Excess Electron Densities Research Articles

The lower dayside ionosphere of Mars from 14 years of MaRS radio science observations

This work uses a subset of “quiet” MaRS ionospheric dayside observations (MaRSquiet, 2004–2017) and a 1-D photochemical model (IonA-2) to investigate the potential formation processes of the excess electron densities merged with the base of the main ionosphere (Mm). 42% of the investigated MaRS observations contain identified Mm, which occur in a large variety of shapes ranging from smoothly decreasing electron densities to peak structures below the base of M1. The Mm appear over the full range of accessible solar zenith angles (50° - 90°) and are found between approximately 70 and 110 km altitude. Their base is found on average deeper in the atmosphere than the base of the averaged undisturbed MaRS electron density profiles. This indicates a dependence of the Mm formation on energy sources that penetrate deep into the atmosphere. This is supported by a strong positive correlation with increasing solar activity when solar flares, coronal mass ejections, and enhanced short solar X-ray and Ly-α intensities are more common. No relationship is found between the Mm occurrence rate and the magnitude/inclination of the weak crustal crustal magnetic field in MaRSquiet.Investigations with the IonA-2 photochemical model for undisturbed and flare conditions show that the ionization of the local neutral atmosphere by solar X-ray radiation <2 nm provides a satisfying explanation for detected Mm features with smoothly decreasing electron densities below the M1 base in combination with moderate slopes of the lower Mm region αMm and altitudes of the lower boundary hL,S. While sufficient ionization energy reaches the region of interest during flares, no Mm features with peaks below the M1 base occur in any of the model electron density profiles. This supports the conclusion that the subgroup of merged excess electron densities with peaks or intermediate features (Mi) below the M1 base must have an origin different from the sole variability of solar X-ray radiation during undisturbed and solar flare conditions. The size of the identified Mm makes an exclusive meteoric origin of the Mm peak structures unlikely.It is indicative from the IonA-2 model results that the general increase/decrease of solar X-ray <2 nm leads to a correlated response of the Mm region. The sporadic occurrence of the merged excess electron densities in the MaRS observations is therefore assumed to be a combination of observational (increased observation noise level compared to the available amount of X-ray radiation <2 nm, shift of the lower baseline by ionospheric deviations from radial symmetry) and environmental (e.g. variations in solar X-ray) factors.

Real Space Refinement of Crystal Structures with Canonical Distributions of Electrons

SummaryRecurring groups of atoms in molecules are surrounded by specific canonical distributions of electrons. Deviations from these distributions reveal unrealistic molecular geometries. Here, we show how canonical electron densities can be combined with classical electron densities derived from X-ray diffraction experiments to drive the real space refinement of crystal structures. The refinement process generally yields superior molecular models with reduced excess electron densities and improved stereochemistry without compromising the agreement between molecular models and experimental data.

On the Convergence of the Physicochemical Properties of [n]Helicenes

The results of a series of DFT and DFT−D calculations are reported with the aim to predict the physicochemical properties (equilibrium structures, stabilization energies, redox potentials, excitation and CD spectra, electronic conductivity, and elasticity) of elongating helical structures (i.e., [n]helicenes, n = 1−14). It was shown that many of them are converged at [14]helicene with an interpitch distance of Rpitch = 3.75 A, an incremental stabilization energy of ΔGn = 11 kJ mol-1 (thus suggesting the inherent destabilization of the growing helical structures), an S0−S1 energy difference to the estimated band gap of 2.60 eV, and redox potentials, corresponding to the reduction of [n]helicenes, of E0 = −2.2 V. The elasticity was shown to decrease as a linear function of m (i.e., k ≈ m) with k = 0.01056 a.u. for [14]helicene. Moreover, the conductivity properties were discussed in terms of hole and excess electron densities. Possible implications of the calculated data in nanoscience are discussed.

Effects of density imbalance on the BCS-BEC crossover in semiconductor electron-hole bilayers

We study the occurrence of excitonic superfluidity in electron-hole bilayers at zero temperature. We not only identify the crossover in the phase diagram from the BCS limit of overlapping pairs to the BEC limit of non-overlapping tightly-bound pairs but also, by varying the electron and hole densities independently, we can analyze a number of phases that occur mainly in the crossover region. With different electron and hole effective masses, the phase diagram is asymmetric with respect to excess electron or hole densities. We propose as the criterion for the onset of superfluidity, the jump of the electron and hole chemical potentials when their densities cross.

Static electrification of solid oxide in liquid metal and electrical double layer at the interface

Static electrification of a solid oxide, say a semiconducting oxide in liquid metal, is mainly due to electron transfer between two phases. Excess electrons in the liquid metal phase provided by the oxide give rise to an electrical double layer at the interface. The electrical double layer may be divided into three parts, an immobile inner layer, a compressed diffuse layer, and a flat layer extending into the bulk liquid metal. Differential potential analysis and the induced emf method were used to measure the potential of the compressed diffuse layer and the excess electron density of the flat layer, respectively. Results show that most oxides in liquid metals carry positive charges on their surfaces and the potentials of the compressed diffuse layer are in the range of 3 to 42 μV. Such a low potential implies that the diffuse layer is considerably compressed. The excess electron densities of the flat layer are on the order of 10 22 electrons/m 3 of Hg and their contributions to surface charges of oxide are in the range of 10 17 to 10 18 charges/m 2 for the oxide/mercury systems with a solid density of 0.3 wt% at room temperature.

