Showing papers in "Physica Scripta in 2004"
TL;DR: Angle-resolved photoemission spectroscopy (ARPES) as mentioned in this paper is one of the most direct methods of studying the electronic structure of solids, in particular for those complex systems which cannot be appropriately described within the independent-particle picture.
Abstract: Angle-resolved photoemission spectroscopy (ARPES) is one of the most direct methods of studying the electronic structure of solids. By measuring the kinetic energy and angular distribution of the electrons photoemitted from a sample illuminated with sufficiently high-energy radiation, one can gain information on both the energy and momentum of the electrons propagating inside a material. This is of vital importance in elucidating the connection between electronic, magnetic, and chemical structure of solids, in particular for those complex systems which cannot be appropriately described within the independent-particle picture. The last decade witnessed significant progress in this technique and its applications, thus ushering in a new era in photoelectron spectroscopy; today, ARPES experiments with 2 meV energy resolution and 0.28 angular resolution are a reality even for photoemission on solids. In this paper we will review the fundamentals of the technique and present some illustrative experimental results; we will show how ARPES can probe the momentum-dependent electronic structure of solids providing detailed information on band dispersion and Fermi surface as well as on the strength and nature of many-body correlations, which may profoundly affect the oneelectron excitation spectrum and in turn the macroscopic physical properties.
414 citations
TL;DR: In this article, a full formalism for the calculation of the second-order optical response for semiconductors and insulators is presented, where the expressions for the optical susceptibilities are derived within perturbation theory.
Abstract: We present a full formalism for the calculation of the linear and second-order optical response for semiconductors and insulators. The expressions for the optical susceptibilities are derived within perturbation theory. As a starting point a brief background of the single and many particle Hamiltonians and operators is provided. As an example we report calculations of the linear and nonlinear optical properties of the mono-layer InP/GaP (110) superlattice. The features in the linear optical spectra are identified to be coming from various band combinations. The main features in the second-order optical spectra are analyzed in terms of resonances of peaks in linear optical spectra. With the help of the strain corrected effective-medium-model the interface selectivity of the second-order optical properties is highlighted.
144 citations
TL;DR: The reliability of the assumption of a Maxwell-Boltzmann displacement distribution in a steady state diffusion process has been discussed and the appropriateness of both the treatments relating the diffusion coefficient and the Brownian motion have been questioned.
Abstract: Einstein and Smoluchowski's treatment of the Brownian motion has been discussed. It is shown that although the diffusion equations derived from both treatments are mathematically identical, Smoluchowski's equation defines a somewhat concentration-dependent diffusion coefficient, while Einstein's equation defines a constant diffusion coefficient. The reliability of the assumption of a Maxwell–Boltzmann displacement distribution in a steady state diffusion process has been discussed and the appropriateness of both the treatments relating the diffusion coefficient and the Brownian motion have been questioned.
137 citations
TL;DR: In this paper, a numerical technique for solving some physical problems on a semi-infinite interval is presented, which is based on a rational Legendre tau method and the operational matrices of derivative and product of rational linear Legendre functions are used to reduce the solution of these physical problems to the solutions of systems of algebraic equations.
Abstract: A numerical technique for solving some physical problems on a semi-infinite interval is presented. Two nonlinear examples are proposed. In the first example the Volterra's population model growth is formulated as a nonlinear differential equation, and in the second example the Lane–Emden nonlinear differential equation is considered. The approach is based on a rational Legendre tau method. The operational matrices of derivative and product of rational Legendre functions are presented. These matrices together with the tau method are utilized to reduce the solution of these physical problems to the solution of systems of algebraic equations. The method is easy to implement and yields very accurate results.
117 citations
TL;DR: In this article, the authors give a personally flavored expose of static density functional theory (DFT), starting from standard many-body physics at a very elementary level and then gradually introducing the basic concepts of DFT.
