Asymmetric Confinement Research Articles

Vertical Electric-Field-Induced Switching from Strong to Asymmetric Strong–Weak Confinement in GaAs Cone-Shell Quantum Dots Using Transparent Al-Doped ZnO Gates

The first part of this work evaluates Al-doped ZnO (AZO) as an optically transparent top-gate material for studies on semiconductor quantum dots. In comparison with conventional Ti gates, samples with AZO gates demonstrate a more than three times higher intensity in the quantum dot emission under comparable excitation conditions. On the other hand, charges inside a process-induced oxide layer at the interface to the semiconductor cause artifacts at gate voltages above U≈ 1 V. The second part describes an optical and simulation study of a vertical electric-field (F)-induced switching from a strong to an asymmetric strong–weak confinement in GaAs cone-shell quantum dots (CSQDs), where the charge carrier probability densities are localized on the surface of a cone. These experiments are performed at low U and show no indications of an influence of interface charges. For a large F, the measured radiative lifetimes are substantially shorter compared with simulation results. We attribute this discrepancy to an F-induced transformation of the shape of the hole probability density. In detail, an increasing F pushes the hole into the wing part of a CSQD, where it forms a quantum ring. Accordingly, the confinement of the hole is changed from strong, which is assumed in the simulations, to weak, where the local radius is larger than the bulk exciton Bohr radius. In contrast to the hole, an increasing F pushes the electron into the CSQD tip, where it remains in a strong confinement. This means the radiative lifetime for large F is given by an asymmetric confinement with a strongly confined electron and a hole in a weak confinement. To our knowledge, this asymmetric strong–weak confinement represents a novel kind of quantum mechanical confinement and has not been observed so far. Furthermore, the observed weak confinement for the hole represents a confirmation of the theoretically predicted transformation of the hole probability density from a quantum dot into a quantum ring. For such quantum rings, application as storage for photo-excited charge carriers is predicted, which can be interesting for future quantum photonic integrated circuits.

Macrotransport of active particles in periodic channels and fields: Rectification and dispersion.

Transport and dispersion of active particles in structured environments, such as corrugated channels and porous media, are important for the understanding of both natural and engineered active systems. Owing to their continuous self-propulsion, active particles exhibit rectified transport under spatially asymmetric confinement. While progress has been made in experiments and particle-based simulations, a theoretical understanding of the effective long-time transport dynamics in spatially periodic geometries remains less developed. In this paper, we apply generalized Taylor dispersion theory to analyze the long-time effective transport dynamics of active Brownian particles (ABPs) in periodic channels and fields. We show that the long-time transport behavior is governed by an effective advection-diffusion equation. The derived macrotransport equations allow us to characterize the average drift and effective dispersion coefficient. For the case of ABPs subject to a no-flux boundary condition at the channel wall, we show that regardless of activity, the average drift is given by the net diffusive flux along the channel. For ABPs, their activity is the driving mechanism that sustains a density gradient, which ultimately leads to rectified motion along the channel. Our continuum theory is validated against direct Brownian dynamics simulations of the Langevin equations governing the motion of each ABP.

Effect of magnetic and electric field on the ground state properties of weak coupling piezoelectric polaron in a quantum well with asymmetric Gaussian confinement

In this study, we employed the linear combination operation and a modified Lee-Low-Pines unitary transformation method to explore the ground state properties of an interacting electron coupled with piezo-acoustic phonons within a quantum well featuring asymmetric Gaussian confinement potential well under the influence of magnetic and electric fields. We calculated various properties, including the ground state energy, ground state binding energy, magnetic moment, magnetic susceptibility, and polarizability of the weakly coupled piezoelectric polaron. The effects of the magnetic and electric fields, electron-phonon weak coupling strength, Debye cut-off wavenumber, and confinement depth and range on these properties were thoroughly analyzed. Our findings reveal that the ground state energy decreases with increasing the Debye wavenumber, confinement range, and electron-phonon weak coupling strength. In contrast, the ground state binding energy shows the opposite trend, increasing with these parameters. Furthermore, the magnetic and electric fields, confinement depth and range, electron-phonon coupling strength, and Debye wavenumber are critical factors that significantly impact the magnetic and optical properties of the piezoelectric polaron.

