Empty States Research Articles

Atomic-scale study of Si-doped AlAs by cross-sectional scanning tunneling microscopy and density functional theory

Silicon (Si) donors in GaAs have been the topic of extensive studies since Si is the most common and well understood n-type dopant in III-V semiconductor devices and substrates. The indirect bandgap of AlAs compared to the direct one of GaAs leads to interesting effects when introducing Si dopants. Here we present a study of cross-sectional scanning tunneling microscopy (X-STM) and density functional theory (DFT) calculations to study Si donors in AlAs at the atomic scale. Based on their crystal symmetry and contrast strengths, we identify Si donors up to four layers below the (110) surface of AlAs. Interestingly, their short-range local density of states (LDOS) is very similar to Si atoms in the (110) surface of GaAs. Additionally we show high-resolution images of Si donors in all these layers. For empty state imaging, the experimental and simulated STM images based on DFT show excellent agreement for Si donor up to two layers below the surface.

Buckling failure analysis for buried subsea pipeline under reverse fault displacement

Buckling failure analysis of buried subsea pipeline under reverse fault displacement was investigated by the Vector Form Intrinsic Finite Element method in this paper, considering nonlinear soil–pipeline interaction in slit with sand and the multifold nonlinearities of nonlinear material, large deformation, large displacement and self-collision/contact. Two typical conditions discussed were the operation condition and the empty waiting state during installation, maintenance, and operation. Effects of soil properties, pressure loadings, diameter–thickness ratios and dip angles on local buckling formation, tensile strain and cross-sectional distortion were investigated. The results show that subsea pipelines obtain a large deformation capacity and the flexure curve shape is usually S-shape. The external pressure is more unfavorable than the internal pressure since it aggravates the compression strains but alleviates the tension strains. A small fault dip angle results in serious axial compression and can lead to local collapse in the empty waiting states. For big diameter–thickness ratios or great pressure level, external pressure will cause local collapse and even buckle propagation along the pipeline. Seven different modes of collapse and propagation are discovered in the empty waiting state. They can be classified by the first collapse location, buckle propagation direction and further propagation sustainability. The cross-sectional shapes after collapse and during propagation are mainly determined by the compression or tension state around the circum. These results can be used for the development of performance-based design methodologies for buried subsea pipelines.

Surface Electron-Hole Rich Species Active in the Electrocatalytic Water Oxidation.

Iridium and ruthenium and their oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotentials observed for Ru- and Ir-based anodes and the high corrosion resistance of Ir-oxides. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nanofabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrOx centers (i.e., oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally IrV species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (μ1-O and μ1-OH) that are responsible for water activation and oxidation. Oxygen bound to three iridium centers (μ3-O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a μ1-OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface-to-bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species.

Visualization of Frontier Molecular Orbital Separation of a Single Thermally Activated Delayed Fluorescence Emitter by STM.

Because the spatial distribution of frontier molecular orbitals (FMOs) regulates the thermally activated delayed fluorescence (TADF) property, researchers synthesize TADF emitters by designing their FMO distribution. However, it remains challenging to clarify how the FMO distribution is altered at molecular interfaces. Thus, visualizing the FMOs at molecular interfaces helps us to understand the working behavior of TADF emitters. Using scanning tunneling microscopy (STM), we investigated the electronic structure of a single TADF emitter, hexamethylazatriangulene-triazine, at molecule-metal and molecule-insulating film interfaces. FMOs at the molecule-metal interface were not spatially confined to the donor-acceptor moieties because of hybridization. Meanwhile, FMOs at the molecule-insulator interface exhibited spatially separated filled and empty states confined to each moiety, similar to the calculated gas-phase FMOs. These observations illustrate that the molecule-environment interaction alters the spatial distribution of FMOs, proving that STM is a powerful tool for studying TADF molecules.

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis

Electric-field tuning of the magnetic properties of bilayer VI3 : A first-principles study

The magnetic properties of two-dimensional VI3 bilayer are the focus of our first-principles analysis, highlighting the role of trigonal crystal-field effects and carried out in comparison with the CrI3 prototypical case, where the effects are absent. In VI3 bilayers, the empty a1g state - consistent with the observed trigonal distortion - is found to play a crucial role in both stabilizing the insulating state and in determining the inter-layer magnetic interaction. Indeed, an analysis based on maximally localized Wannier functions allows to evaluate the interlayer exchange interactions in two different VI3 stackings (labelled AB and AB'), to interpret the results in terms of virtual-hopping mechanism, and to highlight the strongest hopping channels underlying the magnetic interlayer coupling. Upon application of electric fields perpendicular to the slab, we find that the magnetic ground-state in the AB' stacking can be switched from antiferromagnetic to ferromagnetic, suggesting VI3 bilayer as an appealing candidate for electric-field-driven miniaturized spintronic devices.

