Real Metal Research Articles

Current transport and formation of energy structures in narrow Au/n-GaAs Schottky diodes

The current transport and formations of potential barrier height in narrow Au/n-GaAs Schottky diodes (SD) with a contact surface in length of 200 μm, width of 1 and 4 μm have been investigated. It was determined that features of current transport are in good agreements with the thermionic emission theory in the forward bias as like high-quality conventional (flat) SD. Features of current transport in the reverse bias also is well described by thermionic emission theory, but it has specific features unlike I– V characteristics flat SD. Forward bias of narrow SD current–voltage ( I– U) characteristics are represented by straight lines in semi-logarithmic scale in a wide range, nearly nine order of current up to 0.7 V with near unit ideality factor. In the beginning of the reverse voltage, the current practically was extremely low, by increasing in voltage the current jump in steps approximately for 3–4 order in voltage of 3–4 V, then current increases linear for 3–5 order in semi-logarithmic scale by increasing in voltage up to nearly 7 V. Numerical values of parameters such as the saturation currents, the operating barrier height, ideality factor, dimensionless factor are obtained. The correlations between ideality factor and dimensionless factor were meaningful. The energy diagrams of narrow SD have been drawn in absence and presence of forward and reverse voltage. It is found that electronic processes in narrow SD are well described by energy model of real narrow metal–semiconductor contacts. The additional electric field arising in near contact area of the semiconductor because of creating contact potential difference between contact surface and to it adjoining free surfaces of the metal and semiconductor.

Holes in a Kondo lattice

Real metals invariably contain impurity atoms, and at concentrations of parts per million, nonmagnetic impurities have a barely detectable influence on the electronic properties of the vast majority of metals. On the other hand, pronounced changes can arise from magnetic impurities, such as iron impurity atoms in gold. At room temperature the magnetic moment carried by iron's 3d electrons is decoupled from the electrically conducting electrons of gold and produces a magnetic susceptibility expected for atomic iron moments, but at much lower temperatures the iron moments are compensated exactly by the collective antiferromagnetic alignment of spins carried by conduction electrons. The inherently quantum many-body process of moment compensation, called the Kondo effect (1), builds up a resonance in the electronic density of states (2), giving the conduction electrons a heavy mass. An extreme limit of the Kondo-impurity effect is found in intermetallic compounds that are built structurally from a periodic lattice of 4f- or 5f-electron Kondo atoms, such as cerium or uranium. Except for the very heavy mass of their itinerant charge carriers and the physics that underlies this mass enhancement (3), these Kondo-lattice or heavy-electron metals at very low temperatures are physically equivalent to a simple metal like gold. Writing in PNAS, Hamidian et al. (4) address the question of what happens when a magnetic atom U is replaced by a nonmagnetic impurity atom Th in the Kondo lattice formed in URu2Si2. Unlike the negligible influence of Th impurities in gold, the Th defect induces strong nanoscale electronic heterogeneity in URu2Si2 that is revealed in spectroscopic images obtained with a scanning tunneling microscope (STM). In effect, the nonmagnetic atom “digs a hole” in the strongly correlated electron state of URu2Si2, with consequences that ripple well beyond the immediate …

Transmission characteristics of circular metallic waveguides for terahertz radiation

Transmission characteristics of oversized circular metallic waveguides excited by linearly polarised Gaussian laser beams in the terahertz range (4 — 28 THz) are studied theoretically andexperimentally. Calculating the transmission characteristics, we have determined the conditions of applicability of the method of the eigenoscillations in the approximations of a real metal by an idealmetal or dielectric, depending on the transmitted radiation frequency. The existence of the transition region is established in the behaviour of the electrodynamic properties of metallic waveguidesin the frequency range of 7.5 — 15 THz.

The electromagnetics of light transmission through subwavelength slits in metallic films

By numerically calculating the relevant electromagnetic fields and charge current densities, we show how local charges and currents near subwavelength structures govern light transmission through subwavelength apertures in a real metal film. The illumination of a single aperture generates surface waves; and in the case of slits, generates them with high efficiency and with a phase close to -π with respect to a reference standing wave established at the metal film front facet. This phase shift is due to the direction of induced charge currents running within the slit walls. The surface waves on the entrance facet interfere with the standing wave. This interference controls the profile of the transmission through slit pairs as a function of their separation. We compare the calculated transmission profile for a two-slit array to simple interference models and measurements [Phys. Rev. B 77(11), 115411 (2008)].

