Zero Electron Research Articles

Threshold Photoionization and Density Functional Theory Investigations of Small Lanthanum Monoxide Clusters, LanO (n = 2-10).

Small lanthanum monoxide clusters, LanO (n = 2-10), have been studied by laser threshold photoionization (PI) spectroscopy and density functional theory (DFT). Ionization energies (IEs) gradually decrease with cluster size with oscillatory behavior at certain sizes at n = 4, 7, and 9, and the IEs are slightly lower than the corresponding bare Lan clusters. Mulliken charge analysis of the neutral and cation clusters indicates that the threshold PI occurs from a delocalized metal-based 5d orbital. The stable geometric structures and IEs for the LanO (n = 2-8) clusters have been determined from DFT. Zero electron kinetic energy and PI spectra have been calculated based on DFT calculations and compared with the experimental PI spectra. On the basis of favorable agreement between the simulated and experimental PI spectra, reliable ground-state geometric structures have been assigned. The O atom is negatively charged, equivalent to one electronic charge mainly transferred from La atoms bonded to it. Chemical bonding between the La and O atoms is ionic, whereas La atoms are mostly covalently bonded. No direct correlation is found between the electronic properties like the IE, electron affinity, and HOMO-LUMO gap and the stability of these clusters.

Theoretical Linkage of Thermionic, Field, and Space-Charge Limited Emission for a Vacuum Crossed-Field Gap

Crossed-field devices, such as magnetrons and crossed-field amplifiers, where an external magnetic field is orthogonal to the electric field across the gap, are used in radars, communications systems, and material processing. Improving the efficiency and power density of crossed-field devices requires a robust understanding of electron emission, particularly when multiple emission mechanisms may contribute, such as for thermionic and field emission. Previous studies of nonmagnetic diodes have theoretically identified the transitions, or nexuses, between multiple electron emission mechanisms by examining the relevant equations defining each individual mechanism and then derived exact theoretical solutions that combined these mechanisms and approached the standard emission equations under appropriate limits. This work extends those studies by incorporating the orthogonal magnetic field and assessing the transitions between the Richardson–Laue–Dushman (RLD) formulation for thermionic emission, the Fowler–Nordheim (FN) equation for field emission, the space–charge limit given by both the general (for nonzero electron injection velocity) and (zero electron injection velocity) Child–Langmuir (CL) (generalized CL (GCL) and CL, respectively) laws for space–charge limited current, and the space–charge limited current for a noninsulated (magnetic field below the Hull cutoff) and insulated (magnetic field above the Hull cutoff) diode. We derive exact solutions for the current density for these mechanisms and demonstrate their asymptotic behavior under relevant conditions. Furthermore, we plot the exact solution of current density <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$J$ </tex-math></inline-formula> as a function of applied voltage <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> by incorporating the general thermo-field emission theory for the current source and demonstrate the transition between each emission mechanisms with increasing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> for various temperatures and fixed ratios of magnetic field to the Hull cutoff. We also construct nexus plots to identify transitions between the various regimes for different conditions.

Design and Simulation of High-Aspect-Ratio Sheet Beam EIK at 0.22 THz

To improve the output performance of 0.22 THz extended-interaction klystrons (EIKs), we utilized a 20:1-aspect-ratio sheet beam to drive the circuit. The applicable high-frequency structure is discussed to obtain effective characteristic impedance, which is conducive to stronger beam-wave interaction. In addition, the loaded waveguide and the tuning scheme in the input cavity are proposed to provide good electric field uniformity over the whole beamwidth. Moreover, the nonuniform period output circuit based on the phase dynamic synchronization technology is optimized to further enhance power. Meanwhile, considering the feasibility of cathode performance, the beam current density in the tunnel is limited to 330 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The designed four-cavity interaction circuit is comprised of two 9-gap idler cavities and 11-gap input and output cavities. With the operating parameters of 16.2 kV beam voltage and 0.66 A beam current, the particle-in-cell (PIC) simulation predicts a stable output power of 700 W with a 39 mW input excitation. The corresponding gain and 3 dB bandwidth are 42.5 dB and 500 MHz, respectively. An asymmetric single-stage depressed collector is also designed to recover the spent beam energy from the interaction circuit. The collector efficiency is 81.1% with zero electron reflux rate while taking secondary electrons into consideration, and the overall tube efficiency is 26.9%.