Abstract: In these notes I have given a personally flavored exposeof static density- functional theory (DFT). I have started from standard many-body physics at a very elementary level and then gradually introduced the basic concepts of DFT. Successively more advanced topics are added and at the end I even discuss a few not yet published theories. The discussion represents many of the personal views of the author and there is no attempt at being comprehensive. I fully realize that I am often 'unfair' in treating the achievements of other researchers. Many topics of standard DFT are deliberately left out like, e.g., time- dependence, excitations, and magnetic or relativistic effects. These notes represent a compilation of a series of lectures given at at the EXC!TING Summer School DFT beyond the ground state at Riksgransen, Sweden in June of 2003. 1. Background Many reviews and articles on the topic of density- functional theory (DFT) start by the proclamation that, over the past so and so years, DFT has become, by far, the most prominent tool for the calculation of the ground-state properties of electronic systems. Although somewhat vacuous the statement is definitely true. In fact, DFT calculations of the electronic properties of real materials have nowadays turned into an extensive industrial endea- vor. The idea of using the density as the basic variable for the description of the energies of electronic systems goes back almost to the advent of quantum mechanics and the realization that the solution of the full equation of Schrodinger was beyond reach in most cases. The statistical atom of Gombas (1) and the approximations by Thomas (2) and Fermi (3) were early attempts in this direction. Then, of course, came the Hohenberg and Kohn (4) theorems in the mid sixties followed by the work by Kohn and Sham (5). They demonstrated that the electron density of a fully interacting system could actually, in a rigorous way, be obtained from simple one-electron theory. At that time, most researchers involved with the calculation of the electronic properties of atoms, molecules, and solids where strongly influenced by the school of J. C. Slater. Properties were calculated from one-electron theory using a statistical approximation but only for the effect of exchange and correlation. The latter was obtained in 1951 by Slater (6) as an average over the Fermi sea of the self-energy of the homogeneous electron gas treated within the Hartree-Fock approximation. As a matter of fact, from a numerical point of view, that theory was not very different from modern DFT within the local-density approximation (LDA).
108 citations
TL;DR: It is concluded that plasma can efficiently kill bacteria without harming the cells, and also influence the cells without causing cell death (necrosis) and in future it will lead to applications like skin and caries treatment.
Abstract: This paper surveys the research activities at the Eindhoven University of Technology (The Netherlands) in the area of biomedical applications of gas discharge plasmas. A non-thermal atmospheric plasma source (the plasma needle) has been developed, and its interactions with living mammalian cells and bacteria are studied. It is concluded that plasma can efficiently kill bacteria without harming the cells, and also influence the cells without causing cell death (necrosis). In future it will lead to applications like skin (wound) and caries treatment.
83 citations
TL;DR: In this article, beryllium is seeded into a deuterium plasma to simulate the flow of the scrape-off layer plasma from the first wall to the divertor in ITER.
Abstract: Experiments at UC San Diego PISCES-B, in collaboration with EFDA, are investigating the influence of beryllium impurities on deuterium plasma erosion of graphite material. The experiments are designed to reduce uncertainties in the prediction of tritium retention in redeposited mixed-materials expected in future burning plasma devices. Earlier PISCES-B experiments hinted that small amounts of beryllium surface impurities on graphite could reduce the rate of chemical erosion. In the present experiment, beryllium is seeded into a deuterium plasma to simulate the flow of the scrape-off layer plasma from the first wall to the divertor in ITER. Plasma containing low levels of Be impurities have so far been investigated and a greater reduction of carbon erosion is found. For beryllium concentrations above ~ 0.1%, a thin coating of beryllium is observed to form on the surface of the graphite. This beryllium layer suppresses chemical and physical erosion of the underlying carbon. The ITER divertor plasma is expected to contain percent levels of beryllium impurities, so similar beryllium deposition may occur on the ITER divertor plates. Future experiments at PISCES will quantify the formation rate of redeposited carbonaceous films and their suppression, and aim at better simulating ITER conditions including higher operating temperature ranges for the graphite targets, and larger beryllium impurity fractions.
83 citations
TL;DR: In this paper, a discussion of subtleties in handling continuum states with regard to their normalization, orthogonality, and completeness has been presented, together with some of the simplest derivations of the Fano formula.
Abstract: The general profile of a resonance, as given by Fano in 1961, is well known throughout physics. This Comment provides some of the simplest derivations of this formula, together with a discussion of subtleties in handling continuum states with regard to their normalization, orthogonality, and completeness. Inadequate appreciation of these subtleties has led to misconceptions and errors in analysis. Recent, novel applications of the Fano formula in condensed matter systems include systems with decoherence and lack of time reversal symmetry which make the resonance profile parameter complex.
