic effects and its impact found to be negligible for this particular barrier formation mechanism.

Photovoltaic efficiency at maximum power of a quantum dot molecule

In this work, the behavior of the efficiency at the maximum power of a quantum dot molecule, acting as a device for photovoltaic conversion, is investigated. A theoretical approach using a master equation, considering the effect of the energy offsets, and the width of the quantum barrier, identify realistic physical conditions that enhance the photovoltaic response of the photocell. By mapping the effect of the control of the energy offsets of the nanostructure, a condition for gain in 30% of maximum power delivered per molecule if compared with a single quantum dot is demonstrated. Studying the behavior as a function of temperature, the physical system exhibits gain when compared to the Chambadal-Novikov-Curzon-Ahlborn efficiency at maximum power, without exceeding Carnot's efficiency, as expected from the second law of thermodynamics.

Excitons in nonpolar ZnO/BeZnO quantum wells: Their binding energy and its dependence on the dimensions of the structures

Binding energy of the 1s exciton in the nonpolar ZnO/BexZn1−xO quantum well for x up to 0.3 has been determined by means of fractional dimension approach. Electron and hole wave functions have been computed by solving Schrödinger equation for each particle. The results have been represented as a function of the dimension of the structure. The effects of the optical phonon interactions and the image charges on the binding energy have been considered also. Their contribution leads to a fractional change as much as 0.53 and the binding energy increases up to nearly 140 meV within the considered ranges of the parameters. The effect of the changes in the barrier width has been found negligible, except for the case of very thin quantum wells.

Artificial Synapses Based on WSe2 Homojunction via Vacancy Migration.

Artificial synapses based on two-dimensional (2D) transition metal dichalcogenides (TMDs) materials have attracted wide attention to boost the development of neuromorphic computing in recent years. Various structures have been adopted to build 2D-material-based artificial synapses. In lateral- and vertical-structures, the realization of synaptic function mainly results from the migration of the defects and vacancies, which requires the strong ion diffusion ability. Here, we successfully demonstrate an artificial synapse based on lateral WSe2 homojunction. The migration of Se vacancies from the thin region to the thick region has been promoted by applying negative gate voltage, resulting in n-type doping in the thick region due to the accumulation of Se vacancies, which would diminish the barrier width of the metal-semiconductor junctions in the thick region. Consequently, the transformation from a high-resistance state (HRS) to a low-resistance state (LRS) is achieved. Significantly, our device can efficiently emulate the biological synaptic functions with a large synaptic weight change. Additionally, the transition from short-term memory (STM) to long-term memory (LTM) can be accomplished with a simpler structure, which would be beneficial to realizing the large-scale integration of transistor-based artificial synapses.

Pseudospin-one particles in the time-periodic dice lattice: a new approach to transport control

The controlling of the transmission in the pseudospin-one Dirac–Weyl systems offers a rich tool to study new concepts of massive Dirac electron tunneling by means of a time-dependent potential. The time-periodic potential is one of the experimental techniques to have more control over the tunneling effect. In this paper, we study the transmission coefficient for different sidebands to obtain total transmission. We show how the super Klein tunneling under special conditions is independent of the incidence angle, oscillation amplitude, frequency, and barrier width. We consider a band gap opening with different locations of the flat band and modulate the resonances by tuning free parameters in our system.

Tunneling effect in phosphorene through double barriers

We study the transport properties of charge carriers in phosphorene with a mass term through double barriers. The solutions of the energy spectrum are obtained and the dependence of the eigenvalues on the barrier potentials and wave vectors in the x-direction is numerically computed. Using the boundary conditions together with the matrix transfer method, we determine transmission and the conductance of our system. These two quantities are analyzed by studying their main characteristics as a function of the physical parameters along the armchair direction. Our results show the highly anisotropic character of phosphorene and the signature of Klein tunneling at normal incidence. Moreover, it is found that the transmission and conductance display oscillatory behaviors in terms of the barrier width under suitable conditions.