Structure and electron affinity of (1 1 [formula omitted] 0)–X 4H–SiC surface

Control over the chemical termination of SiC surfaces is of great importance for the material in high temperature and hostile environment applications, where 4H-SiC in particular enjoys frequent use in areas including sensors and MOSFETs. Despite this, a wealth of surface specific data for the 4H polytype is yet to be accounted for, even under the most basic of terminations. Detailed structural and surface electronic properties highlight areas in which the use of SiC can expand and excel. In this work, we report density functional calculations on the effect of electron affinity upon adsorption of a variety of elements and their stability on the non-polar (112¯0) surface. We find that monolayer hydrogen and lithium termination work to reduce the electron affinity to 1.86 eV and 0.08 eV respectively, whereas for fluorine and chlorine it increases to large positive values of 5.05 eV and 3.90 eV respectively. All reactions with the unterminated surface are exothermic. In addition, lithium termination generating a near zero electron affinity makes it an exciting potential application in the field of emission. Notably, the increased efficiency of cold cathode devices with negative electron affinity make this surface a candidate for further work into functionalised SiC.

An electronic avalanche model for metal–insulator transition in two dimensional electron gas

In this paper, we present an electronic avalanche model for the transport of electrons in the disordered two-dimensional (2D) electron gas which has the potential to describe the 2D metal–insulator transition (MIT) in the zero electron–electron interaction limit. The disorder is considered to be uncorrelated-Coulomb noise with a uniform distribution. In this model we sub-divide the system to some virtual cells each of which has a linear size of the order of phase coherence length of the system. Using Thomas-Fermi-Dirac theory we propose some simple energy functions for the cells and using the thermodynamics of 2DEG we develop some rules for the charge transfer between the cells. A second order transition line arises from our model with some similarities with the experiments. The compressibility of the system also diverges on this line. We characterize this (disorder-driven) phase transition which is between the non-percolating phase and the percolating phase (in which the system shows metallic behavior) and obtain some geometrical critical exponents. The fractal dimension of the exterior frontier of the electronic avalanches on the transition line is compatible with the percolation theory, whereas the other exponents are different. The exponents are robust against disorder in the low disordered 2DEGs and change considerably in the high disordered ones.

Calculation of the Energy of Excited States with Zero Electron Orbital Angular Momenta

Within the framework of the variational method, as the result of a numerical calculation we have found the energy of the excited state and the screening constant of the helium atom in the electron configuration 1s 2s, which is metastable with respect to the single-photon transition to the 1s2 state, and also in the 1s 3s configuration. The results of numerical calculation of the energy are in approximate agreement with other values given for these states in the literature which are available to us. For the first time, numerical calculations have also been carried out for 1s ns configurations in the range of values n = 4, …, 9, in which the corresponding values of the energy En and the screening constant σn were obtained. It was found, in particular, that σn and |En| for the excited states in the range n = 2, 3, …, 9 fall monotonically with increasing n, where σn → 0 in the formal limit n → ∞, and En tends to the energy of a hydrogenlike atom with nuclear charge (2e), as it should be in accordance with the physical meaning of these quantities. The results of the calculations are illustrated graphically. The present study has important methodological significance in terms of the development and application of the basic principles of quantum mechanics to the helium atom.

