Additional External Field Research Articles

Optical gain and entanglement through dielectric confinement and electric field in InP quantum dots.

Quantum dots are widely recognized for their advantageous light-emitting properties. Their excitonic fine structure along with the high quantum yields offers a wide range of possibilities for technological applications. However, especially for the case of colloidal QDs, there are still characteristics and properties which are not adequately controlled and downgrade their performance for applications which go far beyond the simple light emission. Such a challenging task is the ability to manipulate the energetic ordering of exciton and biexciton emission and subsequently control phenomena such as Auger recombination, optical gain and photon entanglement. In the present work we attempt to engineer this ordering for the case of InP QDs embedded in polymer matrix, by means of their size, the dielectric confinement and external electric fields. We employ well tested, state of the art theoretical methods, in order to explore the conditions under which the exciton-biexciton configuration creates the desired conditions either for optical gain or photon entanglement. Indeed, this appears to be feasible for QDs with small diameters (1 nm, 1.5 nm) embedded in a host material with high dielectric constant and additional external electric fields. These findings offer a new design principle which might be complementary to the well-established type II core-shell QDs approach for achieving electron-hole separation.

Study of characteristics elementary atomic processes in the neon-like multicharged ions plasma within an energy approach

We present the theoretical foundations of an advanced relativistic approach to computing the main energy, spectral characteristics of radiative-collisional processes in the plasma (in particular, the Debye plasma) of atomic as the example, neon-like) ions with simultaneous, quantitatively consistent consideration of the complex relativistic, interelectron exchange-correlation and plasma environment effects. The approach is based on the combination of a relativistic energy approach (S-matrix Gell-Mann and Low formalism), the relativistic gauge-invariant many-body perturbation theory with optimized Dirac-Fock-Sturm and Debye-Hückel approximations with accounting for the plasma environment effects with possible generalization on the presence of an additional external electromagnetic field. The fundamental point of our approach is the selection of the optimized Dirac-Fock-Sturm zeroth approximation and application of the consistent procedure for constructing a one-quasiparticle representation (basis’s of relativistic wave functions) in compliance with the principle of gauge invariance, in particular, by minimizing a gauge-noninvariant contributions to the radiative widths of the atomic (ionic) levels due to the complex exchange-correlation effects. The electron-ion collision strength and the dioelect5ron capture rate etc are determined within the presented theory.

Multiphysics Modeling of Impurity Transport for FNSF Startup Scenario with ERO2.0

This study focuses on performing a multiphysics study using the ERO2.0 and UEDGE codes for two standard double null configurations for the Fusion Nuclear Science Facility: (a) 100% recycling and (b) 99% recycling. Results show that the main contributor to tungsten erosion along the divertor plates is impurities from the midplane waveguides. In addition, the standard high-recycling case (100% recycling) shows a significantly higher buildup of impurities along the divertor tiles during the startup phase, which can lead to a higher increase of energy loss in the plasma during steady-state operation. Last, for high recycling, anomalous diffusion can dominate over parallel field diffusion. The work performed in this study can be iteratively applied to a full operation scenario with additional physics such as those from neutrals, wall shaping, and additional external fields.

Combining sensors and actuators with electrowetting-on-dielectric (EWOD): advanced digital microfluidic systems for biomedical applications.

The concept of digital microfluidics (DMF) enables highly flexible and precise droplet manipulation at a picoliter scale, making DMF a promising approach to realize integrated, miniaturized "lab-on-a-chip" (LOC) systems for research and clinical purposes. Owing to its simplicity and effectiveness, electrowetting-on-dielectric (EWOD) is one of the most commonly studied and applied effects to implement DMF. However, complex biomedical assays usually require more sophisticated sample handling and detection capabilities than basic EWOD manipulation. Alternatively, combined systems integrating EWOD actuators and other fluidic handling techniques are essential for bringing DMF into practical use. In this paper, we briefly review the main approaches for the integration/combination of EWOD with other microfluidic manipulation methods or additional external fields for specified biomedical applications. The form of integration ranges from independently operating sub-systems to fully coupled hybrid actuators. The corresponding biomedical applications of these works are also summarized to illustrate the significance of these innovative combination attempts.

