Inhomogeneous Distribution Of Electron Density Research Articles

Anomalous electronic energy losses in protons passing through Gd thin films

We report a theoretical analysis of recent data on the unexpectedly high electronic energy loss protons in a polycrystalline gadolinium target. These results led the authors of this data to conclude that the free electron model used to analyse these results fails. In this work we provide a quantitative explanation of the experimental results, using an approach based on density functional theory within the framework of the free electron gas model. We performed semi-classical trajectory simulations (SCTS) and employ the local-density-approximation model (LDA), using an inhomogeneous electron density distribution and the polycrystalline character of Gd samples. The good agreement obtained, delineates the need of considering a realistic description of a particle trajectory and the effective electron density ‘seen’ along it, whose description remains within a FEG model.

Effects of an inhomogeneous electron density distribution on the hydrogen distribution in TiZrTaNbAl multi-principal element alloys

A body-centered cubic equiatomic TiZrTaNbAl multi-principal element alloy (MPEA) with elemental fluctuations was investigated to further understand the relationship between the microstructure and hydrogen distribution. In this study, a composition dependence of the hydrogen distribution was observed in the TiZrTaNbAl MPEA. An inhomogeneous electron density distribution of the MPEA was revealed by advanced differential phase-contrast scanning electron microscopy (DPC-STEM) for the first time. The results showed that the electron density has a significant effect on the hydrogen distribution in TiZrTaNbAl MPEAs. This work provides new insight into the design of materials with high hydrogen storage capacity and high hydrogen embrittlement resistance.

The free electron model and the electronic energy losses of protons at low velocities interacting with polycrystalline tantalum

In this letter, we report experimental data and theoretical work on the electronic energy loss and energy loss straggling of protons transmitted through self-supported thin films of tantalum in a polycrystalline tetragonal phase . Low-energy protons with energies below 10 keV and Ta films with nominal thickness of 6, 9 and 12 nm are used. The aim of this work is to understand the unexpected values of the Ta stopping power for low-energy proton backscattering reported recently, which are far from the prediction of the standard free electron gas model and semi-empirical approaches. This had led the authors to conclude the failure of the free electron model. Our transmission measurements confirm these experimental results. In this work, a qualitative discussion and quantitative explanation of our experimental results is given, using an approach based on the density functional theory within the framework of the free electron gas (FEG) model. We performed semiclassical deterministic trajectory simulations and employ the local density approximation model, using an inhomogeneous electron density distribution and the polycrystalline character of Ta samples.

Construction of eight mixed-valence pentanuclear CuI4CuII clusters using ligands with inhomogeneous electron density distribution: synthesis, characterization and photothermal properties.

To enhance light absorption in the visible region for the utilization of sunlight, eight mixed-valence polynuclear CuI/CuII clusters have been synthesized for evaluating their photothermal conversion performance. They are fabricated considering the ligand's electron density distribution inhomogeneity using 1,2,3-triazole (3N) or tetrazole (4N) and different mono-phosphine ligands. We report here the synthesis, crystal structure, characterization, optical properties, and photothermal conversion performance of these clusters. X-ray crystal structures reveal that those pentanuclear clusters are neutral clusters with octahedrally-coordinated copper(II) ion being surrounded by four tetrahedrally coordinated copper(I) ions. Interestingly, with the introduction of the mixed-valence centers, these compounds show additional light absorption centers in 350-600 nm via the IVCT transition mechanism, compared with our previously reported Cu(II) compounds. These clusters show excellent photothermal conversion performance, with an average equilibrium temperature (∼60 °C) and a temperature increment (∼40 °C), which are also superior to Cu(II) complexes (the average equilibrium temperature ∼55 °C). This work proves that it is possible to design and prepare new polynuclear mixed-valence CuI/CuII clusters for achieving high-performance photothermal conversion materials.