Observation of Polar Plumes at High Solar Altitudes

Using the Large-Angle Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft, we have imaged polar plumes extending 30 R☉ from disk center in the image plane and ~45 R☉ in three-dimensional space, a factor of 2-3 farther than previous imaging measurements and well into the constant-velocity regime of wind flow. We find that the plumes maintain their overall linear morphology and density enhancement to at least this altitude range. Using LASCO photometry and a modeled cylindrical plume geometry, we derive the density excess within the plumes 30 R☉ above the Sun (in three dimensions). At this altitude, the plumes are (2-4) × 103 cm-3 above the background interplume density, with an estimated plasma β of order 300. The excess electron densities are a factor of 20-30 greater than the average total electron density estimates obtained from extrapolation of in situ measurements by Ulysses at 1 AU. The contrast between the high plume excess densities that we observe and the uniformity of the wind seen by Ulysses may best be explained by wind models that include horizontal mixing in the lower heliosphere between 45 R☉ and Ulysses's altitude of ~200 R☉.

Electron Density Dependence of the Excitonic Absorption Thresholds of GaAs Quantum Wells

We study the evolution of the interband optical excitation spectra of remotely doped GaAs/Al0.33Ga0.67As quantum wells (QWs) with excess electron density using photoluminescence excitation (PLE) and electroreflectance (ER) spectroscopies. At the lowest electron densities the PLE spectra resemble those of undoped QWs, with a discrete peak due to the 1s neutral exciton (X) and a higher energy band due to the 2s/continuum. At very dilute excess electron densities the excitons bind a single excess electron to form a trion or negatively charged exciton (X–), producing a second peak in the PLE a few meV below that due to X. With increasing density the X– peak shifts to higher energy, tracking the position of the k-vector conserving Fermi edge transition observed on the high-energy side of the photoluminescence band, to form the single threshold seen at high carrier density. The peak originating from X at low density shifts more rapidly to higher energy with increasing density and weakens, although it can be resolved to densities of up to 10 cm–2. On the other hand the 2s/continuum threshold disappears at the lowest excess electron densities.

Rate-equation analysis of transient photoconductivity for exponential localized-state distribution

The time dependence of excess free and trapped electron densities based on multiple trapping in a continuous exponential density of states (DOS) was simulated by solving numerically multiple trapping rate equations. It was found that the time for establishment of thermal equilibrium between free and trapped electrons did not necessarily correspond to the release time for the particular level. The results of this simulation indicate that the model developed by Orenstein and Kastner provides a good fit to the kinetic data but significantly overestimates the recombination lifetime.

Donor Impurities as a Probe of Electron Correlations in a Two-Dimensional Electron Gas in High Magnetic Fields

Many-electron effects have been observed in far-infrared magnetospectroscopic studies of GaAs/AlGaAs multiple quantum wells (QWs), $\ensuremath{\delta}$ doped with Si donors in both well and barrier centers. With increasing excess electron densities in the QWs the negative donor ion $({D}^{\ensuremath{-}})$ singlet and triplet transitions are substantially blueshifted, exhibiting cusps at integer and fractional filling factors. At high magnetic fields the many-electron system appears to approach a collection of isolated two-electron ${D}^{\ensuremath{-}}$ ions. Exact diagonalization studies of donor and confined electrons show the importance of electron correlations and localization at high magnetic fields in understanding this behavior.

Small‐angle X‐ray scattering on latexes

AbstractSmall‐angle X‐ray scattering (SAXS) is a tool which allows the study of the structure and the interaction of polymer latexes with great accuracy. The low electron density of the polymers used for the synthesis of latex particles as e.g. polystyrene allows the matching of the contrast by adding sucrose to the serum. Thus, scattering intensities measured at different contrast, i.e., at different excess electron densities can be evaluated (contrast variation). This yields precise information on the radial electron density of the particles. In this article recent SAXS‐investigations on latex particles are reviewed. It is demonstrated that core‐shell latexes can be analyzed precisely by contrast variation. The same method can be applied to swollen latex particles to examine the polymer concentration near the boundary to the water phase. Here it is shown that the depletion of the polymer molecules near this boundary is very small which points to a minute wall‐repulsion effect. Since the excess electron density of polystyrene latex particles in water is very small, the scattering from adsorbed layers of surfactants dominates the measured intensity in this particular system. Therefore the adsorption equilibrium of a given surfactant as well as the competitive adsorption of two different surfactants on a polystyrene latex can be assessed by SAXS.