Aspects of Bose-Einstein condensation in a charged boson system over the dielectric surface

We study theoretically a gas consisting of charged bosons (ions) over the flat dielectric surface at low temperatures and its tendency to form a state with a Bose-Einstein condensate. For the stability of a system, an additional external electric field, which keeps charges at the dielectric surface, is introduced. The formalism is developed in the framework of a self-consistent-field approach, which combines the quasiclassical description in terms of the Wigner distribution functions and the quantum-mechanical approach by employing the Gross-Pitaevskii equation. We predict a formation of the state with a Bose-Einstein condensate and determine the near-critical physical characteristics of the system. It is shown that the thermal and condensate components become spatially separated under these conditions. We discuss the limitations of the developed semiclassical approach and prospects for the pure quantum-mechanical treatment of the problem.

Self-bound droplet clusters in laser-driven Bose-Einstein condensates

We investigate a two-dimensional Bose-Einstein condensate that is optically driven via a retro-reflecting mirror, forming a single optical feedback loop. This induces a peculiar type of long-range atomic interaction with highly oscillatory behavior, and we show here how the sign of the underlying interaction potential can be controlled by additional optical elements and external fields. This additional tunability enriches the behavior of the system substantially, and gives rise to a surprising range of new ground states of the condensate. In particular, we find the emergence of self-bound crystals of quantum droplets with various lattice structures, from simple and familiar triangular arrays to complex superlattice structures and crystals with entirely broken rotational symmetry. This includes mesoscopic clusters composed of small numbers of quantum droplets as well as extended crystalline structures. Importantly, such ordered states are entirely self-bound and stable without any external in-plane confinement, having no counterpart to other quantum-gas settings with long-range atomic interactions.

Measuring the frequency chirp of extreme-ultraviolet free-electron laser pulses by transient absorption spectroscopy

High-intensity ultrashort pulses at extreme ultraviolet (XUV) and x-ray photon energies, delivered by state-of-the-art free-electron lasers (FELs), are revolutionizing the field of ultrafast spectroscopy. For crossing the next frontiers of research, precise, reliable and practical photonic tools for the spectro-temporal characterization of the pulses are becoming steadily more important. Here, we experimentally demonstrate a technique for the direct measurement of the frequency chirp of extreme-ultraviolet free-electron laser pulses based on fundamental nonlinear optics. It is implemented in XUV-only pump-probe transient-absorption geometry and provides in-situ information on the time-energy structure of FEL pulses. Using a rate-equation model for the time-dependent absorbance changes of an ionized neon target, we show how the frequency chirp can be directly extracted and quantified from measured data. Since the method does not rely on an additional external field, we expect a widespread implementation at FELs benefiting multiple science fields by in-situ on-target measurement and optimization of FEL-pulse properties.

External Electric Fields Interrupt the Concerted Cope Rearrangement of Semibullvalene.

The topic of this paper is whether the mechanism of the degenerate Cope rearrangement of semibullvalene can be affected by the presence of electrostatic fields. Herein, we report that the shape of the energy surface, as demonstrated by an "interrupted" (stepwise) mechanism, is altered in the presence of a copper cation, Cu+. Natural bond-orbital and block-localized wave-function energy decomposition analyses suggest that orbital and electrostatic interactions play a major role in altering the shape of the energy surface. Applying additional external electric fields (EEFs) induces a significant change to the energy surface with Cu+ present but negligible effects in the absence of Cu+. These findings are consistent with recent studies that demonstrate that EEFs more readily stabilize/destabilize systems with larger, more polarizable, dipole moments.

Structure of two-dimensional electron clusters with various shapes, at low temperatures

The molecular dynamics method is used to study the structure of two-dimensional clusters that can be created in electronic systems above the surface of superfluid helium, using triangular, square, hexagonal and round holding electrode geometries. Clusters with a fixed surface density of 108 cm−2, but different numbers of particles (from 3 to 406) are considered. The spatial configurations of particles in clusters are calculated corresponding to a temperature of 10−2 K. The shape of the cluster significantly affects the arrangement of the electrons inside. If the symmetry of the cluster is consistent with the triangular symmetry of the electronic crystal, then for some numbers of particles, clusters with ideal defect-free packing are possible; otherwise, the clusters contain defects. An additional external field applied to the cluster significantly changes the configuration of its particles.