Hall effect and symmetry breaking in the nonmagnetic metal LuB12 with dynamic charge stripes

A comprehensive study of magnetoresistance and Hall effect has been performed for the set of the single crystals of non-magnetic metal LuB12 with the Jahn-Teller instability of the boron cage and dynamic charge stripes forming along <110> direction. Anomalous positive contribution to Hall effect for particular direction of magnetic field H//[001] is found in the single crystals of LuB12 of the highest quality. This contribution arising at T~ 150 K is shown to increase drastically when approaching the disordered ground state below 60 K. The Hall effect anomaly is shown to appear in combination with the peak of magnetoresistance. The various scenarios allowing for the topology of Fermi surface, anisotropy of relaxation time for charge carriers and the filamentary structure of fluctuating charge stripes are proposed to explain the features of magnetotransport in this metal with inhomogeneous distribution of electron density. The origin of SdH oscillations, which are observed in this non-equilibrium metal with electron phase separation and strong charge carrier scattering, is discussed.

High‐TC Interfacial Ferromagnetism in SrMnO3/LaMnO3 Superlattices

AbstractHeterostructures of strongly correlated oxides demonstrate various intriguing and potentially useful interfacial phenomena. LaMnO3/SrMnO3 superlattices are presented showcasing a new high‐temperature ferromagnetic phase with Curie temperature, TC ≈360 K, caused by electron transfer from the surface of the LaMnO3 donor layer into the neighboring SrMnO3 acceptor layer. As a result, the SrMnO3 (top)/LaMnO3 (bottom) interface shows an enhancement of the magnetization as depth‐profiled by polarized neutron reflectometry. The length scale of charge transfer, λTF ≈2 unit cells, is obtained from in situ growth monitoring by optical ellipsometry, supported by optical simulations, and further confirmed by high resolution electron microscopy and spectroscopy. A model of the inhomogeneous distribution of electron density in LaMnO3/SrMnO3 layers along the growth direction is concluded to account for a complex interplay between ferromagnetic and antiferromagnetic layers in superlattices.

Propagation of terahertz waves in a magnetized, collisional, and inhomogeneous plasma with the scattering matrix method

The interaction between terahertz waves and a magnetized, collisional, and inhomogeneous plasma is analyzed theoretically in this paper. Using the scattering matrix method, this investigation mainly takes account of the absorbance, transmittance, and reflectance corresponding to some key parameters of the plasma, such as electron density, collision frequency, plasma thickness, and magnetic field strength. Furthermore, three electron density profiles, the uniform, parabolic, and double exponential distributions, are employed to describe the effects of the plasma density on the propagation of the terahertz waves. Specifically, the propagation characteristics induced by homogeneous and inhomogeneous electron density distribution models are compared. Simulation results demonstrate that these key parameters influence the propagation characteristics of terahertz waves in plasma. In addition, a plasma density with double exponential distribution is more advantageous for the transmittance than the uniform and parabolic distributions. These results provide a significant theoretical basis for blackout communication to a certain extent.

Diversity of charge orderings in correlated systems.

The phenomenon associated with inhomogeneous distribution of electron density is known as a charge ordering. In this work, we study the zero-bandwidth limit of the extended Hubbard model, which can be considered as a simple effective model of charge ordered insulators. It consists of the on-site interaction U and the intersite density-density interactions W_{1} and W_{2} between nearest neighbors and next-nearest neighbors, respectively. We derived the exact ground state diagrams for different lattice dimensionalities and discuss effects of small finite temperatures in the limit of high dimensions. In particular, we estimated the critical interactions for which new ordered phases emerge (laminar or stripe and four-sublattice-type). Our analysis show that the ground state of the model is highly degenerated. One of the most intriguing finding is that the nonzero temperature removes these degenerations.

