Emission Widths Research Articles

Critical current in a two-dimensional non-magnetically insulated crossed-field gap with monoenergetic emission

Prior studies have developed theories for the maximum permissible current, or critical current, for one-dimensional planar and cylindrical crossed-field diodes where the magnetic field is below the Hull cutoff, meaning that an electron emitted from the cathode reaches the anode. Here, we develop semi-empirical and analytical models to predict the critical current for a two-dimensional (2D) planar diode with nonzero monoenergetic initial velocity. The semi-empirical method considers the geometry, nonzero initial velocity, and magnetic field as multiplicative corrections to the Child–Langmuir law for space-charge limited current in a one-dimensional planar diode with an initial velocity of zero. These results agree well with 2D particle-in-cell (PIC) simulations using the over-injection method to assess virtual cathode formation for different emission widths, magnetic field strengths, and initial velocities. The analytical solution agrees better with PIC results because it accounts for the coupling of the magnetic field, geometry, and initial velocity that the semi-empirical approach does not.

Uniform space-charge-limited current for a two-dimensional planar emitter with nonzero monoenergetic initial velocity

While characterizing space-charge-limited current (SCLC) is important for numerous applications, no analytical solutions for SCLC with monoenergetic initial velocity exist for two-dimensional (2D) geometries. Here, we derive approximate closed-form solutions for uniform SCLC with monoenergetic emission of electrons in a 2D planar diode, where emission is restricted to a long patch of width W for electrodes separated by a distance D. We also derive a semiempirical approach for estimating the SCLC for these cases by treating the geometric and velocity correction factors as multiplicative corrections to the SCLC for a one-dimensional vacuum diode given by the Child–Langmuir (CL) law. We show that the SCLC for a finite patch with nonzero velocity can exceed the CL law by three orders of magnitude. The theoretically calculated SCLCs for various emission widths and initial velocities in the 2D diode agree well with particle-in-cell simulations using the over-injection method in XOOPIC; they agree with the semiempirical relationship for lower initial velocities. In the limit of high initial velocity, the geometry and velocity corrections to the CL law cannot be decoupled, invalidating the assumption of the semiempirical approach and causing it to diverge from the theoretical solution and XOOPIC simulations. These results provide valuable estimates for determining the onset of virtual cathode formation for photocathodes and thermionic cathodes, which operate in the over-injection regime to avoid beam quality degradation.

Modelling observable signatures of jet-ISM interaction: thermal emission and gas kinematics

ABSTRACT Relativistic jets are believed to have a substantial impact on the gas dynamics and evolution of the interstellar medium (ISM) of their host galaxies. In this paper, we aim to draw a link between the simulations and the observable signatures of jet-ISM interactions by analyzing the emission morphology and gas kinematics resulting from jet-induced shocks in simulated disc and spherical systems. We find that the jet-induced laterally expanding forward shock of the energy bubble sweeping through the ISM causes large-scale outflows, creating shocked emission and high-velocity dispersion in the entire nuclear regions (∼2 kpcs) of their hosts. The jetted systems exhibit larger velocity widths (>800 km s−1), broader Position-Velocity maps and distorted symmetry in the disc’s projected velocities than systems without a jet. We also investigate the above quantities at different inclination angles of the observer with respect to the galaxy. Jets inclined to the gas disc of its host are found to be confined for longer times, and consequently couple more strongly with the disc gas. This results in prominent shocked emission and high-velocity widths, not only along the jet’s path, but also in the regions perpendicular to them. Strong interaction of the jet with a gas disc can also distort its morphology. However, after the jets escape their initial confinement, the jet-disc coupling is weakened, thereby lowering the shocked emission and velocity widths.