Terahertz surface plasmon polaritons on a conductive right circular cone: Analytical description and experimental verification

We report on an analytical solution of Maxwell's equations for the propagation of surface plasmon polaritons on a right circular cone. The problem was solved for THz frequencies in real metals and was therefore derived using the Leontovich approximation, which is valid for media with small surface impedances. The solution also accounts for both surface plasmon polaritons that are axisymmetric and those that have an angular structure in a plane normal to the cone's axis. This was an important consideration since it is crucial for describing surface phenomena such as surface-enhanced absorption, fluorescence, and Raman scattering. Our findings predict a total reflection of surface plasmon polaritons at the cone's apex, which was experimentally verified by an absence of light emitted from a heated cone's tip into the far-field region.

Modeling the effect of the microstructure of compacted graphite iron on chip formation

The unique mechanical and physical properties of compacted graphite iron (CGI) have awarded the material such desirable and increasing demands in both automotive and locomotive industries. The graphite round edges combined with irregular graphite boundaries highly enhance crack arrest resistance within the matrix and participate into the good adhesion of graphite–matrix interface, compared to gray cast iron. However, the praised mechanical performance of compacted graphite iron (CGI) compared to gray iron, and its superior thermal properties compared to nodular iron (ductile iron) have come with CGI's relative poor machinability. Finite element simulation of the microstructure of CGI will provide better understanding of the behavior of the metal during machining and will establish a good foundation of CGI machining optimization. Modeling of the microstructure of CGI chip considering the three main constituents of the metal; graphite, pearlite, and ferrite, was possible using the accumulated plastic strain fracture criterion. Although there is no distinctive boundary line between adiabatic shearing and surface crack initiation chip formation principles in real metal cutting, chip formation simulation showed that it was predominantly due to crack initiation and propagation. Cracks initiated at either the graphite particles or the graphite–matrix interface promoted by the fracture of surface graphite particles then progressed through the matrix. The characteristic segmental chip produced in CGI machining was mainly driven by the presence of graphite embedded in the matrix. Chip segments formation initiated at the graphite particles (or graphite–matrix interface) then progressed towards the chip-tool tip. Modeling of the graphite/matrix interface was based on the utilization of cohesive zone elements. Comparison between the modeled CGI microstructure and graphite-free modified microstructure highlights the significant role of graphite on chip characteristics. Comparison between the simulated segmental chip and real CGI chip validated the proposed simulation and proposes it as a valid foundation for further CGI machining optimization.

Oscillation dynamics of metal wire sensor elements

An attempt is made to present a number of models and their properties, physically and mathematically. They constitute the most important part of the physical theory of a metal wire sensor element (a real metal wire). Possible further developments for effective modelling of the oscillation dynamics of this type of sensor element are discussed.

Optimal algorithm for constructing the embedded atom method potential for liquid metals

An optimal algorithm has been developed for calculating the embedding potential in the embedded atom method (EAM) with the aim of describing not only the temperature-dependent density of a liquid metal but also its energy up to its critical temperature. The algorithm is based on the unification of the form of the potential and calculation of its parameters from known density and energy data for the liquid metal. The basis of the algorithm is the use of least squares fitting of the pressure and energy in molecular dynamics simulations to data for a series of states of the liquid along an isobar. To describe liquid potassium, the pair contribution to the potential is represented by a power series in interparticle distance. Data on the properties of potassium at 343, 473, 723, 1000, 1500, 2000, and 2200 K were used. The embedding potential was expanded in terms of 1 − ρ, where ρ is the effective electron density in the EAM. In least squares fitting, fifteen equations were included: eight for the energy and seven for the pressure. The number of unknown coefficients was seven. Iterative calculations allow one to find optimal expansion coefficients and construct equilibrium models through molecular dynamics simulations. It is shown that the discrepancy in energy between simulations and the real metal at high temperatures can be eliminated by taking the electron excitation energy into consideration. The difference between the actual energy of a metal and the energy obtained in EAM simulations is very close to the contribution of the electron heat capacity.