Density inhomogeneities and Rashba spin-orbit coupling interplay in oxide interfaces

There is steadily increasing evidence that the two-dimensional electron gas (2DEG) formed at the interface of some insulating oxides like LaAlO3/SrTiO3 and LaTiO3/SrTiO3 is strongly inhomogeneous. The inhomogeneous distribution of electron density is accompanied by an inhomogeneous distribution of the (self-consistent) electric field confining the electrons at the interface. In turn this inhomogeneous transverse electric field induces an inhomogeneous Rashba spin-orbit coupling (RSOC). After an introductory summary on two mechanisms possibly giving rise to an electronic phase separation accounting for the above inhomogeneity, we introduce a phenomenological model to describe the density-dependent RSOC and its consequences. Besides being itself a possible source of inhomogeneity or charge-density waves, the density-dependent RSOC gives rise to interesting physical effects like the occurrence of inhomogeneous spin-current distributions and inhomogeneous quantum-Hall states with chiral “edge” states taking place in the bulk of the 2DEG. The inhomogeneous RSOC can also be exploited for spintronic devices since it can be used to produce a disorder-robust spin Hall effect.

Study of scattering cross section of a plasma column using Green's function volume integral equation method

In this article, the electromagnetic wave scattering from plasma columns with inhomogeneous electron density distribution is studied by the Green's function volume integral equation method. Due to the ready production of such plasmas in the laboratories and their practical application in various technological fields, this study tries to find the effects of plasma parameters such as the electron density, radius, and pressure on the scattering cross-section of a plasma column. Moreover, the incident wave frequency influence of the scattering pattern is demonstrated. Furthermore, the scattering cross-section of a plasma column with an inhomogeneous collision frequency profile is calculated and the effect of this inhomogeneity is discussed first in this article. These results are especially used to determine the appropriate conditions for radar cross-section reduction purposes. It is shown that the radar cross-section of a plasma column reduces more for a larger collision frequency, for a relatively lower plasma frequency, and also for a smaller radius. Furthermore, it is found that the effect of the electron density on the scattering cross-section is more obvious in comparison with the effect of other plasma parameters. Also, the plasma column with homogenous collision frequency can be used as a better shielding in contrast to its inhomogeneous counterpart.

On the phase diagram of the extended Hubbard model with intersite density–density interactions in the atomic limit

Abstract The charge ordering is a phenomenon associated with inhomogeneous distribution of electron density occurring mostly in strongly correlated materials such as transition metal oxides or organic conductors. The extended Hubbard model (EHM) is one of the simplest model for description of this phenomenon. The full phase diagram of the EHM with intersite density–density interactions W 1 and W 2 (nearest- and next-nearest-neighbour, respectively) in the atomic limit as a function of the chemical potential has been derived in the variational approach, which treats the on-site interaction exactly and the intersite interactions within mean-field approximation. The results for arbitrary values of model parameters (in the two-sublattice assumption) reveal that the diagram has very complex structure including various (multi-)critical points. A variety of the transitions between different phases, in particular with long-range charge-order, has been found to occur on the diagram. The results presented are rigorous ones in the high-dimension limit for any W 1 and W 2 ≤ 0 .

Modeling of a Plasma Antenna with Inhomogeneous Distribution of Electron Density

The distribution of the electron density along a plasma antenna can influence the antenna’s performance. But little has been done in this regard in former studies. In this paper, a model of a practical plasma antenna with an inhomogeneous distribution of electron density is founded according to the transmission-line equivalent theory of a metal monopole, from which the current distribution and the radiation pattern of a plasma antenna with appropriate parameters are calculated. The results show that the electrical current distribution, the maximum radiation direction, and the beamwidth of a plasma antenna vary with electron density distributions. To validate the model, the plasma antenna with the same parameters is also simulated based on electromagnetic software HFSS. It is found that the results from the two ways are almost consistent.