78 citations
TL;DR: In this article, the effects of ions on the structure and dynamics of the hydrogen bonds in liquid water were studied using femtosecond two-color mid-infrared spectroscopy.
Abstract: We study the effects of ions on the structure and dynamics of the hydrogen bonds in liquid water. As a technique we use femtosecond two-color mid-infrared spectroscopy, since this technique allows a clear distinction of the dynamics of the first solvation (hydration) shell of water molecules from the dynamics of bulk water. We find that water molecules in the first hydration shell of the halogenic anions Cl−, Br− and I− show much slower hydrogen-bond dynamics than water molecules in the pure liquid. We also observe that the first hydration shell shows very slow collective orientational dynamics, and forms a rigid, long-living structure. Finally, we find that ions have surprisingly little effect on the hydrogen-bond dynamics of water molecules outside the first hydration shell, which implies that ions do not enhance or weaken the hydrogen-bond network of liquid water.
72 citations
TL;DR: In this article, the authors compared a fluid neutral model with a kinetic neutral model and showed that by iterative inclusion of additional effects, the agreement between the two is improved by iteratively adding additional effects.
Abstract: Edge plasma modelling is discussed with particular emphasis on the comparison of a fluid neutral model with a kinetic neutral model. By iterative inclusion of additional effects, the agreement between the two is improved. Of particular importance is the proper treatment of neutrals at the core boundary, the proper implementation of a neutral flux limit, and the modification of the ion heat boundary condition to include the neutral contribution. In the end, the agreement in the upstream profiles of electron and ion temperatures, and of electron and neutral core densities is very satisfactory. In addition, the effects of parallel ion and electron heat flux limiters, of transport ballooning to the low field side, and of drifts are investigated.
65 citations
TL;DR: In this article, the initial stages of blistering in W due to low-energy He implantation have been investigated by molecular dynamics simulations and it was found that an important mechanism for growth of small He bubbles (containing up to about 10 atoms) is the formation of (111) self-interstitial crowdion atoms.
Abstract: The initial stages of blistering in W due to low-energy He implantation have been investigated by molecular dynamics simulations. The calculations were carried out on a block of (001) W at 0 K, for 100 eV He ions impinging on it. This energy is far below the threshold energy needed by the projectiles to create lattice displacements in W, which is about 0.50 keV. By visual inspection of the motion of W atoms during implantation, we find that an important mechanism for growth of small He bubbles (containing up to about 10 atoms) is the formation of (111) self-interstitial crowdion atoms. The formation of the interstitial relieves the high internal pressure in the He bubbles.
TL;DR: In this article, the authors compared co-deposition, material mixing and fuel inventory on plasma facing and side surfaces of tiles, i.e., JET and TEXTOR components.
Abstract: Plasma facing components from JET and TEXTOR were studied. The emphasis was on the comparison of co-deposition, material mixing and fuel inventory on plasma facing and side surfaces of tiles, i.e. ...
TL;DR: In this paper, the authors consider the pump wave enhanced fluctuation spectrum that was derived in the seventies and compare it with the present paper. But they do not consider the effect of fluctuations.
Abstract: The present paper reconsiders the pump wave enhanced fluctuation spectrum that was derived in the seventies.
TL;DR: In this paper, the authors describe applications to the environmental and medical fields and it is noted that the basic approaches are very similar in both areas Environmental diagnostics discussed includes satellite imagery, long-path absorption atmospheric spectroscopy, differential absorption lidar, gas correlation imaging and fluorescence lidar imaging of vegetation and historical building facades.
Abstract: Optical spectroscopy, and in particular laser spectroscopy, provides numerous possibilities for advanced diagnostics and multi-spectral imaging with real-time capability We describe applications to the environmental and medical fields and it is noted that the basic approaches are very similar in both areas Environmental diagnostics discussed includes satellite imagery, long-path absorption atmospheric spectroscopy, differential absorption lidar, gas correlation imaging and fluorescence lidar imaging of vegetation and historical building facades The medical field is illustrated by photodynamic therapy, laser-induced fluorescence diagnostics, scattering spectroscopy and optical mammography, gas in scattering media absorption spectroscopy and imaging with laser-produced x-rays
TL;DR: In this article, a statistically averaged noise spectrum computed with a particle-in-cell (PIC) code for a simple single-Maxwellian and unmagnetized electron plasma is compared to both, the thermal noise spectrum for the corresponding real plasma and the noise spectrum we would anticipate from our numerical scheme.