Electromagnetic energy density in dispersive and dissipative media

The description of the energy density associated with an electromagnetic field propagating through matter must treat two different phenomena: dispersion, the variation of the refractive index with frequency, and dissipation, the loss of field energy by absorption. In many cases, as in common dielectrics, the dispersive medium is essentially transparent, so dissipation can be neglected. For metals, however, both dispersion and dissipation must be taken into account, and their respective contributions vary significantly with the frequency of the electromagnetic field. Plasmonic structures such as slits, holes, and channel waveguides always involve surfaces between dielectrics and metals, and the energy density in the vicinity of the interface figures importantly in the dynamic response of these structures to light excitation in the visible and near-infrared spectral regions. Here we consider the electromagnetic energy density propagated on and dissipated at real metal–dielectric surfaces, including the important surface plasmon polariton, the wave guided by the interface. We show how the “stored energy” oscillates over an optical cycle between the plasmonic structure and the propagating surface mode, while the dissipated energy continues to accumulate over the same period. We calculate these energy densities for the case of the silver–air interface (using two datasets for silver permittivity commonly cited in the research literature) over a range of frequencies corresponding to the range of wavelengths from 200 to 2000 nm.

Investigation of nano patches in Ni/n-Si micro Schottky diodes with new aspect

High-quality Ni/n-Si Schottky barrier diodes (SBDs) with low reverse leakage current were produced using a molybdenum (metal) mask. An identical preparation of the diodes shows a diode-to-diode variation in the ideality factor and barrier height parameters. Topological, phase and potential distribution investigations of the surface before and after preparation of the thin metal film for ohmic and Schottky contacts with a Conducting Probe Atomic Force Microscope (CP-AFM) show that thin metal film deposited on Ni/n-Si Schottky diode (SD) consists of many nano patches. Investigation of the patches in the ohmic contact side shows that these ohmic contacts also consist of nano patches in the range 20–40 nm with a variation in the potential distribution reaching about 1 V, resulting in different electrical behaviors in different regions of the contact surface. Even with sufficient care in the preparation and cleaning of the contact surface there is a potential difference of about 0.5 V between different parts of the semiconductor surface, which is one of the sources of different electrical behaviors of the substrate. These patches are sets of parallelly connected and electrically cooperating nano contacts with size between 20 and 40 nm. The barrier heights (BH) and ideality factor ( n) are affected by the potential distribution on the surface. There is a spot field between the patches with different local work functions, which are in direct electrical contact with the surrounding patches. It is shown that in the real metal–semiconductor (MS) contacts, patches with quite different configurations, various geometrical sizes and local work functions are randomly distributed on the metal surface. The relation between ideality factor ( n) and potential barrier height (BH) depends on the distribution of patches and their work functions.

Homogeneous phase and multi-phase approaches for modeling radiative transfer in foams

The aim of the present study is to investigate the suitability of two different continuum-based approaches for the modeling of the radiative transfer in two types of foams lying in the geometric optic regime: open cell metal foams and closed cell polymer foams. The two approaches are the commonly used Homogeneous Phase Approach (HPA) and the Multi-Phase Approach (MPA) which is rather new in the field of radiative transfer. For both approaches, the radiative properties involved in their respective frameworks are determined using newly developed Ray-Tracing methods applied to 3-D meshes representing the porous structures of open cell or closed cell foams. The 3-D meshes have been obtained from X-Ray Tomography applied to real metal and polymer foams. The radiative properties determined are used to compute the transmittances and reflectances of one-dimensional slabs of foams by the two approaches. They are compared with the results of a baseline Monte Carlo simulation in order to evaluate, for the two types of foams, the suitability of each method. It appears that both approaches are globally appropriate for predicting the radiative transfer in open cell metal foams although they are not able to match exactly the directional distribution of the transmittances and reflectances. For polymer foams, the accuracy of the HPA is demonstrated whereas the MPA provides significant differences with the reference MC simulations.