Edge effects on the intrinsic magnetism in ferromagnetic triangular phosphorene quantum dots using fully spin-polarized calculations

This project involves discovering the electronic and magnetic properties of nanometer-sized phosphorene structures with triangular shapes in both zigzag and armchair termination types. The goal is to discuss the relationship between the electronic states belonging to the different conditions of these phosphorene quantum dots and their intrinsic magnetic properties. For this purpose, we consider electronic interactions utilizing the spin-polarized density functional theory calculations, and then the results compare with the data generated from tight-binding calculations. Both descriptions yield mid-gap states in the spectrum of ferromagnetic structures. Our results in non-spin computations without any geometry optimization were matched by tight-binding calculations which shows that the tight-binding method is an inefficient approximation in analyzing the optimized spin samples. Unlike graphene, in our spin-polarized calculations, we have obtained empty mid-gap states in the spectrum of ferromagnetic triangular phosphorene quantum dots. The edge atoms of these structures are known as the magnetic atoms with an unequal contribution of spin up and spin down. To prevent deforming the initial structures, the dangling bonds at the edge atoms were passivated in two types, fully hydrogenated and partial passivation with oxygen atoms. Oxygen doping was required for introducing magnetism to the non-spin edges of the fully hydrogenated case.

Human Dignity in the Counseling Perspective

Human dignity is a human nobility that distinguishes it from other creatures on earth. The purpose of writing this article is to describe human dignity in the perspective of counseling. The method used in this writing is literature study. The data collection technique in this article is the documentation technique. The data analysis technique used is content analysis technique. Based on the results of the author's search, humans are not born in an empty state where humans are given a noble spirit by God, the proof of which is the Qur'an as in its content, humans are created in the best possible shape and perfect. Human dignity is influenced by the conditions chosen by the human being, but if it is taken from the word nobility, humans must do and become a better person.

Abnormal functional connectivity within the prefrontal cortex in interstitial cystitis/bladder pain syndrome (IC/BPS): A pilot study using resting state functional near-infrared spectroscopy (rs-fNIRS).

To investigate the abnormalities of functional connectivity (FC) within the prefrontal cortex (PFC) of patients with interstitial cystitis/bladder pain syndrome (IC/BPS) based on resting state functional near-infrared spectroscopy (rs-fNIRS) data using FC matrix analysis. Ten patients with IC/BPS (females, 9; mean age, 56.9 ± 12.432 years) and 15 age- and gender-matched healthy controls (HC) (females, 12; mean age, 55.067 ± 7.46 years) participated in this rs-fNIRS study. Two rs-fNIRS scans were performed (when the bladder was empty and when the desire to void was strong). The Pearson's correlation coefficient between the time series of the 22 channels was calculated to obtain a 22 × 22 FC matrix for each subject. A two-sample t-test (p < .05) was performed to compare group differences in the FC matrix between patients with IC/BPS and HC. FC was significantly decreased within the PFC in the IC/BPS group based on a two-sample t-test (p < .05) compared with HC. FC decreased in a wider range of brain regions during the strong desire to void state (4 brain regions and 28 edges) when compared with the empty bladder state (3 brain regions and 18 edges). FC abnormalities in IC/BPS patients may lead to frontal lobe disorders involved in processing sensory integration, motivation drive, emotional control, and decision-making whether to urinate, leading to urinary control dysfunction manifested as typical clinical IC/BPS symptoms. Our results may provide new insight into the pathogenesis of IC/BPS and new brain biomarkers for diagnosis.