Reconstructing subsurface sandbody connectivity from temporal evolution of surface networks

AbstractCharacterization of groundwater aquifers and hydrocarbon reservoirs requires an understanding of the distribution and connectivity of subsurface sandbodies. In deltaic environments, distributary channel networks serve as the primary conduits for water and sediment. Once these networks are buried and translated into the subsurface, the coarse‐grained channel fills serve as primary conduits for subsurface fluids such as water, oil or gas. The temporal evolution of channels on the surface therefore plays a first‐order role in the 3D permeability and connectivity of subsurface networks. Land surface imagery is more broadly available than topographic or subsurface data, and time‐series imagery of river networks can hold useful information for constraining the shallow subsurface. However, these reconstructions require an understanding of the degree to which channel bathymetry and river kinematics affect connectivity of subsurface sandbodies. Here, we present a novel method for building synthetic cross sections using overhead images of an experimental delta. We use principal components analysis to extract river networks from surface imagery, then couple this with an inverse‐CDF method to estimate channel bathymetry, to generate a time‐series of synthetic delta topography. This synthetic topography is then transformed, accounting for deposition and subsidence, to produce synthetic stratigraphy that differentiates coarse‐grained channel fill from overbank and offshore deposition. We find that large‐scale subsurface architecture is relatively insensitive to details of channel bathymetry, but instead is primarily controlled by channel location and kinematics. We analyse the connectivity of sand bodies and the geometries of barriers to flow and find that periods of rapid sea‐level rise have more variability in sand body connectivity. We also find that barrier width decreases downstream during all sea‐level phases. Our method generates synthetic stratigraphy that closely resembles the large‐scale architecture and 2‐dimensional connectivity of the real stratigraphy built during the experiment it was based on. We anticipate that it will be broadly applicable to other experimental and field‐scale scenarios.

Calculation of threshold current density and efficiency for multiple quantum wire of 〖Hg〗_(0.5) 〖Cd〗_(0.5) Te/〖Hg〗_(0.8) 〖Cd〗_(0.2) Te/CdTe laser

This study of semiconductor laser structure works on reducing the value of the threshold current density to improve the working conditions of the laser device. In this paper shown that the threshold current density and Optical confinement factor Г depends on Barrier width (B), and Number of well (Nw). We found any increase in Barrier width and Number of well is enhanced important parameters of diode laser such as threshold current density and confinement factor that's clearly for (Nw=2, B=40 nm) then (Г=0.175, ), while for (Nw=2, B=60 nm), that give results are (Г=0.195, ). On the other hand when changing Nw=3, the results become for (B=40 nm and B=60 nm), as follows (Г=0.275, and (Г=0.295, respectively. This investigated contributed to calculating the efficiency and work of the laser device. We found the value of slope efficiency equal (45 %) and the external quantum efficiency (50%).

Influence of the active region structure of the resonant tunneling diode on the critical points of its current-voltage characteristic

The article proposes a study of the effect of the structure of the active region of a resonant tunneling diode on the critical points of its current-voltage characteristic. The basic configuration of a resonant tunneling diode, which is a structure of a quantum well with a nanometer-sized double barrier including two contacts, and a region with strongly doped contacts made of a semiconductor with a relatively small band gap, is disclosed and illustrated. It is emphasized that since the characteristic dimensions of the structure of the quantum well with a double barrier are comparable to the wavelengths of electrons, the wave nature of electrons leads to such quantum phenomena as interference, tunneling, energy quantization, etc. the double barrier causes resonant tunneling phenomena, which form the basis for the operation of the resonant-tunneling diode. It is emphasized that repeated reflection causes destructive or constructive interference depending on the wavelength of a particular electron. For electrons with a certain wavelength that promotes constructive interference, a transfer probability close to unity can be found at energies corresponding to these wavelengths. The modification of the active region of the resonant tunnel diode with a barrier height of 0.3 - 0.4 eV is mathematically substantiated. The dependence of the transmission coefficient is found by solving the Schrödinger equation in one electron approximation without taking into account the scattering effects. The calculation of the volt-ampere characteristic of the resonant-tunnel diode was performed at temperatures of 100 and 300 K. The given volt-ampere characteristics were obtained without taking into account the effects of electron scattering. However, it should be noted that the main influencing factor is the resonant tunneling through the second level, for which the peak of the transmission coefficient is much wider and higher. However, in gallium doped arsenide, the fact of electron scattering can significantly affect the value of the transmission coefficient and the value of current. It is established that an increase in the width of quantum wells leads to a significant decrease in the densities of peak currents and valley currents, and an increase in the width of potential barriers leads to a slight decrease in the current density of the first peak and current densities of the second peak and valley.

Optimization of gain region in mid-IR ( ≈ 5 μm) QCL.