Active Micromotor Systems Built from Passive Particles with Biomimetic Predator-Prey Interactions.

Inspired by chasing-escaping behaviors of predator and swarming prey in nature, here we demonstrate a concept to create active micromotor systems from two species of passive microparticles with biomimetic predator-prey interactions. In this concept, the biomimetic predator-prey interactions are established in a binary particle system comprising the diffusiophoretic attractive microparticles (prey particles) and the diffusiophoretic repulsive ones (predator particles). In the absence of additional chemical fuels and external fields, the predator particles are attracted by and constantly chase the swarming prey particles, which, in response, escape from the former and show dynamic group reconfigurations because of the local repulsion. Based on this concept, various synthetic active micromotor systems have been demonstrated, including active ZnO-TiO2, Ag3PO4-TiO2, and ZnO-AgBr micromotor systems. As the predator and prey particles are powered by each other through the biomimetic predator-prey interactions, the concept proposed here provides an advanced method to develop not only a class of single micromotors powered by passive particles or "solid fuels" but also micromotor swarms capable of manipulating "moving cargo". In addition, it also illustrates a proof-of-concept implementation of intelligent micro/nanomotor systems composed of heterogeneous individuals with complementary or cooperative functions.

The effect of electron heating on hydrocarbon plasma decay after high-voltage nanosecond discharge

An experimental study of plasma decay in the afterglow of a high-voltage nanosecond discharge in ethane:argon mixtures was performed under conditions when the mean electron energy significantly exceeded the mean energy of neutral particles at room temperature and pressures from 2 to 10 Torr. Electrons were heated during the plasma decay (i) by retarding electron thermalization through a strong dilution of the hydrocarbon with argon and (ii) by the application of an additional external electric field. Using the microwave interferometer, the temporal evolution of the electron density during plasma decay was studied in the range from 5 × 1010 to 3 × 1012 cm−3. The analysis of the experimental data allowed us to obtain the dependence of the effective recombination coefficients of electrons with hydrocarbon ions on the electron temperature. A simplified kinetic scheme was proposed for the description of electron and ion reactions during the plasma decay for our experimental conditions. Calculations on the basis of this scheme showed that the plasma decay is controlled by the dissociative recombination of molecular and cluster hydrocarbon ions.

Modified hydrodynamic equations of quantum liquid, taking into account quantum shell effects

In this paper, the modified hydrodynamic equations of a quantum system consisting of a degenerate electron gas and a classical ion gas, which take into account quantum shell effects, are derived. Two approaches are considered. The first is based on the modification of hydrodynamic equations. The modified equations take into account the shell effects by introducing an additional external field into the equations of motion for the electronic component. The second approach is based on a simplified description of the DFT modeling of the electron distribution for a given ionic distribution. In deriving the self-consistent system of equations, the Green’s function method is used in the coordinate representation, previously successfully applied in analyzing quantum shell effects in a gas bubble of submicron size.

In situ surface‐enhanced Raman spectroscopic monitoring electrochemical and surface plasmon resonance synergetic catalysis on dehydroxylation of PHTP at Ag electrodes

AbstractSurface plasmon resonance (SPR)‐driven heterogeneous catalytic reactions have attracted considerable interests. However, the application of SPR was restricted in few types of surface reaction; the extension of SPR‐catalyzed reaction still remains significant challenge. The introduction of additional external fields is expected to generate the synergetic effect for improving the performance of SPR catalytic reaction. Herein, the roughened Ag electrode played as a substrate for introducing the second external field, that is, electrochemical control. A probe molecule, p‐hydroxythiophenol (PHTP), was preanchored onto Ag electrodes and surface‐enhanced Raman spectroscopy was employed to monitor the surface processes. It demonstrated that the PHTP underwent the dehydroxylation reaction to produce the thiophenol (TP) by the appropriate excitation line and potentials. It disappeared on Ag roughened electrodes without electrochemical control or on a smooth charged surface attached with shell‐isolated nanoparticles. It suggested that the dehydroxylation reaction was contributed by the synergetic effects of localized SPR and applied potential. The results revealed that the efficiency of dehydroxylation was critically depended on wavelength and power of laser, potential, as well as solution pH. The hydrogen sources were essential for dehydroxylation and protonated hydroxyl group promoted the process for producing TP, thus, the dehydroxylation of PHTP was more favorable in acidic solution than that in a neutral case, while it was inhibited completely in alkaline environment. With the appropriate potential, the size of the nanostructures on Ag electrode surface and the corresponding surface plasmon band resulted in the higher efficiency with matched laser wavelength at about 532 nm. The synergetic effects of SPR and potential allowed the extension of the SPR catalysis to a new catalogue of surface reaction, that is, dehydroxylation. It was beneficial to develop the SPR catalysis as a promising approach in the surface chemistry.