Critical analysis of the response function in low-dimensional materials

The presence of sharp peaks in the real part of the static dielectric response function are usually accepted as indication of charge or spin instabilities in a material. However, there are misconceptions that Fermi surface (FS) nesting guarantees a peak in the response function like in one-dimensional systems, and, in addition, response function matrix elements between empty and occupied states are usually considered of secondary importance and typically set to unity like in the free electron gas case. In this work, we explicitly show, through model systems and real materials, within the framework of density functional theory, that predictions about the peaks in the response function, using FS nesting and constant matrix elements yields erroneous conclusions. We find that the inclusion of the matrix elements completely alters the structure of the response function. In all the cases studied other than the one-dimensional case we find that the inclusion of matrix elements washes out the structure found with constant matrix elements. Our conclusion is that it is imperative to calculate the full response function, with matrix elements, when making predictions about instabilities in novel materials.

Carbon stoichiometry and mechanical properties of high entropy carbides

The search for new materials via compositional exploration has recently led to the discovery of entropy stabilized and high entropy ceramics. The chemical diversity in the cation sublattice of high entropy ceramics has led to many enhanced properties and applications such as reversible energy storage, low temperature water splitting, amorphous-like thermal transport in crystalline solids and enhanced mechanical properties. This work describes the synthesis and mechanical properties of high entropy (HfNbTaTiZr)Cx thin films as a function of carbon content. The nature of the bonding and microstructure evolves as the material transforms from metallic to ceramic to nanocomposite with variations in the quantity and types of carbon, yielding large variations in the film hardness. Through multiple characterization techniques and first principles investigations, we separate the roles of microstructure and bonding characteristics in the mechanical property development of (HfNbTaTiZr)Cx thin films. This study presents a strategy to establish the bonding, structure, and property relationships in chemically disordered high entropy ceramics, largely based on the relative populations of filled or empty antibonding states for which there are new abilities to do so in high configurational entropy systems that exhibit high solubility of diverse cations while retaining rocksalt structure.

(Invited) Electro-Optical Techniques to Measure Traps in Organic-Based Devices: Why the Methods Originally Developed for Silicon-Based Devices Must be Modified

Electrical techniques to detect traps are a crucial part of the development of a new electronic materials and devices. Since most of our electronic technology relies on silicon, the well-established methods to measure traps were originally developed for silicon-based devices. The intense work developed in the 60’s to map all electronic defects in silicon brough the technology to today’s standards. Among the techniques developed are the deep level transient spectroscopy, photo-induced current spectroscopy, and admittance spectroscopy. Blood and Orton wrote and excellent textbook on these methods [1]. The methodology behind these techniques is the following: Traps are first filled using a voltage pulse or a light pulse, then the kinetics of the charge carrier release is measured usually as function of temperature. This allow us to determine the trap signature (energetic depth and cross-capture section). In organic semiconductors this methodology often does not apply, this is because the traps are extremely deep. A trap with an energetic depth of 0.1 eV is considered a very deep trap in silicon, but in organic electronics, is just a shallow trap. Traps in an organic semiconductor may have an energetic depth higher than 0.5 eV they can be so deep that a charge carrier may take minutes, hours or even days to be released. This makes unpractically the use of the popular methods to study traps in silicon. To detect the presence of the traps and their effects on the device performance the standard methods developed for silicon must be modified.In this contribution we present a methodology to detect the presence of deep traps in organic based devices. This methodology relies on the measuring the trap filing process and not the trap emptying process. To perform this analysis, the traps must be first emptied, so the device is first brough into a state called “trap emptied state”. This state is then frozen to a specific low temperature and the trap filling recorded at that temperature. To reach a “trap empty state” several methods can be used, namely high temperature while the device is kept under short circuited conditions and/or light exposure for certain types of traps. This methodology was used to detect the effect of traps in organic- based transistors. Transistor devices produced by spin coating and by ink-jet printing methods were studied and presented in this work. The use of this methodology revels the presence of metastable traps that only become active above certain temperatures (200 K, 240 K, and 310 K). This type of metastable traps is only observed organic thin film transistors.[1] J. W. Orton and P. Blood “Electrical Characterization of Semiconductors: Measurement of Minority Carrier Properties (Techniques of Physics)”, Academic Press (November 11, 1990).

Ions substitution effect on the magnetic phase transition temperature of 0.5BiFeO3-0.5Pb1-xSrxTiO3 multiferroic solid solutions

0.5BiFeO3-0.5Pb1-xSrxTiO3 (0 ≤ x ≤ 1) solid solution compositions were synthesized using solid phase reactions route from high-purity oxides. Mössbauer studies showed that for 0.5BiFeO3-0.5PbTiO3 (x = 0) the magnetic phase transition temperature T M value was about 130 K. Substitution of Sr for Pb led to a dramatic decrease in T M, though the Fe content did not change with x and for 0.5BiFeO3-0.5SrTiO3 (x = 1) T M was only ≈20 K. This dramatic difference in T M values seems to be due to additional contribution of the magnetic superexchange between Fe3+ ions via the empty 6p states of Bi3+ and Pb2+ ions to the overall superexchange.