Non-equilibrium Green's function (NEGF) formalism is used to optimize the gain region of a quantum cascade laser (QCL) tailored to emit radiation at ∼5 µm wavelength, originally designed by Evans et al. [Appl. Phys. Lett., 88,051105(2006)10.1063/1.2171476]. The optimization strategy uses electron-photon selfenergies to find characteristics of devices under the "operating conditions," i.e., interacting with the laser field. These conditions can be quite different from the one when the device is in no-lasing state and the unsaturated gain is being optimized. The saturation caused by the optical field can push the structure from strong to weak coupling conditions, what changes laser parameters in a non-linear manner. Moreover, the NEGF method does not require any phenomenological parameters (such as, e.g., the phase relaxation times), so the quantities dependent on these parameters are determined solely on physical grounds. The use of the above procedure for the structure under investigation shows that the increase of the quantum efficiency by 24% and the output power by 83% in comparison to the original design can be achieved when the widths of injection and extraction barriers are changed to their optimal values.

Improvement of tunneling magnetoresistance and spin-valley polarization in magnetic silicene superlattices induced by structural disorder

It is well known that the unavoidable variation of the heights and widths of the barriers (structural disorder) in semiconductor superlattices and superlattices based on two-dimensional materials degrades important physical properties such as the electronic transport and the thermoelectric response. Here, we show that the structural disorder contrary to what is expected improves the magnetoresistive and spin-valleytronic properties of magnetic silicene superlattices. We reach this conclusion by studying the impact of the random variations of the width and strength of the magnetic barriers on the tunneling magnetoresistance and spin-valley polarization. The theoretical treatment is based on a Dirac-like Hamiltonian, the transfer matrix method, and the Landauer-B\"uttiker formalism to describe silicene electrons, to obtain the transmittance, and to obtain the conductance, respectively. Our results indicate that structural disorder effects improve the magnetoresistance response and the spin-valley polarization by eliminating the conductance oscillations caused by the periodic magnetic modulation as well as by differentiating the response of the conductance for the parallel and antiparallel magnetization configuration. These results could be useful in designing versatile devices with magnetoresistive and spin-valleytronic capabilities. Particularly, magnetic silicene superlattices with moderate structural disorder are more convenient than perfect or nearly perfect ones.

Manifestation of deformation and nonlocality in α and cluster decay

It is common to study the strong decay of a heavy nucleus as a tunneling phenomenon where the $\alpha$ ($^4$He) or a light nuclear cluster tunnels through the Coulomb barrier formed by its interaction with the heavier daughter nucleus. The position and width of the Coulomb barrier are determined by the total interaction potential between the two daughter nuclei. We examine the effects of including nonlocality and deformation in the interaction potential by calculating the half-lives, $t_{1/2}$, and thereby phenomenological preformation factors of several nuclei which have the possibility of decaying by emitting either an $\alpha$ or a light nuclear cluster. The effect of deformation manifests itself by a decrease in $t_{1/2}$ for all the decays studied. The effect of nonlocality is studied within two different models of the nuclear potential: the energy-independent but angular momentum ($l$) dependent Mumbai (M) potential and the energy-dependent Perey-Buck (PB) potential. The nonlocal nuclear interaction leads to a decrease in all half-lives studied. Though the decrease is larger due to the Perey-Buck potential, half-lives evaluated using the Mumbai potential show a strong sensitivity to the $l$ value in the decay. This feature of the $\alpha$ and cluster decay half-lives can provide a complementary tool in addition to scattering data which are more commonly used to fix the parameters of a nonlocal potential in literature.

Effect of Fatigue Fracture on Resistive Switching of ZnO and NiO Stacking Films

The ZnO and NiO stacking films are prepared by sol–gel spin‐coating method to explore the impact of bending on device resistive switching. Compared with others, the device with NiO/ZnO/NiO/indium tin oxide/PET stacking structure exhibits a higher ON/OFF ratio. After bending 1000 times, the switching performance is reduced by two orders of magnitude due to the branch breakage of the conductive filaments. The finite element studies indicate that stress mutation between the ZnO and NiO functional layers leads to microcrack formation and interfacial delamination, increasing the interfacial barrier and depletion width. The cracks propagate along the grain boundary and thus result in more traps, which may act as electron blocking layers to restrict the carrier transport. The scanning electron microscopy observation shows that when the crack density increases to about 0.03 μm−1, the device is broken down. Under threshold bending cycles as simulated by Comsol Multiphysics, the grain boundary is completely torn leading to fatigue fracture. The proposed work provides some information for improving the flexibility of memory devices.