Abstract: Thermal noise measurements by space-borne antennas are commonly used to determine plasma parameters like the electron density and the plasma temperature from the noise spectra. It would be desirable to have a controlled experiment in which noise from a plasma with known properties is sampled in space and in time and which results can then be used to reproduce the satellite measurements. Here we examine the possibility to use particle-in-cell (PIC) simulations as such an experiment. In this work we present a statistically averaged noise spectrum computed with a PIC code for a simple single-Maxwellian and unmagnetized electron plasma and we compare it to both, the thermal noise spectrum for the corresponding real plasma and the noise spectrum we would anticipate from our numerical scheme. We find that we can produce noise fields with sufficiently low amplitudes to keep the plasma in a linear regime. We show that the simulation noise at low and at large wave numbers differs not only from thermal noise of a physical plasma but also from the numerical noise we would expect from our numerical scheme. We explain the drop of the noise power at low wave numbers by our initial conditions. We find experimentally the relation that connects the theoretical noise spectrum for our simulation code with that we actually measure, provided that the phase velocity of the noise is less than the maximum velocity of the computational particles.
TL;DR: In this article, a self-switching device (SSD) is realized by tailoring the boundary of a narrow semiconductor channel to break its symmetry, which results in a strongly nonlinear I-V characteristic, resembling that of a conventional diode.
Abstract: A new type of nanometer scale nonlinear device, called self-switching device (SSD) is realized by tailoring the boundary of a narrow semiconductor channel to break its symmetry. An applied voltage V not only changes the potential profile along the channel direction, but also either widens or narrows the effective channel width depending on the sign of V. This results in a strongly nonlinear I-V characteristic, resembling that of a conventional diode. Because the structure resembles a diode-connected FET (gate and drain shorted), we have modeled the device as a sideways turned FET, so that the trench width t corresponds to insulator thickness tox and conducting layer thickness Z (inside the semiconductor!) corresponds to channel width W.
TL;DR: Theoretical and numerical studies of the nonlinear amplitude modulation of DA waves propagating in an unmagnetized three component, weakly-coupled, fully ionized plasma consisting of electrons, positive ions and charged dust particles, considering perturbations oblique to the carrier wave propagation direction are presented in this article.
Abstract: Theoretical and numerical studies are presented of the nonlinear amplitude modulation of dust–acoustic (DA) waves propagating in an unmagnetized three component, weakly-coupled, fully ionized plasma consisting of electrons, positive ions and charged dust particles, considering perturbations oblique to the carrier wave propagation direction. The stability analysis, based on a nonlinear Schrodinger-type equation (NLSE), shows that the wave may become unstable; the stability criteria depend on the angle θ between the modulation and propagation directions. Explicit expressions for the instability rate and threshold have been obtained in terms of the dispersion laws of the system. The possibility and conditions for the existence of different types of localized excitations have also been discussed.
TL;DR: The history of codeposition, the tritium experience of TFTR and JET and thetritium removal rate required to support ITER's planned operational schedule are reviewed and the merits and shortcomings are discussed with particular emphasis on oxidation and laser surface heating.
Abstract: Tritium removal is a major unsolved development task for next-step devices with carbon plasma facing components. The 2–3 order of magnitude increase in duty cycle and associated tritium accumulation rate in a next-step tokamak will place unprecedented demands on tritium removal technology. The associated technical risk can be mitigated only if suitable removal techniques are demonstrated on tokamaks before the construction of a next-step device. This article reviews the history of codeposition, the tritium experience of TFTR and JET and the tritium removal rate required to support ITER's planned operational schedule. The merits and shortcomings of various tritium removal techniques are discussed with particular emphasis on oxidation and laser surface heating.
TL;DR: The spectrum of ten times ionized xenon, Xe XI, was observed in the region 105-157 A with a low-inductance vacuum spark and a 107 m grazing-incidence spectrograph as mentioned in this paper.