Reacting Flow Prediction of the Low-Swirl Lifted Flame in an Aeronautical Combustor With Angular Air Supply

Abstract The development of lean-burn combustion systems is of paramount importance for reducing the pollutant emissions of future aero engine generations. By tilting the burners of an annular combustor in circumferential direction relative to the rotational axis of the engine, the potential of increased combustion stability is opened up due to an enhanced exhaust gas recirculation between adjacent flames. The innovative gas turbine combustor concept, called the short helical combustor (SHC), allows the main reaction zone to be operated at low equivalence ratios. To exploit the higher stability of the fuel-lean combustion, a low-swirl lifted flame is implemented in the staggered SHC burner arrangement. The objective is to reach ultralow NOx emissions by complete evaporation and extensive premixing of fuel and air upstream of the lean reaction zone. In this work, a modeling approach is developed to investigate the characteristics of the lifted flame in an enclosed single-burner configuration, using the gaseous fuel methane. It is demonstrated that by using the large eddy simulation method, the shape and liftoff height of the flame are adequately reproduced by means of the finite-rate chemistry approach. For the numerical prediction of the lean lifted flame in the SHC arrangement, the focus is on the interaction of adjacent burners. It is shown that the swirling jet flow is deflected toward the sidewall of the staggered combustor dome, which is attributed to the asymmetrical confinement. Since the stabilization mechanism of the low-swirl flame relies on outer recirculation zones, the upstream transport of hot combustion products back to the flame base is studied by the variation of the combustor confinement ratio. It turns out that increasing the combustor size amplifies the exhaust gas recirculation along the sidewall, and increases the temperature of recirculating burned gases. This study emphasizes the capability of the proposed lean-burn combustor concept for future aero engine applications.

Magnetic field modulation of polaron relaxation coherence effects in III-V semiconductor Gaussian quantum wells

We have theoretically investigated the effects of the cyclotron frequency of magnetic field, the initial depth and the range of the asymmetrical Gaussian confinement potential on the vibrational frequency, ground state energy, excited state energy and coherence time of polaron in III-V semiconductors by using a linear combination operator and unitary transformations methods. Through numerical calculations, we find some interesting phenomena that the polaron decreases the energy level and releases the energy of a phonon to jump. From the point of view of the energy level of the polaron, which is αℏωLO energy lower than that of the electron, and the transition of the polaron to the ground state will have a change in lattice position, and the lattice relaxation phenomenon will appear. The theoretical study of the quantum coherence effect of polaron energy levels will provide guidance for the development of optoelectronic devices and the transmission of quantum information.

Effective dynamics for a spin-1/2 particle constrained to a curved layer with inhomogeneous thickness

We derive an effective Hamiltonian for a spin-1/2 particle confined within a curved thin layer with non-uniform thickness using the confining potential approach. Our analysis reveals the presence of a pseudo-magnetic field and effective spin–orbit interaction (SOI) arising from the curvature, as well as an effective scalar potential resulting from variations in thickness. Importantly, we demonstrate that the physical effect of additional SOI from thickness fluctuations vanishes in low-dimensional systems, thus guaranteeing the robustness of spin interference measurements to thickness imperfection. Furthermore, we establish the applicability of the effective Hamiltonian in both symmetric and asymmetric confinement scenarios, which is crucial for its utilization in one-side etching systems.

Stimulated Emission in the InAs/InAsSb/InAsSbP Heterostructures with Asymmetric Electronic Confinement

Stimulated Emission in the InAs/InAsSb/InAsSbP Heterostructures with Asymmetric Electronic Confinement

Polaron level spectrum in double-potential quantum wells

The description of macroscopic systems of quasi-particles is a bright spot in quantum physics, providing an accurate description of the physics development. The current study presents a theoretical study of the energy level spectrum of weakly coupled polarons in polar crystals (GaAs, PbSe, PbTe) in double potential quantum wells (parabolic and asymmetric Gaussian confinement potentials) using the linear combination operator and two simple unitary transformations. Rigorous electron-phonon coupling interaction results are obtained under the weak coupling limitation. The results indicate that the polaron energy is relatively lower than that of the electron and the coupling strength affects the energy difference between polaron and electron. The influence of quantum wells’ confinement potential on energy is also discussed. These findings provide the possibility for potential application of polaron.

Solvent effect on equilibrium organization of confined polymers.