D− Impurities in a Quasi-Two-Dimensional System: Statistics and Screening

AbstractFar infrared spectroscopic studies of electron-density- and magnetic-field-dependenceof manyelectron effects on silicon donor impurities confined in GaAs quantum wells are presented. At low excess electron densities, transitions from D− singlet and triplet states are observed. Temperature- and polarization-dependence measurements show that the relative absorption strengths of various spectroscopic features (D0, D−, and CR) are in qualitative agreement with a statistical calculation in thermal equilibrium in high magnetic fields. At large excess electron densities, the “D−” transition energy shifts to higher energy when electron density is increased, and the magnetic-field dependence of the transition energy exhibits discontinuities in slope at integer filling factors. The spectroscopic features and their relation to excess free carriers and the role of screening and correlation are presented and discussed.

Optical Absorption of the Soliton Lattice

AbstractThe optical conductivity for the soliton‐lattice solution of the continuum approximation to the Su‐Schrieffer‐Heeger model of doped polyacetylene is calculated for different excess electron densities. The optical sum rule is verified in each case. The resulting shifts of the absorption maxima are compared with experiment. Implications of the finiteness of Δ0/W (gap/bandwidth) on the nature of the phase transition and the absorption are discussed.

A 13C NMR and CNDO/2 Calculation Study of the Transannular Interaction in [2.2]Cyclophanes

Abstract The relationships between the aryl carbon chemical shifts and the CNDO/2 electron densities for several [2.2]cyclophanes are reported. Plots of the chemical shifts against the excess total electron densities indicate additional downfield shifts of 6–9 ppm on the average for those aryl carbons that are in close proximity to other aryl carbons. A plot of the 13C shielding constant, calculated using the Karplus-Pople theory, against the chemical shift shows a downfield shift of a similar magnitude for the proximate carbons relative to the others. It is found that this difference can be essentially eliminated simply by adjusting the ΔE parameter in the Karplus-Pople equation from 10.0 eV to 9.7 eV for the proximate carbon atoms.

Extreme accumulation layers on ZnO surfaces due to He + ions

Accumulation layers with excess surface electron densities of up to 10 14 cm −2 have been produced on the (001) face of ZnO single crystals by an electrical discharge in helium atmosphere. Hall effect measurements show that at these extreme surface electron densities both the electron density and the mobility are temperature independent from room temperature down to 1.6 K.

Anomalous behaviour of electron mobility in space charge layers

The Hall effect on polar faces of ZnO has been measured at 80 K. Surface excess electron densities between 10 7 and 10 13 cm −2 have been achieved by photolysis, annealing or hydrogen exposure. The surface electron mobility as a function of carrier density shows a modulated structure. Possible scattering mechanism are discussed.

ROCKET STUDIES OF ISOLATED IONOSPHERIC IRREGULARITIES

It has been shown previously that ground-based observations of the relative phase, or direction-of-arrival, of radio signals propagated through the ionosphere from artificial satellites provide a useful extension to existing techniques for the study of ionospheric irregularities. This technique has now been extended to observations of the auroral ionosphere using small transmitters ejected from two rockets launched at Fort Churchill, Manitoba. Observations were also made on beacon signals from the Alouette I satellite as it passed over Fort Churchill and these revealed large irregularities in the F region but no similar irregularities in the E region. Nevertheless, when transmitters were placed between the E and the F regions, large E-region irregularities (1–2 km in diameter) having excess electron densities of the order of 2 × 1011 m−3 were clearly revealed. The background ionization, though not measured, was probably of about the same magnitude. These preliminary observations raise a number of questions which need further investigation.

Some comments on Linvill's lumped models for semiconductor devices

The Linvill models and the more conventional equivalent circuits can be used to make roughly equally accurate predictions of terminal behaviour of semiconductor devices. An extreme case, the diode storage time, illustrates this point. The principal feature of the Linvill models, not found in other models, is that current is expressed in terms of excess hole and electron densities. Specifically, the distributed current in a semiconductor device is approximated in the Linvill model by a finite number of currents in corresponding current nodes, and each of the nodes is associated with a particular elementary volume. The relation between the hole- and electron-charge density and the current in any elementary volume is represented by elements of a lumped equivalent circuit.When the models are applied to a practical problem, it is not possible to use the very large number of lumped elements which would be needed closely to model the distributed current, and a small number of model elements is used. In such a ‘simplified’ model, the one-to-one correspondence between model space and device space is obscure, and, furthermore, conditions based on this correspondence are not sufficient to obtain correctly the model element values. Using, in the last analysis, arbitrary configurations, the model elements must be calculated in such a manner that they match some set of terminal characteristics. A further practical drawback is that the lumped elements cannot be measured directly because they are expressible in terms of hole and electron concentrations, themselves not measurable.Without altering the intent of the Linvill approach, both of the above difficulties can be mitigated. One can represent the currents in the device by an alternative set of lumped elements in which the notion of volume is absent. Such elements are not necessarily associated with physical volumes and no contradictions are encountered with the simplified models. The alternative set of elements proposed in this paper can, furthermore, often be measured in terms of current and voltage. A last by no means negligible advantage is that a special symbolism is not required.