Abstract: The spectrum of ten times ionized xenon, Xe XI, was observed in the region 105–157 A with a low-inductance vacuum spark and a 107 m grazing-incidence spectrograph About 200 lines belonging to the 4d8–(4d75p + 4d74f + 4p54d9) transition array were observed 140 of these lines are in the region 130–140 A, which is of importance for extreme ultraviolet (EUV) lithography The spectrum was interpreted by means of Hartree–Fock calculations and orthogonal parameters using scaling factors for the energy parameters obtained by extrapolation along the Rh II isoelectronic sequence All 9 levels of the 4d8 configuration and 123 levels of the 4d75p + 4d74f + 4p54d9 configurations were established Transition probabilities for all observed lines were calculated with fitted values of the energy parameters A value of 1847200 ± 1600 cm-1 (22902 ±020 eV) was obtained for the ionization energy
TL;DR: A survey of resonance and threshold phenomena in low-energy electron collisions with molecules and clusters is presented in this article, where the authors highlight threshold and resonance phenomena in electron scattering and attachment channels, as observed in recent high resolution experiments and characterized by improved theoretical models.
Abstract: A survey of resonance and threshold phenomena in low-energy electron collisions with molecules and clusters is presented. Following an introduction into the role of resonances to promote vibrational excitation and anion formation through electron attachment, we discuss recent progress in achieving high energy resolution, using optimized conventional setups and photoelectron methods (the latter achieving sub-meV energy widths in attachment studies). Discussing selected cases, we highlight threshold and resonance phenomena in electron scattering and attachment channels, as observed in recent high resolution experiments and characterized by improved theoretical models. The threshold behaviour for dissociative electron attachment proceeding through s-wave and p-wave capture is demonstrated for CCl4 and Cl2, respectively. Threshold peaks, vibrational Feshbach and outer-well resonances as well as boomerang-type oscillatory structures are discussed for the polar molecules HF, HCl, and CH3I. Narrow vibrational Feshbach resonances, observed in cluster anion formation due to electron attachment to molecular clusters of CO2 and N2O, and their size-dependent redshifts, illustrating the effects of solvation, are discussed and explained as diffuse weakly-bound electron states. We briefly address the recent observation and the relevance of vibrational resonances in positron-molecule collisions at energies below 0.5 eV, as observed in positron annihilation. We conclude with a brief summary and mention some perspectives for future work.
TL;DR: The phase diagram of magnetron sputter-deposited Al-Cu thin films is simpler than the equilibrium diagram as discussed by the authors, and the films are single?Cu phase, the microstructure consists of a mixture of the solid solution?Al phase and an intermetallic compound phase, and an unexpected (Cu3Al) phase for 49.07 to 66.64 at.%Cu films.
Abstract: The phase diagram of magnetron sputter-deposited Al-Cu thin films is simpler than the equilibrium diagram. Above 86.17 at.%Cu, the films are single ?Cu phase. Below, the microstructure consists of a mixture of the solid solution ?Al phase and an intermetallic compound phase, the previous ?(Al2Cu) phase for the 1.8 to 45.99 at.%Cu films and an unexpected (Cu3Al) phase for 49.07 to 66.64 at.%Cu films. We note the appearance of a phase separation (?Al + ?Cu + Cu3 Al) in 66.64 at.%Cu films. The microhardness and the young modulus of the sputtered films increase regularly with Cu concentration reaching a maximum (H ? 8000 MPa and E ? 200 GPa). This phenomenon of strengthening of aluminium by means of copper is essentially due to a combination of solid solution effects and grain size refinement.
TL;DR: In this article, the authors review the status of and comment on current developments in the field of two-photon decay in atomic physics research and emphasize the importance of measuring the shape of the continuum emission as a probe of relativistic effects in the strong central fields found in heavy atomic systems.
Abstract: We review the status of and comment on current developments in the field of two-photon decay in atomic physics research. Recent work has focused on two-photon decays in highly-charged ions and two-photon decay of inner-shell vacancies in heavy neutral atoms. We emphasize the importance of measuring the shape of the continuum emission in two-photon decay as a probe of relativistic effects in the strong central fields found in heavy atomic systems. New experimental approaches and their consequences will be discussed.
TL;DR: In this article, a D-dimensional Schrodinger equation with a Coulomb plus inverse-square potential was analyzed in great detail, and the relationship between the energy E(n,l,D) and the dimension D was analyzed.