The equilibrium organization of two polymers in strong confinement has been extensively investigated in the scope of simple models in the recent past, after it was suggested that the entropic repulsion between polymers could be a mechanism for chromosome separation in living cells. Despite many efforts, how solvent quality can manipulate separation or compaction for the formation of domains in a two-polymer model system within symmetric or asymmetric confinement still remains elusive. This paper presents some important findings for a two-polymer system within box-like (symmetric) and triangle-like (asymmetric) confinement with equal areas, in terms of structural transitions such as from a single globule to individual globules, non-monotonic change in polymer size, variation in the free-energy barrier with separation, etc. By using the exact enumeration method on a two-dimensional self-avoiding walk model of polymers, our results reveal that the polymers have a greater preference to form individual globules rather than a 'micelle'-like single globule in triangular confinement compared to box-like confinement. Additionally, our results suggest that asymmetric confinement can act as an ideal tool to trap polymers in poor solvents due to the large free-energy barrier.

Model studies on motion of respiratory droplets driven through a face mask

Face masks are used to intercept respiratory droplets to prevent spreading of air-borne diseases. Designing face masks with better efficiency needs microscopic understanding on how respiratory droplets move through a mask. Here we study a simple model on the interception of droplets by a face mask. The mask is treated as a polymeric network in an asymmetric confinement, while the droplet is taken as a micrometer-sized tracer colloidal particle, subject to driving force that mimics the breathing. We study numerically, using the Langevin dynamics, the tracer particle permeation through the polymeric network. We show that the permeation is an activated process following an Arrhenius dependence on temperature. The potential energy profile responsible for the activation process increases with tracer size, tracer bead interaction, network rigidity and decreases with the driving force and confinement length. A deeper energy barrier led to better efficiency to intercept the tracer particles of a given size in the presence of driving force at room temperature. Our studies may help to design masks with better efficiency.

Asymmetrical Gaussian Potential Effects on Strongly Coupled Magnetopolaron Properties in Triangular Confinement Potential Quantum Wells

In this research, the existence of an asymmetrical Gaussian confinement potential (AGCP) along the quantum well (QW) growth direction and of a parabolic potential perpendicular to the polar coordinate direction were considered. The magnetic field and temperature properties of the longitudinal optical (LO)-phonon mean number, ground-state energy (GSE), ground-state binding energy (GSBE) and vibrational frequency (VF) of strongly coupled magnetopolarons in triangular confinement potential QWs (TCPQWs) were investigated according to the quantum statistical theory as well as the linear combination operator and unitary transformation methods. We obtained analytical expressions for the GSE, GSBE, VF and LO-phonon mean number as functions of the applied magnetic field, temperature, AGCP barrier height, AGCP range, polar coordinate system’s polar angle and polar coordinate system’s confinement strength. It was demonstrated by the calculated numerical results that the GSE, GSBE, VF and LO-phonon mean number varied with the related physical quantities. The obtained theoretical results are expected to provide a reference for future research on polarons.

Reversible and Nonvolatile Manipulation of the Spin-Orbit Interaction in Ferroelectric Field-Effect Transistors Based on a Two-Dimensional Bismuth Oxychalcogenide

The spin-orbit interaction (SOI) offers a nonferromagnetic scheme to realize spin polarization through utilizing an electric field. Electrically tunable SOIs through electrostatic gates have been investigated; however, the relatively weak and volatile tunability limits their practical applications in spintronics. Here, we demonstrate the nonvolatile electric field control of the SOI via constructing ferroelectric Rashba architectures, i.e., two-dimensional ${\mathrm{Bi}}_{2}{\mathrm{O}}_{2}\mathrm{Se}/\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$-${\mathrm{PbTiO}}_{3}$ ferroelectric field-effect transistors. The experimentally observed weak antilocalization (WAL) cusp in ${\mathrm{Bi}}_{2}{\mathrm{O}}_{2}\mathrm{Se}$ films implies the Rashba-type SOI that arises from the asymmetric confinement potential. Significantly, taking advantage of the switchable ferroelectric polarization, the WAL-to-weak-localization-transition trend reveals the competition between spin relaxation and the dephasing process, and the variation of carrier density leads to a reversible and nonvolatile modulation of the spin-relaxation time and the spin-splitting energy of ${\mathrm{Bi}}_{2}{\mathrm{O}}_{2}\mathrm{Se}$ films by this ferroelectric gating. Our work provides a scheme to achieve nonvolatile control of the Rashba SOI with the utilization of ferroelectric remanent polarization.