Electron Polarons in Lithium Niobate: Charge Localization, Lattice Deformation, and Optical Response

Lithium niobate (LiNbO3), a material frequently used in optical applications, hosts different kinds of polarons that significantly affect many of its physical properties. In this study, a variety of electron polarons, namely free, bound, and bipolarons, are analyzed using first-principles calculations. We perform a full structural optimization based on density-functional theory for selected intrinsic defects with special attention to the role of symmetry-breaking distortions that lower the total energy. The cations hosting the various polarons relax to a different degree, with a larger relaxation corresponding to a larger gap between the defect level and the conduction-band edge. The projected density of states reveals that the polaron states are formerly empty Nb 4d states lowered into the band gap. Optical absorption spectra are derived within the independent-particle approximation, corrected by the GW approximation that yields a wider band gap and by including excitonic effects within the Bethe–Salpeter equation. Comparing the calculated spectra with the density of states, we find that the defect peak observed in the optical absorption stems from transitions between the defect level and a continuum of empty Nb 4d states. Signatures of polarons are further analyzed in the reflectivity and other experimentally measurable optical coefficients.

Influence of the bias voltage on the photoluminescence intensity and spectral responsivity of the GaN Schottky barrier photodetector

The effects of the bias voltage on photoluminescence (PL) intensity and the spectral responsivity are studied for an Au/Ni/undoped GaN/n+-GaN structure Schottky barrier photodetector. Near-band-gap PL of GaN quenches at low reverse bias but enhances at high reverse bias. Under high reverse bias, holes are accumulated in the region of the GaN adjacent to the Ni/Au. Only electrons below empty states at top of valence can be excited to the conduction band in this region, which reduces the absorption of near-band-gap luminescence. The decrease of the spectral responsivity for near the band gap under higher reverse bias also supports this assumption.

Determination of dynamic loading of bearing structures of freight wagons with actual dimensions

The determination of the dynamic loading of the bearing structures of the main types of freight wagons with the actual dimensions under the main operating conditions is carried out. The inertial coefficients of the bearing structures of the wagons are determined by constructing their spatial models in the SolidWorks software package. Two cases of loading of the bearing structures of the wagons – in the vertical and longitudinal planes – have been taken into account. The studies were carried out in a flat coordinate system. When modeling the vertical loading of the bearing structures of wagons, it was taken into account that they move in the empty state with butt unevenness of the elastic-viscous track. The bearing structures of the wagons are supported by bogies of models 18-100. The solution of differential equations of motion was carried out by the Runge-Kutta method in the MathCad software package. When determining the longitudinal loading of the bearing structures of wagons, the calculation was made for the case of a shunting collision of wagons or a "jerk" (tank wagon). The accelerations acting on the bearing structures of the wagons are determined. The research results will help to determine the possibility of extending the operation of the bearing structures of freight wagons that have exhausted their standard service life. It has been established that the indicators of the dynamics of the load-carrying structures of freight wagons with the actual dimensions of the structural elements are within the permissible limits. So, for a gondola wagon, the vertical acceleration of the bearing structure is 4.87 m/s2, for a covered wagon – 5.5 m/s2, for a flat wagon – 5.8 m/s2, for a tank wagon – 4.25 m/s2, for a hopper wagon – 4.5 m/s2. The longitudinal acceleration acting on the bearing structure of a gondola wagon is 38.25 m/s2, for a covered wagon – 38.6 m/s2, for a flat wagon – 38.9 m/s2, for a tank wagon – 27.4 m/s2, for a hopper wagon – 38.5 m/s2. This makes it possible to develop a conceptual framework for restoring the effective functioning of outdated freight wagons. The conducted research will be useful developments for clarifying the existing methods for extending the service life of the bearing structures of freight wagons that have exhausted their standard resource