Structure and properties of (AlB2)n and (MgB2)n (n = 1, …, 10) clusters

A systematic search for energetically lowest lying structures of neutral (AlB2)n and (MgB2)n clusters with n = 1, …, 10 is performed using density functional theory within a multistep hierarchical algorithm specially adapted for the global optimization of relatively large structures. For obtained clusters, different physical properties (energetic, electrostatic, electronic, and thermodynamic) are determined. The variation of these properties with increasing cluster size is discussed in detail. The bulk values of binding energy, specific zero point energy, ionization potential, electron affinity, collision diameter and formation enthalpy for aluminum and magnesium diborides have been obtained by means of physically sound extrapolation of the calculated data to the particles of infinite size. The temperature-dependent thermodynamic functions of (AlB2)n and (MgB2)n clusters, such as enthalpy, entropy, specific heat capacity, and reduced Gibbs energy, are evaluated with allowance for vibrational anharmonicity and for the existence of excited electronic states. The appropriate data are fitted to seven-parameter NASA (Chemkin) polynomials. The approximations of the reduced Gibbs energy applicable for extrapolation towards large clusters and even small nanoparticles are also elaborated.

Three-particle electron-hole complexes in two-dimensional electron systems

Three-particle complexes consisting of two holes in the completely filled zero electron Landau level and an excited electron in the unoccupied first Landau level are investigated in a quantum Hall insulator. The distinctive features of these three-particle complexes are an electron-hole mass symmetry and the small energy gap of the quantum Hall insulator itself. Theoretical calculations of the trion energy spectrum in a quantizing magnetic field predict that, besides the ground state, trions feature a hierarchy of excited bound states. In agreement with the theoretical simulations, we observe new photoluminescence lines related to the excited trion states. A relatively small energy gap allows the binding of three-particle complexes with magnetoplasma oscillations and formation of plasmarons. The plasmaron properties are investigated experimentally.

Reprint of Low-energy electron potentiometry

In a lot of systems, charge transport is governed by local features rather than being a global property as suggested by extracting a single resistance value. Consequently, techniques that resolve local structure in the electronic potential are crucial for a detailed understanding of electronic transport in realistic devices. Recently, we have introduced a new potentiometry method based on low-energy electron microscopy (LEEM) that utilizes characteristic features in the reflectivity spectra of layered materials [1]. Performing potentiometry experiments in LEEM has the advantage of being fast, offering a large field of view and the option to zoom in and out easily, and of being non-invasive compared to scanning-probe methods. However, not all materials show clear features in their reflectivity spectra. Here we, therefore, focus on a different version of low-energy electron potentiometry (LEEP) that uses the mirror mode transition, i.e. the drop in electron reflectivity around zero electron landing energy when they start to interact with the sample rather than being reflected in front of it. This transition is universal and sensitive to the local electrostatic surface potential (either workfunction or applied potential). It can consequently be used to perform LEEP experiments on a broader range of material compared to the method described in Ref[1]. We provide a detailed description of the experimental setup and demonstrate LEEP on workfunction-related intrinsic potential variations on the Si(111) surface and for a metal-semiconductor-metal junction with external bias applied. In the latter, we visualize the Schottky effect at the metal-semiconductor interface. Finally, we compare how robust the two LEEP techniques discussed above are against image distortions due to sample inhomogeneities or contamination.

Nonlinear Electrostatic Waves in PIE and PI Plasmas With Field-Aligned Shear Flow

Nonlinear low-frequency ion acoustic (IA) wave and pair plasma convective cell (PPCC) mode are investigated in pair-ion (PI) plasmas with and without electrons in the presence of sheared flows. IA wave forms both KdV solitons and dipolar vortices in PI-electron plasma, and their amplitude depends upon electron density and sheared flow. But when electron density is too low, the structures disappear, because IA does not exist in pure PI plasma. On the other hand, PPCC mode cannot form pulse-type solitary structures in the pair plasma, while it forms dipolar vortices due to finite vorticity. The nonlinear analysis has been performed assuming the presence of electrons in fullerene PI plasma and limit of zero electron density is also considered for a comparison.