Electromagnetic field effect on weak-coupling piezoelectric polaron in an asymmetrical Gaussian confinement potential quantum well

The properties of an electron weakly coupled to piezo-acoustic phonon in asymmetrical Gaussian confinement potential quantum well (AGCPQW) subject to external electric field (EF) and magnetic field (MF) has been investigated using the Lee-Low-Pines unitary transformation and linear combination operation methods. We have obtained the ground state energy (GSE) and the ground state binding energy (GSBE) of piezoelectric polaron. The effects of the EF, the MF, the range of the asymmetrical Gaussian confinement potential (RAGCP), Debye cut-off wavenumber (DCOW) and the electron–phonon coupling strength on the GSE and the GSBE are also analyzed. It is found that the GSE is an increasing function of the EF and the cyclotron frequency (CF), whereas it is a decreasing one of the RAGCP, the DCOW and electron–phonon coupling strength. The GSBE is an increasing function of the DCOW and the electron–phonon coupling strength. It is also an aggrandizing function with decreasing of the RAGCP, whereas it is a decayed one of the EF and CF. It is shown that the EF, the RAGCP, the MF, the DCOW and electron–phonon coupling strength are important factors that have great influence on the properties of the piezoelectric polaron in AGCPQW.

Classical Yang Mills equations with sources: Consequences of specific scalar potentials

Some well known gauge scalar potential very often considered or used in the literature are investigated by means of the classical Yang Mills equations for the SU(2) subgroups of Nc=3. By fixing a particular shape for the scalar potential, the resulting vector potentials and the corresponding color-charge distributions are found. By adopting the spherical coordinate system, it is shown that spherically symmetric solutions, only dependent on the radial coordinate, are only possible for the Abelian limit, otherwise, there must have angle-dependent component(s). The following solutions for the scalar potential are investigated: the Coulomb potential and a non-spherically symmetric generalization, a linear potential A0(r→)∼(κr), a Yukawa-type potential A0(r→)∼(Ce−r/r0/r) and finite spatial regions in which the scalar potential assumes constant values. The corresponding chromo-electric and chromo-magnetic fields, as well as the color-charge densities, are found to have strong deviations from the spherical symmetric configurations. We speculate these types of non-spherically symmetric configurations may contribute (or favor) for the (anisotropic) confinement mechanism since they should favor color charge-anti-charge (or three-color-charge) bound states that are intrinsically non spherically symmetric with (asymmetric) confinement of fluxes. Specific conditions and relations between the parameters of the solutions are also presented.

Temperature effect of strong-coupling polaron in RbCl quantum wells with double asymmetric confined potentials

In this study, the ground state properties of strong-coupling polaron in RbCl quantum wells (QWs) by using double asymmetric confined potential were theoretically investigated based on effective mass approximation. Ground state binding energy (GSBE), ground state energy (GSE), vibrational frequency (VF), and mean LO phonon number (MLOPN) were obtained as functions of asymmetric confinement potential parameters in QWs by using the unitary transformation method and linear combination operator. GSBE, GSE, VF and MLOPN were found to be increasing functions of asymmetric semi-exponential confinement potential (ASECP) parameter [Formula: see text] and decreasing functions of parameter [Formula: see text]. GSE was rapidly increased with the increase of confinement strength. GSBE, GSE, VF and MLOPN increase as the increasing (decreasing) of anisotropic parabolic confinement potential strength (confinement length). In addition, the temperature effect was evaluated by using quantum statistical theory. With the increase of temperature, GSE and GSBE decreased while VF and MNLOP increased.

Magnetic field and anisotropic parabolic potential effects on ground state binding energy of impurity magnetopolaron in asymmetrical semi-exponential quantum wells

We performed the ground state binding energy (GSBE) and vibrational frequency (VF) of strong-coupling impurity magnetopolaron (SCIM) in asymmetrical semi-exponential quantum wells (ASEQWs) containing a hydrogen-like impurity in the center exposed to a uniform magnetic field through Lee-Low-Pines unitary transformation and linear combination operation method based on effective mass approximation. The results were expressed as functions of GSBE and VF for different magnetic field cyclotron frequency (MFCF) and Coulombic impurity potential (CIP) strength, the positive criteria of ASEQWs along the growth direction of QW and effectual confinement lengths of APCP along the coordinates were analyzed. Our findings revealed that the VF and GSBE were enhanced by increasing magnetic field cyclotron frequency and the parameter of asymmetrical semi-exponential confinement potential (ASECP), which was increased by reducing APCP effectual confinement length along the directions of coordinates. The above observations could be because of quantum size confinement effects.