Abstract: A D-dimensional Schrodinger equation with a Coulomb plus inverse-square potential is carried out. The relationship between the energy E(n,l,D) and the dimension D is analyzed in great detail. It is shown that the E(n,0,D) first decreases for D (0,2] and then increases for D ≥ 2. The energy E(n,l,D) is almost independent of the quantum number l for large D, but the quantum number l plays some role in the energy E(n,l,D) when the dimension D is not too large.
TL;DR: In this paper, the surface area of the wall that is covered with carbon measures about 15 m2 (2 to 4 m2 of which in close interaction with the plasma) and the required gas injection to maintain the prescribed density remains constant during the whole discharge.
Abstract: In the new CIEL configuration of Tore Supra, all the plasma facing components are actively cooled. The surface area of the wall that is covered with carbon measures about 15 m2 (2 to 4 m2 of which in close interaction with the plasma). Steady-state plasma conditions up to 4 min 25 s have been maintained in this configuration. In these experiments, the required gas injection to maintain the prescribed density remains constant during the whole discharge. The exhausted flux is also constant and equal to 40 ÷ 50% of the injected flux. Therefore, 50 ÷ 60% of the injected particles remain trapped in the vessel, the total retention being proportional to the plasma duration. Since the amount of gas recovered between shots or by He-glow discharges does not always balance the injected gas, it follows that a quantity of deuterium remains indefinitely trapped in the vessel, which appears as an infinite reservoir. This reservoir is believed to be dominated by co-deposited layers, as observed in several places of the vessel. The thickest deposits (up to 800 µm) are observed on the leading edge of the neutralizers of the pump limiter. They display a column-like shape (typical growth rate ~20 nm/s) and have a graphite-like structure. Their deuterium concentration is D/C ~ 1%. Conversely, in regions that are shadowed from the direct plasma flux, the deposits show a smoother shape and their deuterium content is typically ~10 ÷ 15%.
TL;DR: In this article, a kinetic Monte Carlo (KMC) model is used to simulate the migration and clustering of transmutant helium gas atoms and ultimately determine the role of helium in mediating the long term aging of primary defects (vacancies, self-interstitial atoms and their clusters) produced in displacement cascades.
Abstract: Structural materials in fusion reactors will operate in harsh radiation conditions including high displacement rates from 14 MeV neutrons with accompanying high levels of hydrogen and helium production and will experience severe property degradation Predicting their in-service performance requires a detailed understanding of the mechanisms of defect accumulation and microstructure evolution The physical processes involved in radiation damage are inherently multiscale, spanning more than 15 orders of magnitude in length and 24 orders of magnitude in time Here, we describe a kinetic Monte Carlo (KMC) model to simulate the migration and clustering of transmutant helium gas atoms and ultimately determine the role of helium in mediating the long term aging of primary defects (vacancies, self-interstitial atoms and their clusters) produced in displacement cascades The results illustrate the mechanisms responsible for the formation of vacancy-He clusters, in particular the high mobility of small vacancy-He clusters, which often lead to cluster growth through cluster–cluster coalescence events
TL;DR: In this article, the generalized Hulthen potential (GHP) is shown to be a shape invariant potential with a translation of parameters, and the exact energy spectrum of this potential is obtained in terms of a shape-invariant approach.
Abstract: It is shown that the generalized Hulthen potential (GHP) is a shape invariant potential with a translation of parameters. The exact energy spectrum of this potential is obtained in terms of a shape invariant approach. The supersymmetric WKB approximation gives the same result.
TL;DR: In this article, negative photoresist SU-8 is used both as the structural material and as the adhesive material for fabrication of structures for microelectromechanical systems (MEMS).
Abstract: An adhesive bonding method for fabrication of structures for microelectromechanical systems (MEMS) has been investigated. Negative photoresist SU-8 is used both as the structural material and as the adhesive material. Ultraviolet initiated crosslinking of SU-8 was investigated for bonding of small pillars as well as large uniform areas. Fabrication of thermally- and electrically insulated structures was also done and is reported here. Insulated structures are fabricated with SU-8 as roof, floor and sidewalls of the structure to ensure uniform wetting and surface electrochemistry for microfluidic applications.
TL;DR: In this article, the effect of series resistance on the relationship between barrier heights and ideality factors of inhomogeneous Schottky barrier diodes has been clearly shown, and a laterally homogeneous SBH value of approximately 0.770 eV has been obtained by the barrier inhomogeneity model.