Density-functional analysis of the electronic structure of tris-bipyridyl Ru(II) sensitisers

Abstract Excited state transitions and energies of a series of [Ru(bpy) 3 ] 2+ type complexes incorporating the ligand, 4,4′-bis-phosphonato(methyl)-2,2′-bipyridine ( dmpbpy ) was investigated, and the influence of this organometallic ligand on the electronic structure of the complexes was examined using Time-Dependent Density Functional Theory (TD-DFT). Experimental data and the theoretical TD-DFT calculations were presented to support the effect of non-equivalent ligand substitution on spectral and molecular orbital (MO) energy properties on this class of tris-chelate surface sensitisers. For the series of complexes studied, it was identified that the lowest lying LUMO states were consistently found to reside on the ligand 2,2′-bipyridine ( bpy ) for gas phase calculations. As an implication of this, it was suggested that this could impact the effectiveness of these complexes as surface sensitisers in PEC cell applications such as the dye-sensitised solar cell (DSC) due to the lower probability of the excited state electron residing on a ligand anchored to the semiconductor substrate. However, further calculations in a solvation medium showed that the electron withdrawing nature of PO 3 H 2 on dmpbpy saw the lowest lying LUMO states are populated on dmpbpy . This inhomogeneous distribution of electron density across non-equivalent ligands may have implications for further ‘spectral tuning’ of surface sensitisers. Despite the TD-DFT gas phase calculations not being corrected for solvent/media effects, the three longest wavelength bands associated with known charge transfer phenomena were identified. The symmetry allowed ν ¯ MLCT in the visible region was assigned as a π ψ ∗ ← dπ transition, the mid-UV spectrum ν ¯ LC was assigned π ψ ∗ ← π in origin. Whilst the near-UV shoulder on the blue side of ν ¯ MLCT showed dσ ← dπ and π ∗ ← dπ transitional character and was tentatively described as ν ¯ MC/MLCT . UV–Vis absorption spectra calculated for solvated analogues containing dmpbpy indicated that the low energy transitions associated with the MLCT are subject to bathochromic shift due to solvent polarity by 0.062 eV (500 cm −1 ) compared with the gas phase calculations, which is more highly correlated to the observed experimental transitions.

Positronium formation in semicrystalline poly(ethylene terephthalate)

The probability of positronium (Ps) formation in semicrystalline poly(ethylene terephthalate) (PET), has been investigated by means of positron annihilation lifetime spectroscopy (p.a.l.s.). The p.a.l.s. data are correlated with the structural characteristics of the samples, such as crystalline content, interlamellar spacing and lamella thickness. The results show an approximately linear decrease of the intensity ( I 3) of the long-lived orthopositronium ( o-Ps) component with an increase in the crystalline content up to 53%, while the lifetime remains practically constant. It seems that the other structural parameters do not affect the Ps formation. The decrease of I 3 cannot be completely accounted for if one assumes that o-Ps localizes in the amorphous phase only. It is suggested that the o-Ps also forms in the crystalline regions, due to the inhomogeneous electron density distribution along the c-axis of the PET lattice.

Charge transfer in an inhomogeneous two-dimensional electron system in the arrangement of Laughlin's gedanken experiment.

We have investigated the charge transfer due to the change of magnetic flux in samples with a circular gate which allowed us to create an inhomogeneous electron density distribution. It has been shown that at a negative gate voltage, when the filling factors in gated and ungated regions of a sample are close to integers, nonequilibrium accumulation of charge can take place in the gated region. This gives rise to charge transfer through the sample corresponding to Hall conductivity of the ungated region with the larger number of occupied Landau levels

Very Large Array-RATAN 600 observations of a solar active region

The Very Large Array (VLA) and the RATAN 600 were used to observe a solar active region on two consecutive days around the time of a partial solar eclipse in July 1990. VLA synthesis maps at 2.0, 3.5, and 6.2 cm wavelength reveal bright (Tb = 0.2 − 2.2 × 106 K), compact (θ = 10″–40″) sources above the penumbra of the leading sunspot while maps at 20 cm wavelength reveal an extended (θ ≈ 4.5′) looplike structure (Tb ≈ 106 K) between the dominant spots. Total flux and brightness temperature spectra of both components were obtained by the RATAN at nine wavelengths between 1.7 and 21 cm. The relatively-flat spectrum of the extended emission is attributed to the optically thin thermal brems Strahlung of electrons trapped in a magnetic loop at coronal temperatures. Step-spectrum sunspot-associated emission is attributed to thermal gyroresonance radiation at different heights along the leg of a loop joining regions of opposite magnetic polarity. Comparisons with predicted distributions of gyroresonance radiation indicate that the compact sunspot-associated sources lie at heights of h = 2500–17500 km above the photosphere. Although potential fields of sufficient strength appear to exist at coronal heights, differences n the observed and predicted brightness distributions suggest some role for non-potential fields or for an inhomogeneous distribution of electron density or temperature above the sunspot.