Exact quantum-mechanical solution for the one-dimensional harmonic oscillator model asymmetrically confined into the infinite well

We constructed a new model of the non-relativistic quantum harmonic oscillator within the canonical approach. It is asymmetrically confined into the infinitely high potential well. Asymmetrical confinement effect is achieved thanks to the introduction of the effective mass that changes with position. It is shown that the model has exact solutions. Analytical expression of its wavefunctions of the stationary states is expressed through the Jacobi polynomials , whereas, energy spectrum is non-equidistant. At the limit, when confinement parameters a and b , defining the position of two walls of the potential well go to infinity, then obtained expressions of the wavefunctions also recover the wavefunctions expressed through the Hermite polynomials. • An exactly solvable new model of the non-relativistic quantum harmonic oscillator asymmetrically confined into the infinitely high potential well. • Asymmetrical confinement effect is achieved thanks to the introduction of the effective mass that changes with position. • Energy spectrum is non-equidistant. • The model has correct limit to the so-called Hermite oscillator when position of both walls of the potential well go to infinity.

How Superhydrophobic Grooves Drive Single-Droplet Jumping.

Rapid shedding of microdroplets enhances the performance of self-cleaning, anti-icing, water-harvesting, and condensation heat-transfer surfaces. Coalescence-induced droplet jumping represents one of the most efficient microdroplet shedding approaches and is fundamentally limited by weak fluid-substrate dynamics, resulting in a departure velocity smaller than 0.3u, where u is the capillary-inertia-scaled droplet velocity. Laplace pressure-driven single-droplet jumping from rationally designed superhydrophobic grooves has been shown to break conventional capillary-inertia energy transfer paradigms by squeezing and launching single droplets independent of coalescence. However, this interesting droplet shedding mechanism remains poorly understood. Here, we investigate single-droplet jumping from superhydrophobic grooves by examining its dependence upon surface and droplet configurations. Using a volume of fluid (VOF) simulation framework benchmarked with optical visualizations, we verify the Laplace pressure contrast established within the groove-confined droplet that governs single-droplet jumping. An optimal departure velocity of 1.13u is achieved, well beyond what is currently available using condensation on homogeneous or hierarchical superhydrophobic structures. We further develop a jumping/non-jumping regime map in terms of surface wettability and initial droplet volume and demonstrate directional jumping under asymmetric confinement. Our work reveals key fluid-structure interactions required for the tuning of droplet jumping dynamics and guides the design of interfaces and materials for enhanced microdroplet shedding for a plethora of applications.

Stimulated emission in the InAs/InAsSb/InAsSbP heterostructures with asymmetric electronic confinement

The electroluminescent characteristics of the InAs/InAs1-ySby/InAsSbP asymmetric light-emitting diode heterostructures with high InSb mole fraction in the active region (y>0.09) in the temperature range 4.2-300 K have been studied. Stimulated emission was achieved in the wavelength range 4.1-4.2 μm at low temperatures (T<30 K). It was found that the electroluminescence spectra were formed as a result of the superposition of contributions from different channels of radiative recombination of charge carriers near the type II heterointerface. The effect of the quality of the type II InAsSb/InAsSbP heterojunction on the radiative interface transitions with an increase in the content of InSb in the ternary solid solution is considered. Keywords: heterojunctions, InAs, antimonides, electroluminescence, light-emitting diodes.

The effect of parabolic potential on ground state energy and vibration frequency of magnetopolaron in asymmetric Gaussian potential quantum wells

Anisotropy parabolic potential (APP) effects on ground state (GS) energy [Formula: see text] and the vibration frequency (VF) [Formula: see text] of weak-coupled magnetopolaron (MP) in asymmetric Gaussian quantum wells (AGQWs) were investigated using the linear combination operator and unitary transformation method. The obtained results showed that [Formula: see text] and [Formula: see text] were increased by increasing the barrier height [Formula: see text] of AGQWs as well as transverse and longitudinal confined strengths [Formula: see text] and [Formula: see text] of APP and decreased with increase in the asymmetric Gaussian confinement potential (AGCP) range [Formula: see text] and transverse and longitudinal effective confined lengths [Formula: see text] and [Formula: see text] of APP. Thus, the GS energy and VF of MP could be changed by adjusting the confinement parameters of the APP and AGCP. The study of quantum wells’ semiconductor materials has broad potential applications in semiconductor lasers, optoelectronic devices and quantum information.