An improved experimental scheme for simultaneous measurement of high-resolution zero electron kinetic energy (ZEKE) photoelectron and threshold photoion (MATI) spectra

An improved detection scheme is presented for threshold ionization spectroscopy with simultaneous recording of the Zero Electron Kinetic Energy (ZEKE) and Mass Analysed Threshold Ionisation (MATI) signals. The objective is to obtain accurate dissociation energies for larger molecular clusters by simultaneously detecting the fragment and parent ion MATI signals with identical transmission. The scheme preserves an optimal ZEKE spectral resolution together with excellent separation of the spontaneous ion and MATI signals in the time-of-flight mass spectrum. The resulting improvement in sensitivity will allow for the determination of dissociation energies in clusters with substantial mass difference between parent and daughter ions.

Numerical analysis of ion temperature effects to the plasma wall transition using a one-dimensional two-fluid model. II. Asymptotic two-scale limit

A one-dimensional, steady state, two fluid model, presented in Part I [T. Gyergyek and J. Kovačič, Phys. Plasmas 24, 063505 (2017)] is extended to the asymptotic two-scale limit. Separate solutions in the pre-sheath and in the sheath region are presented. Ion temperature is treated as an independent parameter, which is included in the model as a boundary condition. For the pre-sheath solutions, it is shown that when the ion temperature is increased, the ion flow velocity at the boundary of the system must also be increased. A simple relationship between ion temperature and ion flow velocity at the boundary is found. This relationship is the same as the corresponding relationship found in Part I. If ion temperature is increased, both the potential drop and the density drop in the pre-sheath decrease. The same is true for the pre-sheath length. As for the solutions in the sheath scale, it is shown that the ion velocity, electron velocity, and electric field at the sheath edge must all be above a certain minimum value in order to obtain physically acceptable monotonic solutions. It is proposed to select the ion velocity at the sheath edge equal to the ion sound velocity. If, at the same time, the zero electron flow velocity at the sheath edge is selected, the electric field at the sheath edge must be larger than roughly 3 × 10−6, in order to obtain monotonic solutions of the model. The selection of the electron velocity at the sheath edge is elaborated extensively. It is concluded that increased ion temperature improves the shielding of the plasma from the electrode.

Low-energy electron potentiometry

In a lot of systems, charge transport is governed by local features rather than being a global property as suggested by extracting a single resistance value. Consequently, techniques that resolve local structure in the electronic potential are crucial for a detailed understanding of electronic transport in realistic devices. Recently, we have introduced a new potentiometry method based on low-energy electron microscopy (LEEM) that utilizes characteristic features in the reflectivity spectra of layered materials [1]. Performing potentiometry experiments in LEEM has the advantage of being fast, offering a large field of view and the option to zoom in and out easily, and of being non-invasive compared to scanning-probe methods. However, not all materials show clear features in their reflectivity spectra. Here we, therefore, focus on a different version of low-energy electron potentiometry (LEEP) that uses the mirror mode transition, i.e. the drop in electron reflectivity around zero electron landing energy when they start to interact with the sample rather than being reflected in front of it. This transition is universal and sensitive to the local electrostatic surface potential (either workfunction or applied potential). It can consequently be used to perform LEEP experiments on a broader range of material compared to the method described in Ref[1]. We provide a detailed description of the experimental setup and demonstrate LEEP on workfunction-related intrinsic potential variations on the Si(111) surface and for a metal-semiconductor-metal junction with external bias applied. In the latter, we visualize the Schottky effect at the metal-semiconductor interface. Finally, we compare how robust the two LEEP techniques discussed above are against image distortions due to sample inhomogeneities or contamination.

Under pressure, helium forms stable molecules

Chemistry textbook authors may soon have to rewrite sections covering noble gases and chemical inertness. An international research team has reported the synthesis of a helium-sodium compound that’s stable at high pressures (Nat. Chem. 2017, DOI: 10.1038/nchem.2716). Helium’s best known feature is its unwillingness to react. With a stable closed-shell electron configuration, zero electron affinity, and an ionization energy that is higher than that of all other elements, helium defines chemical inertness. To examine the scope of the element’s low reactivity, scientists have searched for years—theoretically and experimentally—for helium-containing molecules. But they have turned up very little: for example, unusual species such as the HeH+ radical, which is stable only in its charged form, and HHeF, a metastable compound. In contrast, helium’s noble gas cousins xenon and krypton long ago showed themselves capable of forming a variety of stable compounds. Still, a team of researchers that includes Artem R. O...