Optimal sensor placement for fixture fault diagnosis using Bayesian network

PurposeFixture failures are the main cause of the dimensional variation in the assembly process. The purpose of this paper is to focus on the optimal sensor placement of compliant sheet metal parts for the fixture fault diagnosis.Design/methodology/approachBased on the initial sensor locations and measurement data in launch time of the assembly process, the Bayesian network approach for fixture fault diagnosis is proposed to construct the diagnostic model. Furthermore, given the desired number of sensors, the diagnostic ability of the sensor set is evaluated based on the mutual information of the nodes. Thereby, a new sensor placement method is put forward and validated with a real automotive sheet metal part.FindingsThe new proposed method can be used to perform the fixture fault diagnosis and sensor placement optimization effectively, especially in a data‐rich environment. And it is robust in the presence of measurement noise.Originality/valueThis paper presents a novel approach for fixture fault diagnosis and optimal sensor placement in the assembly process.

Electrical resistivity of pure transuranium metals under pressure

The influence of electronic structure evolution upon pressure on the temperature dependence of the electrical resistivity of pure Np, Pu, Am, and Cm metals has been investigated within the coherent potential approximation (CPA) for the many-bands conductivity model. The densities of states calculated within the local density approximation accounting for the Hubbard U and spin-orbit coupling (LDA+ U+SO method) for the cubic (bcc- or fcc-) structures with the volumes fitted to the relative volumes of real metal phases were used as a starting point for model investigations of the electrical resistivity. The obtained results were found in good agreement with a number of available experimental data. The origin of large magnitude of the resistivity of the actinides was discussed.

Tuning plasmonic resonances of an annular aperture in metal plate

We present theory to describe the plasmonic resonances of a subwavelength annular aperture in a real metal plate. The theory provides the reflection, including the amplitude and phase, of radially polarized surface plasmon waves from the end faces of the aperture with a significant departure from the perfect electric conductor case due to plasmonic effects. Oscillations in the reflection amplitude and phase are observed. These oscillations arise from transverse resonances and depend on the geometry of the annulus. The theory is applied to the design of various aperture structures operating at the same resonance wavelength, and it is confirmed by comprehensive electromagnetic simulations. The results are contrasted to the perfect electric conductor case and they will be of significant interest to emerging applications in metamaterials, plasmonic sensors, and near-field optics.

Description of the modes governing the optical transmission through metal gratings

An analytical model based on a modal expansion method is developed to investigate the optical transmission through metal gratings. This model gives analytical expressions for the transmission as well as for the dispersion relations of the modes responsible for high transmission. These expressions are accurate even for real metals used in the visible - near-infrared wavelength range, where surface plasmon polaritons (SPP's) are excited. The dispersion relations allow the nature of the modes to be assessed. We find that the transmission modes are hybrid between Fabry-Pérot like modes and SPP's. It is also shown that it is important to consider different refractive indices above and below the gratings in order to determine the nature of the hybrid modes. These findings are important as they clarify the nature of the modes responsible for high transmission. It can also be useful as a design tool for metal gratings for various applications.

Effect of nanopatches on electrical behavior of Ni/n-type Si Schottky diode

Topological surface measurement of thin metal film using a conducting probe atomic force microscope (C-AFM) shows that thin metal film deposited on Ni/n-Si Schottky diode (SD) consists of patches. These patches are sets of parallel connected and electrically cooperating nano-contacts of size between 50 and 100nm. Every individual patch acts as an individual diode with different I– V curve, barrier height (BH) and ideality factor ( n). Between these diodes or patches, there are spot field distributions; the patches with different local work functions are in direct electric contact with surrounding patches. As a result, a potential difference between surfaces of patches, the so-called electrostatic spot field E f , is formed. It is shown that in real metal–semiconductor (MS) contacts, patches with quite different configurations, various geometrical sizes and local work functions are randomly distributed on the surface of metal; hence direction and intensity of spot field are non-uniformly distributed along the surface of metal. There is a linear dependence between barrier height and ideality factor, which is the consequence of reduction of distance of the maximum of BH from the interface. This dependency is the sign of reduction of contribution of a peripheral current.