In the Trenches of the Solar-stellar Connection. IV. Solar Full-disk Scans of C ii, Si iv, and Mg ii by the Interface Region Imaging Spectrograph

Abstract About once a month, the Interface Region Imaging Spectrograph conducts day-long raster scans of the full Sun in three ultraviolet spectral channels. These full-disk mosaics are valuable in the solar context, but provide a unique connection to the distant, unresolved stars. Here, 10 deep-exposure scans (4–8 s per slit step), collected during the peak and decline of sunspot Cycle 24, were analyzed. Spatial spectra (2″ pixels) of resonance lines of C ii (T ∼ 104 K), Si iv (8 × 104 K), and Mg ii (8 × 103 K) were fitted with a pseudo-Gaussian model to track the emission strengths, widths, and shifts in the various surface features that comprise the quiet Sun and active regions. The full-disk mosaic spectra compare well to tracings of solar-twin α Centauri A (HD 128620; G2 V). The contrast between disk-average spectra from cycles MIN and MAX is relatively modest (∼50% in Si iv), but, remarkably, the brightest solar pixels in active regions, at 2″ resolution, exceed the global-average intensities of the most active Sun-like stars, suggesting a deeper solar-stellar connection. Si iv shows a conspicuous bright ring at the limb, whereas optically thicker C ii and Mg ii are suppressed (more so for the latter). The Si iv emission favors the bright knots of the large-scale supergranulation network, while the cooler Mg ii emission is more ubiquitous and C ii intermediate. The non-Gaussian profile of full-disk C iv, similar in formation temperature to Si iv, was previously interpreted as a combination of narrow and broad dynamical components, but the prevalence of the characteristic line shape in the finest resolution spatial pixels of Si iv here provides support for alternative formation scenarios, for example, invoking κ-distributions.

Wigner wave packets: Transmission, reflection, and tunneling

A numerical solution to the time evolution equation of the Wigner distribution function (WDF) with an accuracy necessary to simulate the passage of a wave packet past a barrier is developed, where quantum effects require high accuracy and fine discretization. A wave packet incident on a barrier, a portion of which tunnels through, demonstrates behavior that can define various characteristic transmission and reflection delay (TARD) times useful in the simulation of electron emission. A model for the TARD times is proposed that relies only on the asymptotic maxima of the position $\ensuremath{\rho}(x,t)$ and wave number $\ensuremath{\rho}(k,t)$ densities given by the WDF and applied to a ballistic trajectory model for the (faster) transmitted and (slower) reflected parts. The dependence of the TARD times on barrier width, symmetry, and abruptness is analyzed. For symmetrical barriers with characteristics similar to field emission barrier heights and widths, TARD times are on the order of a fraction of a femtosecond. The TARD times for when tunneling predominates are contrasted to tunneling times in the literature. Use of the TARD times in simulations of field emission in nanogap devices or to model ultrashort pulses generated under rapidly changing conditions for electron sources are proposed.

π-Extended Ladder-Type Conjugated Polymers via BN-Annulation

Two kinds of ladder-type conjugated polymers were concisely synthesized by the formation of single-stranded conjugated polymers via Stille cross-couplings, followed by nitrogen-directed electrophilic borylations at electron-rich aromatic rings. The resulting BN-annulated polymers show good film-forming behaviors and high air and thermal stability. Their structurally shape-persistent rigid backbones render them with π-extended conjugation, allowing for efficient light harvesting in the low-energy regions, and emitting strong fluorescence with narrow emission widths.

R/G/B Micro-LEDs for In-Pixel Integrated Arrays and Temperature Sensing

In this paper, we demonstrate that nitride-based R/G/B micro-LEDs allow temperature sensing through a forward voltage response on both temperature and display devices. In temperature sensing, micro-LEDs operate in current constant mode, exhibiting an excellent performance with a sensitivity of 2.6/2.7/2.2 mV/K for blue/green InGaN micro-LEDs and red AlGaInP micro-LED under an injection current density of 1 mA/cm2, which is about two times lower sensitivity than the sensitivity limit obtained from the Shockley ideal diode equation. It is speculated that the lower sensitivity originated from parasitic resistances. It is also found that the band gaps of ternary InGaN and quaternary AlGaInP obtained from temperature-dependent electroluminescence spectra of red micro-LED agrees with the calculated values by using the semiempirical Varshni relationship. The EL of micro-LEDs is characterized by narrow emission widths of 18–41 nm, leading to wide color gamut covering up to 170/97% of the DCI P3/BT 2020, which are the standard color space of ultrahigh definition television system.