A stable compound of helium and sodium at high pressure.

Helium is generally understood to be chemically inert and this is due to its extremely stable closed-shell electronic configuration, zero electron affinity and an unsurpassed ionization potential. It is not known to form thermodynamically stable compounds, except a few inclusion compounds. Here, using the ab initio evolutionary algorithm USPEX and subsequent high-pressure synthesis in a diamond anvil cell, we report the discovery of a thermodynamically stable compound of helium and sodium, Na2He, which has a fluorite-type structure and is stable at pressures >113 GPa. We show that the presence of He atoms causes strong electron localization and makes this material insulating. This phase is an electride, with electron pairs localized in interstices, forming eight-centre two-electron bonds within empty Na8 cubes. We also predict the existence of Na2HeO with a similar structure at pressures above 15 GPa.

Substrate effect on the interfacial electronic structure of thermally-evaporated CH3NH3PbI3 perovskite layer

The impact of substrate work function on the interfacial electronic structure of thermally-evaporated CH3NH3PbI3 perovskite films on various substrates have been systematically investigated using in-situ ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). On substrates with work function lower than ∼4.43 eV, a Fermi level pinning effect of the lowest unoccupied molecular orbital (LUMO) is observed, resulting in the near zero electron extraction barrier for the CH3NH3PbI3 perovskite solar cells. On the other hand, when substrates with high work function are used, even exceed the highest occupied molecular orbital (HOMO) of CH3NH3PbI3, an almost constant hole extraction barrier of ∼0.88 eV is observed, indicating that the efficiency of hole extraction at these interfaces are low. In order to understand the low hole extraction efficiency at interfaces between CH3NH3PbI3 and these high work function electrodes, the evolution of electronic structures at the interface between CH3NH3PbI3 and MoO3 is further investigated. The charge transfer and dipole formation between CH3NH3PbI3 and MoO3 are deduced from the UPS and XPS results, and the energy level alignment between CH3NH3PbI3 and MoO3 is discussed.

Reconstruction of the electron diffusion region

AbstractWe discuss mathematical tools for the reconstruction of two‐dimensional, time‐independent magnetic field and flow in the electron diffusion region, at a site of antiparallel magnetic reconnection. The basic assumptions are that the ions are stationary and have constant density. The width of the reconnection layer is of the order of the electron gyroradius or the electron inertial length. Our model includes the axial electron pressure term in Ohm's law developed by M. Hesse and coworkers. We demonstrate the feasibility of doing reconstruction of electron magnetohydrodynamic (EMHD) structures for a simplified system with zero electron inertia. The code is benchmarked using an exact solution that has antiparallel unidirectional magnetic fields, plus out‐of‐plane quadrupolar Hall fields, as well as the expected slow electron inflow and rapid exit jets. The inertialess reconstruction is then applied to synthetic data from a 2‐D, particle‐in‐cell, simulation of antiparallel reconnection. We find that the inertialess reconstruction of its electron diffusion region works reasonably well only when the spacecraft path passes close to the center of the reconnection site where the magnetic field is zero and the electron flow has a stagnation point. When the path is located farther away, the effects of electron inertia, and probably also deviations from the Hesse formula, cause the quality of the reconstruction to deteriorate. Electron inertia is included in the theoretical development presented here but requires a more complicated numerical reconstruction code. The development and testing of such a code is underway and will be presented separately.