Hybrid Particle-Field Molecular Dynamics Simulations of Charged Amphiphiles in an Aqueous Environment.

We develop and test specific coarse-grained models for charged amphiphilic systems such as palmitoyloleoylphosphatidylglycerol (POPG) lipid bilayer and sodium dodecyl sulfate (SDS) surfactant in an aqueous environment, to verify the ability of the hybrid particle-field method to provide a realistic description of polyelectrolytes. According to the hybrid approach, the intramolecular interactions are treated by a standard molecular Hamiltonian, and the nonelectrostatic intermolecular forces are described by density fields. Electrostatics is introduced as an additional external field obtained by a modified particle-mesh Ewald procedure, as recently proposed [Zhu et al. Phys. Chem. Chem. Phys. 2016 , 18 , 9799 ]. Our results show that, upon proper calibration of key parameters, electrostatic forces can be correctly reproduced. Molecular dynamics simulations indicate that the methodology is robust with respect to the choice of the relative dielectric constant, yielding the same correct qualitative behavior for a broad range of values. In particular, our methodology reproduces well the organization of the POPG bilayer, as well as the SDS concentration-dependent change in the morphology of the micelles from spherical to microtubular aggregates. The inclusion of explicit electrostatics with good accuracy and low computational cost paves the way for a significant extension of the hybrid particle-field method to biological systems, where the polyelectrolyte component plays a fundamental role for both structural and dynamical molecular properties.

Linear and non-linear dynamics of the free and reference layers in a sub-40 nm magnetic tunnel junction: a micromagnetic study

Micromagnetic simulations are exploited to analyse the spectrum of dynamical spin eigenmodes in a rectangular MgO-based magnetic tunnel junction (MTJ), with sub-40 nm lateral size, as those exploited in current read heads. It is found that, in spite of the reduced lateral dimensions of this structure, there are several eigenmodes in the range between 0 and 20 GHz, whose frequency and spatial character are markedly dependent on the non-uniformity of the internal field, as well as on the characteristics of the external bias field that is provided by an integrated permanent magnet. The frequency evolution of the modes with the intensity of an additional external field is such that it exists a range of values where the two main eigenmodes of the stack get very close to each other and interchange their preferential localization in the free and reference layers. If one moves from analysis of the small-amplitude magnetization precession to larger amplitude dynamics (precession angles exceeding a few degrees, as it occurs in current MTJ sensors), significant non-linear effects appear in the spectra, such as second harmonics and extra-peaks due to mode mixing. In addition, a sizeable low-frequency tail, that can negatively impact the signal-to-noise ratio of read heads and MTJ sensors, is found in the sub GHz range. These results suggest that a detailed understanding of the dynamical properties of these devices is crucial in order to tailor and optimize their performance.

A porous Zn-based metal-organic framework for pH and temperature dual-responsive controlled drug release

A porous metal-organic framework Zn-TBDA (TBDA = 4′-(1H-tetrazol-5-yl) -[1,1′-biphenyl]-3,5-dicarboxylic acid) with one-dimensional channels has been synthesized from zinc ions and bifunctional ligands. The negligible cytotoxicity of Zn-TBDA is confirmed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium (MTT) assay and microscopy observation. The anticancer drug methotrexate (MTX) is loaded into Zn-TBDA successfully via in-situ encapsulation and the loading capability reaches about 12.59 wt% in resulting composite MTX-encapsulated Zn-TBDA. Under different physiological microenvironments, MTX-encapsulated Zn-TBDA exhibits sensitive pH and temperature dual-responsive release manners, revealing its practical value as a smart drug carrier with controlled release in certain site and improved therapeutic efficacy without any additional external fields.

Additional external electromagnetic fields for laser microprocessing of metals.