Emissive Nature and Molecular Behavior of Zero-Dimensional Organic-Inorganic Metal Halides Bmpip2MX4.

Zero-dimensional (0D) organic-inorganic metal halides, with their high stability and broadband emission features, have aroused great interest in optoelectronic applications. Metal halides of the type Bmpip2MX4 (M = Pb, Sn, or Ge; X = I or Br) have 0D disphenoidal coordinated structures that offer an excellent opportunity to investigate their emissive nature and molecular behavior. Herein, the photophysical properties and carrier transport behavior of 0D Bmpip2MX4 metal halides are studied by using density functional theory. Our results indicate that Bmpip2MX4 metal halides present broadband emission widths and significant Stokes shifts. In particular, Bmpip2SnBr4 possesses the largest Stokes shift (1.981 eV) and the shortest exciton self-trapping time, demonstrating the best photoluminescence emission ability. Bmpip2GeI4 exhibits the lowest electron-hole creation energy and the best photoresponse capacity. Moreover, Bmpip2PbI4 demonstrates superior transport capabilities with high carrier mobilities of 4.56 × 10-3 and 2.51 × 10-7 cm2 V-1 s-1 for hole and electron carriers, respectively, which makes it comparable even with typical hole transport materials (e.g., RR P3HT, ∼10-4 cm2 V-1 s-1). These findings highlight exciting opportunities for the future development and application of such kinds of 0D metal halides in optoelectronics.

3D xRAGE simulation of inertial confinement fusion implosion with imposed mode 2 laser drive asymmetry

Low-mode asymmetries represent an important obstacle to achieving high-gain inertial confinement fusion implosions. As a step in learning how to control such effects, an OMEGA experiment with imposed mode 2 laser drive asymmetries was done to study the expected signatures of this type of asymmetry [M. Gatu Johnson et al., PRE 2018]. In the present work, a 3D xRAGE simulation including the stalk mount has been brought to bear on the data from that experiment. Comprehensive comparisons between simulated and measured observables are made. Good agreement between simulated and measured x-ray image-inferred shell trajectories, bang times and neutron emission widths are seen, showing that the hydrodynamics are well captured in the simulation. Asymmetries seen in simulated and measured time-resolved and time-integrated x-ray images and areal densities also compare well, showing impact of both stalk and mode 2. On the other hand, important differences in measured and simulated neutron emission histories, yield, and ion temperature (Tion) asymmetries are seen, suggesting that the simulation is overestimating shock yield. The results clearly demonstrate the importance of considering all asymmetry sources when interpreting measured signatures of asymmetry.

Low-Temperature Electroluminescence Excitation Mapping of Excitons and Trions in Short-Channel Monochiral Carbon Nanotube Devices

Single-walled carbon nanotubes as emerging quantum-light sources may fill a technological gap in silicon photonics due to their potential use as near-infrared, electrically driven, classical or nonclassical emitters. Unlike in photoluminescence, where nanotubes are excited with light, electrical excitation of single tubes is challenging and heavily influenced by device fabrication, architecture, and biasing conditions. Here we present electroluminescence spectroscopy data of ultra-short-channel devices made from (9,8) carbon nanotubes emitting in the telecom band. Emissions are stable under current biasing, and no enhanced suppression is observed down to 10 nm gap size. Low-temperature electroluminescence spectroscopy data also reported exhibit cold emission and line widths down to 2 meV at 4 K. Electroluminescence excitation maps give evidence that carrier recombination is the mechanism for light generation in short channels. Excitonic and trionic emissions can be switched on and off by gate voltage, and corresponding emission efficiency maps were compiled. Insights are gained into the influence of acoustic phonons on the line width, absence of intensity saturation and exciton-exciton annihilation, environmental effects such as dielectric screening and strain on the emission wavelength, and conditions to suppress hysteresis and establish optimum operation conditions.

Controllable Synthesis of All Inorganic Lead Halide Perovskite Nanocrystals with Various Appearances in Multiligand Reaction System.

All inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals (PNCs) exhibit promising applications in light-emitting devices due to their excellent photophysical properties. Herein, we developed a low-cost and convenient method for the preparation of CsPbX3 PNCs in a multiligand-assisted reaction system where peanut oil is applied as a ligand source. The mixed-halide PNCs with tunable optical-band gap were prepared by mixing the single-halide perovskite solutions at room temperature. The resulting PNCs had good monodispersity, with dimensions of 8–10 nm, high photoluminescence quantum yield (96.9%), narrow emission widths (15–34 nm), and tunable emission wavelength (408–694 nm), covering the entire visible spectrum. Additionally, various morphologies of PNCs, such as nanospheres, nanocubes, and nanowires, were obtained by controlling reaction temperature and time, and the amount of oleamine with multiple ligands in peanut oil potentially playing a dominant role in the nucleation/growth processes of our PNCs. Finally, the resulting CsPbBr3 PNCs were employed to develop a white light-emitting diode (WLED), demonstrating the potential lighting applications for our method.

Role of dynamical deformation in pre-scission neutron multiplicity

The light-particle emission probability from an excited compound nucleus depends explicitly on the time-evolution of the system as the available internal excitation energy and, consequently, the particle decay widths depend on the instantaneous deformation of the nucleus. The Langevin dynamical model for fission is employed to extract this deformation dependence in pre-scission particle multiplicities by following the propagation of fissioning trajectories up to scission. The variation of particle decay widths with nuclear deformation is accounted more precisely in comparison to the existing calculations. The number of neutrons emitted from different configurations of the compound nucleus are calculated for a detailed analysis. The deformation dependence of particle emission widths is found to be relevant for highly fissile systems where the dynamics is primarily governed by the saddle to scission motion. This dynamical effect essentially predicts the nuclear shape evolution through evaporated light particles and, for a heavy compound system, simultaneous measurement of neutron multiplicities for fission and evaporation residue events can reveal its intricate nature.

Prompt particle emission in correlation with fission fragments

The de-excitation process of primary fission fragments can be simulated with the FIFRELIN Monte Carlo code leading to an estimation of prompt fission observables such as neutron/gamma multiplicities and spectra in correlation with fission fragments. De-excitation cascades are simulated using the notion of nuclear realization following Becvar terminology generalized to neutron/gamma coupled emission. A nuclear realization is a random set of nuclear levels (energy, spin, parity) in association with partial widths for neutron, gamma or electron emission. Experimental data related to electromagnetic transitions in the discrete level region are taken from RIPL-3 database. When nuclear level structure is completely unknown (in the continuum region), level density and strength function models are used. In between these regions, our partial knowledge of nuclear structure is completed by models up to a fixed maximum level density. In this way the whole available experimental information is accounted for. FIFRELIN is ruled by five free input parameters driving the excitation energy sharing, the rotational energy and the spin distribution of primary fission fragments. These five free parameters are determined to match a target observable such as the average total prompt neutron multiplicity (ν). Once this procedure is completed, the whole set of fission observables can be compared with experimental results. Obviously the number of observables obtained within this code is higher than what is available from measurements. This code can therefore provide useful insights into the compatibility between models and a whole set of fission observables. In the present work the influence of shell corrections is reported on level densities and prompt fission neutron spectra (PFNS). The impact of the input data such as primary fission fragment total kinetic energy (TKE) is also addressed. Average prompt neutron multiplicity as a function of TKE is also estimated for each mass split and compared to recent measurements. The presence of structures in the calculations (especially for light nuclei) is clearly related to the nuclear level scheme. Various situations occur and an overestimation (or underestimation) of the calculated number of emitted neutrons can be correlated to the light or heavy fragment of a pair and to a restricted energy range. In addition prompt fission gamma spectra (PFGS) are estimated for selected fragment mass ranges and compared to recent measurements. In this way the presence of specific gamma-ray transitions can be established.