Abstract: We have studied Cd/n-Si (33 dots) Schottky barrier diodes (SBDs). Si surfaces have been prepared by the usual chemical etching, and the evaporation of metal has been carried out in a conventional vacuum system. The effective Schottky barrier heights (SBHs) and ideality factors obtained from the current–voltage (I–V) characteristics differ from diode to diode although the samples were identically prepared. The SBH for the Cd/n-Si diodes ranged from 0.605 eV to 0.701 eV, and ideality factor n from 1.213 to 1.913. The reason for that the experimental data differ from diode to diode has been analyzed by applying thermionic emission theory (TET) of inhomogeneous Schottky contacts together with standard TET and thus the effect of series resistance on the relationship between barrier heights and ideality factors of the inhomogeneous SBDs has clearly been shown. A laterally homogeneous SBH value of approximately 0.770 eV for Cd/n-Si SDs has been obtained by the barrier inhomogeneity model.
TL;DR: In this paper, a model for the recently discovered PMSE overshoot effect is presented, which is created during radar observations of the mesospheric Polar Mesospheric Summer Echoes (PMSE), when the PMSE strength is modified by artificial electron heating over a short period which in our cases were 20 s.
Abstract: We present a model for the recently discovered PMSE overshoot effect. The overshoot effect is created during radar observations of the mesospheric Polar Mesospheric Summer Echoes (PMSE), when the PMSE strength is modified by artificial electron heating over a short period which in our cases were 20 s. Thereafter the heater is switched off for a long period (160 s) so the plasma and dust charges are allowed to relax back to their undisturbed values. This heater cycling is then repeated throughout the observing run. This results in a series of what we have called overshoot characteristic curves (OCC) which contain information on the state of the PMSE dusty plasma. The form of the OCC strongly support a model where the radar scattering is controlled by many small dust structures within the PMSE region, each of which will produce an electron gradient, with a strength which will depend mainly on the density and size of the dust, the electron density and temperature. It is the electron gradients which cause the PMSE radar scattering. With this model, and assuming an average dust structure to represent the PMSE dust structures, we follow the changes of the dust charging and the electron density through one cycle where the heater is switched on and off, and show how the resulting relative scattering efficiency reproduce observed OCC. We show that we can obtain values for the increase of electron temperature as the heater is switched on, the dust content, and from the relaxation time of the OCC the photodetachment rate or an upper limit to it. Other information may possibly also be obtained from the OCC, such as the electron density from electron heating calculations when the temperature is known, and information on to what degree absorption of plasma during the electron heating phase is important.
TL;DR: Reflection coefficients for carbon and hydrocarbon atoms/molecules on carbon-based surfaces are critically needed for plasma-surface interaction analysis in fusion devices, as carbon will continue to be used in next step devices like ITER.
Abstract: Reflection coefficients for carbon and hydrocarbon atoms/molecules on carbon-based surfaces are critically needed for plasma-surface interaction analysis in fusion devices, as carbon will continue to be used in next step devices like ITER. These have been calculated at different energies and angles with a molecular dynamics code using the Brenner hydrocarbon potential. Hydrogen saturated graphite was prepared by bombarding a graphite lattice with hydrogen, until a saturation at � 0:42 H:C. Carbon at 458 has a reflection coefficient ðRÞ of 0:64 � 0:01 at thermal energy, decreasing to 0:19 � 0:01 at 10 eV. Carbon dimers ðRthermal ¼ 0:51; R>1 eV � 0:10Þ tend to stick more readily than carbon trimers ðRthermal ¼ 0:63; R10 eV ¼ 0:16Þ: Hydrocarbons reflect as molecules at thermal energies and break up at higher energies. The total reflection via these fragments decreases with energy, the number of unpaired electrons, and changing hybridization from sp 3 to sp 2 to sp. The results compare reasonably well with binary collision modeling for higher energies and experimental sticking data at thermal energies. A second surface, representing a ‘‘soft’’ redeposited carbon layer formed by the deposition of hydrocarbons onto a graphite surface, is also analyzed. In general, reflection is lower from the ‘‘soft’’ surface by 0.1–0.2. This reflection data can and has been incorporated in erosion/redeposition codes to allow improved modeling of chemically eroded carbon transport in fusion devices.