Transmission and scattering properties of subwavelength slits in metals

Using an advanced eigenmode-expansion method, we analyze the basic transmission and scattering properties of subwavelength slits in real metals characterized by the complex optical permittivity ${\ensuremath{\varepsilon}}_{m}$. This includes the slit-width, wavelength, and ${\ensuremath{\varepsilon}}_{m}$ dependences of the efficiencies and cross sections of the main transformation processes: (i) transformation of the incident plane wave into the propagating mode, into the surface plasmons, and into diffracted waves in air and (ii) internal reflection of the propagating mode and its transformation to the surface plasmons and diffracted waves. In conjunction with the known perfect-metal-related efficiencies, the established dependences exhibit a wealth of important subwavelength features, including the nontrivial transmission peculiarities. Transition from the case of the periodic array of slits to the single-slit case when increasing the metal-wall width is considered as well. The established characteristics of a single interface between air and perforated metal are sufficient to describe the extraordinary light transmission through subwavelength slits in metal films.

Influence of methane concentration on the electric transport properties in heavily boron-doped nanocrystalline CVD diamond films

ABSTRACTA study is presented on the morphology and electric properties of heavily boron-doped nanocrystalline diamond (B:NCD) thin films (≈150snm) grown with two different C/H-ratios (1% and 5%) and a fixed 5000sppm B/C-ratio in gas phase on fused silica substrates. AFM measurements confirm that a higher C/H-ratio leads to smaller grains and more grain boundaries. Electric transport measurements reveal a higher resistivity and a lower mobility as function of the C/H-ratio for all temperatures measured. The resistivity of the 1% sample is almost not temperature dependent while the 5% sample is much more temperature dependent. The electric transport properties of the grain boundaries, more present in the 5% sample, can be responsible for the difference in transport properties of both samples. The active boron concentration, calculated from the electric transport measurements, is remarkably higher for the 5% sample which indicates there is more boron incorporation for higher C/H-ratios. Although both samples are disordered metals, the 1% sample with the least grain boundaries tends more to the behavior of a highly doped single crystalline diamond film, which behaves like a real metal when heavily boron-doped.

Influence of Electron-Electron Correlations and Lattice Effects on Positron-Electron Enhancement Factors

We present an approach taking into account the effect of electron-electron (e-e) correlations on electron-positron (e-p) momentum density distributions. The approach bases on the modification of the Bethe-Goldstone (B-G) equation for the positron in the electron gas due to self-energy effects. The example calculations have been performed for selected parameters corresponding to simple metals. The calculated dependencies exhibit the increase of the e-p enhancement factors below Fermi momentum, like Kahana enhancements, and a decrease above the Fermi sphere, leading to a many-body “tail” in the e-p momentum density distributions. Moreover, the influence of lattice effects on enhancement factors (EF) is taken into account. This decreases by a few percent the absolute values of the e-p momentum distributions and the corresponding annihilation rates and for real metals such as Mg or Cu evidently improve the agreement with experiment.

Influence of exchange correlation on the symmetry and properties of siderite according to density-functional theory

Density-functional theory (DFT) without spin-orbit coupling (SOC), as implemented in CASTEP, yields structural, energetic, and elastic properties of siderite $({\text{FeCO}}_{3})$ in good agreement with available data from the literature. The electronic ground state is however found ferromagnetic (FM) with a nonzero density of states at the Fermi level, whereas real siderite is antiferromagnetic (AFM) and insulating. We show in both cases how the paradox can be removed. Concerning the metallic/insulating character, a symmetry analysis of the electronic structure performed with the help of ISOTROPY shows that the absence of gap is due to an essential degeneracy of monoelectronic wave functions in the experimental $R\overline{3}c$ symmetry. The small dispersion of individual bands establishes a clear difference with the case of a real metal. Complemented by $\text{DFT}+\text{SOC}$ computations performed using ABINIT this shows the consistency of the DFT description with crystal-field theory data taken from the literature. Thus for DFT the electronic structure of siderite is characterized by correlation, but of exchange origin, as opposed to electrostatic origin. The AFM nature of the ground state is also restored by SOC. Without SOC the total energies of the FM and AFM states are very close. After introducing some modifications in a classical model of superexchange we obtain a satisfactory interpretation of our DFT results in terms of spin polarization in the iron-carbonate bonds, spin-coupling mechanisms between neighboring iron ions and finally of the observed FM over AFM stability. Hence, siderite emerges as an original model system among transition-metal compounds for the application of DFT to open-shell electronic structures.