Electronic structure of graphene/Y2C heterostructure and related doping effect

Until now, many attempts have been made to dope graphene in various ways, but each method turned out to have pros and cons. In this study, to overcome the limitations of doping methods, yttrium hypocarbide (Y2C) is investigated as one prospective material to dope graphene, using density functional theory calculations. In monolayer Y2C, the anionic electrons localized away from Y atomic layers are confirmed to contribute to occupied states near the Fermi level. Next, we investigate the electronic structure of graphene in heterojunction with Y2C. Anionic electrons of Y2C occupy the empty states of graphene in graphene/Y2C heterostructure, which makes the Dirac cone of graphene located at about 1.7 eV below the Fermi level. Such charge transfer of anionic electrons to graphene and the flatness of electric cloud of anionic electrons leads to evenly n-doped graphene in graphene/Y2C heterostructure. This suggests that Y2C is a good candidate to dope graphene.

Effects of addition of chromium and/or nickel oxides on the electrical characteristics of yttrium oxide-doped high-voltage zinc oxide varistors

The effects of Cr2O3 and/or NiO addition to Y2O3-doped Bi-based high-voltage ZnO varistors were investigated, including crystal structure and interface states analysis. Incorporating 0.35 mol% Cr2O3 in Y-doped varistors increased the single-grain varistor voltage VNNGB from 2.6 to 3.3 V and decreased the leakage current density by a factor of 40, from 2 × 10−5 to 5 × 10-7 A/cm2. The nonlinearity index α before and after degradation increased from 21 to 35. 1.2 mol% NiO increased VNNGB to 4 V. Highest varistor voltage was 1500 V for 0.3 mol% NiO. Resistance to electrical degradation improved with optimal amounts of Cr or Ni by reduction of grain boundary oxygen vacancies. Though the donor density was changed prominently by addition of Cr and/or Ni, changes in the barrier height were suppressed by the change in interface state density. As a result, the change in VNNGB was related mainly to the empty interface state under no bias.

Spin distillation cooling of ultracold Bose gases

We study the spin distillation of spinor gases of bosonic atoms and find two different mechanisms in {}^{52}Cr and ^{23}Na atoms, both of which can cool effectively. The first mechanism involves dipolar scattering into initially unoccupied spin states and cools only above a threshold magnetic field. The second proceeds via equilibrium relaxation of the thermal cloud into empty spin states, reducing its proportion in the initial component. It cools only below a threshold magnetic field. The technique was initially demonstrated experimentally for a chromium dipolar gas (Naylor et al. in Phys Rev Lett 115:243002, 2015), whereas here we develop the concept further and provide an in-depth understanding of the required physics and limitations involved. Through numerical simulations, we reveal the mechanisms involved and demonstrate that the spin distillation cycle can be repeated several times, each time resulting in a significant additional reduction of the thermal atom fraction. Threshold values of magnetic field and predictions for the achievable temperature are also identified.

ОЦЕНКА ЭФФЕКТИВНОСТИ РАБОТЫ ТОРМОЗНОГО ОБОРУДОВАНИЯ ИННОВАЦИОННЫХ ГРУЗОВЫХ ВАГОНОВ

The object of researches is brake systems of innovation freight cars. The work purpose consists in the assessment of braking system effectiveness in innovation freight cars and in standard ones at different operation modes. &#x0D; The scientific novelty consists in the definition of the dependence of a slide block depth on the length of wheel pair skidding motion and braking distance dependence upon car speed. During the work there was carried out a retrospective analysis of cases with traffic safety violation connected with the formation of excessive sliders in freight cars in organized trains because of the violation of braking equipment normal operation. By means of the least-squares method there was obtained a linear equation defining the dependence of slider depth upon the skidding length of a wheel pair. As a result of the work there were defined parameters of defect formation on freight car wheels at their skidding, the calculations of braking system efficiency in different freight cars were carried out. &#x0D; In the course of the analysis of freight car braking efficiency there are obtained equations of braking length in the accepted range of speeds at the beginning of braking for empty and loaded states. Minimum values of the design factor of braking block pressing in translation to a train length of 200 shafts in the empty state were 0.2256 for a speed of 40km/h, in the loaded one – 0.1781 for a speed of 120 km/h at the standard values 0.22 and 0.14 respectively. By means of calculations and experiments it is proved that composite braking block pressing force on a shaft exceeds 4tf/shaft, and that of cast-iron – 10tf. Accordingly, a car with the shaft load of 25 tf/shaft and accepted parameters of a braking system may be operated up to the speeds of 120 km/h inclusive without limitations.