New model of the cascade gamma decay of neutron resonances for practitioners: Basic concepts and attainable precision

A model-free extraction of level densities and radiative strength functions for cascade dipole gamma transitions that has been performed to date revealed the need for developing a new model of the deexcitation of compound states of nuclei having arbitrary masses and belonging to any type over a broad range of their excitations. Such a model should provide a practical means for determining thresholds for the breaking of nucleon Cooper pairs below the neutron binding energy, the relationship between the level densities for excitations of the quasiparticle and phonon types, and the emission widths for nuclear-reaction products in excited nuclei. From an analysis of data on the intensities of two-step cascades initiated by radiative thermal-neutron capture, it can be seen that the highest precision in describing available experimental spectra is reached upon taking into account the breaking of three to four Cooper pairs of nucleons.

Determination of human albumin in serum and urine samples by constant-energy synchronous fluorescence method.

A sensitive spectrofluorimetric method using constant-energy synchronous fluorescence technique is proposed for the determination of human albumin without separation. In this method, no reagent was used for enhancement of the fluorescence signal of albumin in the solution. Effects of some parameters, such as energy difference between excitation and emission monochromators (ΔE), emission and excitation slit widths and scan rate of wavelength were studied and the optimum conditions were established. For this purpose factorial design and response surface method were employed for optimization of the effective parameters on the fluorescence signal. The results showed that the scan rate of the wavelength has no significant effect on the analytical signal. The calibration curve was linear in the range 0.1-220.0 µg mL(-1) of albumin with a detection limit of 7.0 × 10(-3) µg mL(-1). The relative standard deviations (RSD) for six replicate measurements of albumin were calculated as 2.2%, 1.7% and 1.3% for 0.5, 10.0 and 100.0 µg mL(-1) albumin, respectively. Furthermore the proposed method has been employed for the determination of albumin in human serum and urine samples.

Spectroscopy of z ∼ 7 candidate galaxies: using Lyman α to constrain the neutral fraction of hydrogen in the high-redshift universe★

Following our previous spectroscopic observations of $z>7$ galaxies with Gemini/GNIRS and VLT/XSHOOTER, which targeted a total of 8 objects, we present here our results from a deeper and larger VLT/FORS2 spectroscopic sample of Wide Field Camera 3 selected $z>7$ candidate galaxies. With our FORS2 setup we cover the 737-1070nm wavelength range, enabling a search for Lyman-$\alpha$ in the redshift range spanning 5.06 - 7.80. We target 22 $z$-band dropouts and find no evidence of Lyman-$\alpha$ emission, with the exception of a tentative detection ($<5\sigma$, which is our adopted criterion for a secure detection) for one object. The upper limits on Lyman-$\alpha$ flux and the broad-band magnitudes are used to constrain the rest-frame Equivalent Widths for this line emission. We analyse our FORS2 observations in combination with our previous GNIRS and XSHOOTER observations, and suggest that a simple model where the fraction of high rest-frame Equivalent Width emitters follows the trend seen at $z=3-6.5$ is inconsistent with our non-detections at $z\sim7.8$ at the 96\% confidence level. This may indicate that a significant neutral HI fraction in the intergalactic medium suppresses Lyman-$\alpha$, with an estimated neutral fraction $\chi_{HI}\sim0.5$, in agreement with other estimates.

Evaluating emission widths of giant dipole resonances in a Fermi liquid model

In the theory of the finite fermi-systems [1], it was shown that giant resonances in nuclei can be consider as the zero-sound excitations which exhaust the large part of the energy-weighted sum rules. In the framework of [1] the solutions of the zero-sound dispersion equation in the symmetric nuclear matter, \omega_s(k), are considered. The method of calculation of these solutions is based on the analytical structure of the polarization operators \Pi(\omega,k). The solutions of the dispersion equation, which are real at small k, become complex with k increasing when the overlapping of the collective and 1p1h modes starts. The imaginary part of \omega_s(k) is the result of the collective zero-sound excitation decay to the real particle-hole pairs and can be compared with the escape width of resonances. We compare the experimental energy and escape width of the giant dipole resonance (GDR) in the nucleus A with Re\omega_s(k) and Im\omega_s(k) taken at a definite wave vector k=k_A.