Symmetry-dependent spin-charge transport and thermopower through a ZSiNR-based FM/normal/FM junction

We investigate the spin-dependent transport and spin thermopower for a zigzag silicene nanoribbon (ZSiNR) with two ends covered by ferromagnets (FMs) under the modulation of a perpendicular electric field, where we take the 6- and 7-ZSiNR to exemplify the effect of the even- and odd-N ZSiNRs, respectively. By using the nonequilibrium Green’s function approach, it is demonstrated that a ZSiNR-based FM/normal/FM junction still shows an interesting symmetry-dependent property although the σ mirror plane is absent for any ZSiNR due to the buckled structure of silicene. The junction with even- or odd-N ZSiNR has very different transport and thermopower behavior, which is attributed to the different parity of π and band wavefunctions under the c2 symmetry operation with respect to the centre axis between two edges, and is linked to the unique symmetry of the band structure for the ribbon. As a result, the magnetoresistance (MR) for the 6-ZSiNR junction with a 100% plateau around zero electron energy is observed, but the plateau is absent for the 7-ZSiNR one. In addition, the spin thermopower also displays the even–odd behaviour. The 6-ZSiNR junction is found to possess superior thermospin performance compared with the 7-ZSiNR one, and its spin thermopower can be improved by one order of magnitude in the absence of an electric field. As the strength of the field increases, the spin thermopower for the 6-ZSiNR junction dramatically decreases, while it notably enhances for the 7-ZSiNR one. Interestingly, the spin thermopower for both junctions is strongly dependent on the strength of magnetisation in FM, and it can be very pronounced with a maximum absolute value of 200 μV K−1by the optimisation of the parameters. However, with the increase in temperature, the spin thermopower for the 6-ZSiNR junction decreases, but the situation for the 7-ZSiNR one is opposite. Finally, the spin figure of merit for the 6-ZSiNR junction is found to be four orders of magnitude larger than that for the 7-ZSiNR one. This even–odd effect is common for N-SiNR, and the result can be regarded as an advance in the understanding of the characteristics of silicene and may be valuable for experimentally designing spin valve and heat spintronic devices based on silicene.

Determination of electron energy probability function in low-temperature plasmas from current – Voltage characteristics of two Langmuir probes filtered by Savitzky–Golay and Blackman window methods

Acceptable data for electron energy probability function (EEPF) measurement in low-pressure plasmas require a small depletion in near-zero electron energy and a large dynamic range in the high-energy regime. The voltage drop across internal resistance and noise from the data acquisition system cause a rounding of the EEPF near the zero electron energy and a reduction in the dynamic range due to a low signal-to-noise ratio of the high-energy regime, respectively, leading to erroneous interpretation in the EEPF measurement. A digital smoothing filter can be employed to reduce the noise signal, but it can also cause additional depletion near the zero energy. In order to obtain reliable EEPF data, a novel technique is proposed using two Langmuir probes with differing collecting areas and the Savitzky–Golay and Blackman window methods. The technique enables the internal resistance effect to be removed using a slope of the current – voltage characteristic taken from the probes. In addition, the Savitzky–Golay and Blackman window methods in the technique apply separately to two regimes of the EEPF, i.e., the near-zero energy regime and the inelastic energy regime, because appropriate smoothing methods that minimize loss and distortion of information differ for the two regimes. This allows one to decrease the noise signal, minimizing the additional depletion near the zero energy. This technique improves the dynamic range of the EEPF from 30 to 55 dB to 65–105 dB and provides more accurate electron density and effective electron temperature from the EEPF, compared to those of the conventional technique.

Ultracold plasma expansion as a function of charge neutrality

Ultracold plasmas (UCPs) are created under conditions of near but not perfect neutrality. In the limit of zero electron temperature, electron screening results in non-neutrality manifesting itself as an interior region of the UCP with both electrons and ions and an exterior region composed primarily of ions. The interior region is the region of the most scientific interest for 2-component ultracold plasma physics. This work presents a theoretical model through which the time evolution of non-neutral UCPs is calculated. Despite Debye screening lengths much smaller than the characteristic plasma spatial size, model calculations predict that the expansion rate and the electron temperature of the UCP interior is sensitive to the neutrality of the UCP. The predicted UCP dependence on neutrality has implications for the correct measurement of several UCP properties, such as electron temperature, and a proper understanding of evaporative cooling of the electrons in the UCP.