Ultra-short pulsed laser processing is a potent tool for microstructuring of a lot of materials. At certain laser parameters, particular periodical and/or quasi-periodical µm-size surface structures evolve apparently during processing. With extended plasmonics theory, it is possible to predict the structure formation, and a systematic technology can be derived to alter the surface for laser processing. In this work, we have demonstrated the modification of the laser processing with applying tailored dynamic surface electro-magnetic fields. Possible improvement in applications is seen in the fields of process efficiency of laser ablation and a superior control of the surface topography.

Electronic spectrum of non-tetrahedral acceptors in CdTe:Cl and CdTe:Bi,Cl single crystals

The electronic spectra of complex acceptors in compensated CdTe:Cl, CdTe:Ag,Cl, and CdTe:Bi,Cl single crystals are studied using low-temperature photoluminescence (PL) measurements under both nonresonant and resonant excitation of distant donor–acceptor pairs (DAP). The wavelength modulation of the excitation source combined with the analysis of the differential PL signal is used to enhance narrow spectral features obscured because of inhomogeneous line broadening and/or excitation transfer for selectively excited DAPs. For the well-known tetrahedral (TD) AgCd acceptor, the energies of four excited states are measured, and the values obtained are shown to be in perfect agreement with the previous data. Moreover, splitting between the 2P3/2 (Г8) and 2S3/2 (Г8) states is clearly observed for AgCd centers located at a short distance (5–7 nm) from a hydrogen-like donor (ClTe). This splitting results from the reduction of the TD symmetry taking place when the acceptor is a member of a donor–acceptor pair. For the Cl-related complex acceptor with an activation energy of ∼121 meV (A-center), the energies of eight excited states are measured. It is shown that this defect produces low-symmetry central-cell correction responsible for the strong splitting of S-like TD shells. The energy spectrum of the Bi-related shallow acceptor with an activation energy of ∼36 meV is measured as well. The spectrum obtained differs drastically from the hydrogen-like set of levels, which indicates the existence of repulsive low-symmetry perturbation of the hydrogen-like Coulomb potential. It is also shown that the spectra of selectively excited PL recorded for a macroscopic ensemble of distant donor–acceptor pairs allow one to detect the low symmetry of acceptors of a given type caused by their complex nature or by the Jahn–Teller distortion. This method does not require any additional (external) field and is applicable to acceptors in diverse zinc-blende compound semiconductors.

Polyatomic ultralong-range Rydberg molecules in external fields

SynopsisWe consider an ultralong-range polytonic Rydberg molecule formed by a Rydberg atom and two heteronuclear diatomic molecules. We focus here on the interaction of a Rb(n = 24, l ≥ 3) Rydberg atom with two KRb diatomic polar molecules in their rotational ground state. The polar diatomic molecules are allowed to rotate in the electric fields generated by the Rydberg electron and core. We investigate the electronic structure of this ultralong-range polytomic Rydberg molecule, and rovibrational properties of the two polar dimers as an additional external field is applied.

MULTIWAVELENGTH OBSERVATIONS AND MODELING OF 1ES 1959+650 IN A LOW FLUX STATE

We report on the VERITAS observations of the high-frequency peaked BL Lac object 1ES 1959+650 in the period 2007-2011. This source is detected at TeV energies by VERITAS at 16.4 standard deviation (sigma) significance in 7.6 hours of observation in a low flux state. A multiwavelength spectral energy distribution (SED) is constructed from contemporaneous data from VERITAS, Fermi-LAT, RXTE PCA, and Swift UVOT. Swift XRT data is not included in the SED due to a lack of simultaneous observations with VERITAS. In contrast to the orphan gamma-ray flare exhibited by this source in 2002, the X-ray flux of the source is found to vary by an order of magnitude, while other energy regimes exhibit less variable emission. A quasi-equilibrium synchrotron self-Compton model with an additional external radiation field is used to describe three SEDs corresponding to the lowest, highest, and average X-ray states. The variation in the X-ray spectrum is modeled by changing the electron injection spectral index, with minor adjustments of the kinetic luminosity in electrons. This scenario produces small-scale flux variability of order >~2 in the HE (E>1 MeV) and VHE (E>100 GeV) gamma-ray regimes, which is corroborated by the Fermi-LAT, VERITAS, and Whipple 10m telescope light curves.