No evidence for Lyman emission in spectroscopy of z &gt; 7 candidate galaxies

We present Gemini/GNIRS spectroscopic observations of 4 z-band (z~7) dropout galaxies and VLT/XSHOOTER observations of one z-band dropout and 3 Y-band (z~8-9) dropout galaxies in the Hubble Ultra Deep Field, which were selected with Wide Field Camera 3 imaging on the Hubble Space Telescope. We find no evidence of Lyman-alpha emission with a typical 5-sigma sensitivity of 5X10^-18erg/cm^2/s, and we use the upper limits on Lyman-alpha flux and the broad-band magnitudes to constrain the rest-frame equivalent widths for this line emission. Accounting for incomplete spectral coverage, we survey 3.0 z-band dropouts and 2.9 Y-band dropouts to a Lyman-alpha rest-frame equivalent width limit > 120Ang (for an unresolved emission line); for an equivalent width limit of 50Ang the effective numbers of drop-outs surveyed fall to 1.2 z-band drop-outs and 1.5 Y-band drop-outs. A simple model where the fraction of high rest-frame equivalent width emitters follows the trend seen at z=3-6.5 is inconsistent with our non-detections at z=7-9 at the ~ 1-sigma level for spectrally unresolved lines, which may indicate that a significant neutral HI fraction in the intergalactic medium suppresses the Lyman-alpha line in z-drop and Y-drop galaxies at z > 7.

Effects of Coriolis and residual neutron–proton interactions in the proton emission from 130Eu

We present for the first time, the non-adiabatic quasiparticle approach to study proton emission from odd–odd deformed nuclei which includes the residual neutron–proton interaction. We perform exact calculations of decay widths for the proton emission from 130Eu where the Coriolis effects are included in both the parent and daughter wavefunctions. Without assuming an exact Nilsson state from where the decay could happen, we could explain the experimentally observed half-life. We discuss the interplay between the Coriolis and the residual np interaction as well. This is significant because whereas most of the studies on odd–odd nuclei concern only the energies, a description of decay can test the details of the wavefunctions which may be strongly influenced by the residual interaction.

Observation and modelling of main-sequence star chromospheres - XIX. FIES and FEROS observations of dM1 stars

We present 187 high-resolution spectra for 62 different M1 dwarfs from observations obtained with the FIbre-fed Echelle Spectrograph (FIES) on the Nordic Optical Telescope (NOT) and from observations with the Fibre-fed Extended Range Echelle Spectrograph (FEROS) from the European Southern Observatory (ESO) data base. We also compiled other measurements available in the literature. We observed two stars, Gl 745A and Gl 745B, with no Ca ii line core emission and Hα line equivalent widths (EWs) of only 0.171 and 0.188 Å, respectively. We also observed another very low activity M1 dwarf, Gl 63, with an Hα line EW of only 0.199 Å. These are the lowest activity M dwarfs ever observed and are of particular interest for the non-local thermodynamic equilibrium radiative transfer modelling of M1 dwarfs. Thanks to the high signal-to-noise ratio of most of our spectra, we were able to measure the Ca ii H&K full width at half-maximum (FWHM) for most of our stars. We find good correlations between the FWHM values and the mean Ca ii line EW for dM1 stars. Then the FWHM seems to saturate for dM1e stars. Our previous models of M1 dwarfs can reproduce the FWHM for dM1e stars and the most active dM1 stars, but fail to reproduce the observations of lower activity M1 dwarfs. We believe this is due to an effect of metallicity. We also investigate the dependence of the Hα line FWHM as a function of its EW. We find that the models globally agree with the observations including subwarfs, but tend to produce too narrow profiles for dM1e stars. We re-investigate the correlation between the Ca ii line mean EW and the absolute magnitude. With our new data that notably include several M1 subdwarfs, we find a slightly different and better correlation with a slope of −0.779 instead of −0.909. We also re-investigate the variations of the Hα line EW as a function of radius and find that the EW increases continuously with increasing radius. This confirms our previous finding that the level of magnetic activity in M1 dwarfs increases with the radius. For the first time, we investigate the Wilson–Bappu correlation for a given spectral type. We find a rather linear correlation for stars of absolute magnitude greater than 9.6, but below this value the FWHM seems to saturate. In fact, we show that these Wilson–Bappu type correlations are activity–FWHM correlations and are due to the diminishing column mass of the transition region with decreasing activity level.