On Soulé-Porod plots of protein X-ray scattering data

It is shown that inhomogeneous electron density distribution as represented either by a spherical model containing smaller spheres or by a sphere with radially periodic density does not produce positive slope in Soule–Porod plots (Ih4 versus h4) of protein small-angle scattering data. It is found that the introduction of small particles into the scattering population can produce the zero or positive slope commonly seen. It is also shown that intersubunit interference is able to account for the low 2θ minimum or dip often observed in Ih4 vs h4 plots of the scattering from multisubunit proteins.

Hydrogen diffusion and appearance potential spectroscopy for examination of electronic structures in alloys of d-metals

From solubility, susceptibility, and electronic specific-heat data, the electronic behaviour of Pd hydride and its alloys is consistent with a 4d-5s overlapping structure with 0.55 electron holes in the d-band.In addition to these collective electronic features one must take into account also the effects of inhomogeneous distribution of electron density in these systems (spatial aspects).There are a number of experiments that perceive local densities of states, e.g., Mössbauer resonances, Appearance Potential Spectroscopy (APS), and consecutive jumps of hydrogen interstitials, which sum up to long-range diffusion processes.Following this line, the evidence for screening effects in Pd alloys is demonstrated by Mössbauer effect measurements, in relation to the paper presented in this issue by Mahnig et al. [36]. The possibility of obtaining more information about local densities of states with the aid of the APS method is pointed out.The increase of the activation energy of hydrogen diffusion in Pd alloys (Ag, V, Ni) is attributed to a local arrangement (screening model) of conduction electrons around the alloying particles and hydrogen.The calculation in terms of screening energies leads to good agreement with the experimental results. Deviations from the suggested model are specified.

Scattering of Microwaves from a Plasma Column in Rectangular Waveguide

The microwave reflection method for the measurement of various parameters of cold plasma is examined. The exact solutions are obtained for the waves scattered by a plasma column inserted in the duct. The probing electric field of TE10 mode is parallel to the axis of the column. The emphasis is placed on the fact that inhomogeneous electron density distribution is fully taken into account. The electron density in the column is assumed to be n(r)=n0(1-r2/ρ2), where ρ is the radius of the column and n0 is the electron density on the axis of the column. The electric permeability is then locally defined with use of n(r). Numerical calculations of reflection coefficients are carried out with the electronic computer OKITAC–5090 and their results are tabulated for such parameters as ωp/ω, ν/ω, ka, and kρ for practical convenience. The electron densities in a positive column determined by the microwave reflection method with use of the tables are compared with those determined by the Langmuir probe method. The comparison is found in satisfactory agreement.

Surface waves on inhomogeneous plasma columns

Quasistatic theory is presented for axially symmetric and dipolar surface modes propagating down an inhomogeneous plasma column enclosed in a glass tube surrounded by free space. The effect of a sheath with no electrons present, radial parabolic electron density distribution, and the combination of these two effects on the dispersion relations of the axially symmetric and the dipolar modes is investigated for lossless, cold and isotropic plasma. Parabolic density variation is treated approximately by a variational technique. Dispersion relations for all three examples of inhomogenoity for both the n = 0 and n =1 modes are calculated for a wide variety of parameters of inhomogeneity. The dipolar mode is seen to be more sensitive to inhomogeneous electron density distribution than the axially symmetric mode for small βa and is thus better suited for experimental verifications than the axially symmetric mode.