Contribution Of Free Electrons Research Articles

Optical Response of fci-ZnMgHo Quasicrystal

The results of an optical spectroscopy study of the icosahedral fci-ZnMgHo quasicrystal are presented. The dielectric function of the quasicrystal was measured by the spectroscopic ellipsometry technique in the spectral range of 0.1–5.0 eV. The analysis of the dielectric function shows that an optical response of fci-ZnMgHo is a superposition of the free-electron Drude-type contribution and that of the interband transitions across the fciZnMgHo pseudogap.

サーモリフレクタンス法によるＴｉＮｘ薄膜の熱拡散率の測定

Thermal diffusivity of titanium nitride (TiNx) films normal to the film surface was measured quantitatively using a thermoreflectance technique. TiNx films were deposited on alkali free glass substrate by reactive rf magnetron sputtering using a Ti metal target and mixture gas of Ar and N2. Nitrogen compositions of the film were determined from plasma energy using Bendavid's relation. The ratio of nitrogen to titanium atoms increased from 0.8 to 1.2 when reactive N2 gas composition changed from 4.7% to 100% at the deposition process. For the stoichiometric film, the thermal diffusivity shows peak value, as well as the electrical conductivity. In contrast, when nitrogen composition becomes to over- or under-stoichiometry, both thermal diffusivity and electrical conductivity rapidly decrease, respectively. The relationship between the thermal diffusivity and the electric conductivity suggests that the free electron contribution is dominant to the heat conduction in the TiNx films.

Threshold effect in the energy-loss straggling of protons channeled in Au⟨100⟩ at very low velocities

The energy-loss straggling of protons channeled in gold crystals in the ⟨100⟩ direction has been experimentally and theoretically studied at the low-velocity limit. The experimental results show a threshold effect in the straggling values, which is similar to that observed earlier in the energy loss. This effect is described by a theoretical model which explains the velocity dependence by considering the individual features of the contributions of free electrons in the conduction band and the nearly free $(5{d}^{10})$ electrons which generate a threshold effect at low energies due to their binding energy. The model predicts a deviation of proportionality with ion velocity of the energy-loss spread and yields a good description of the obtained experimental data.

Infrared absorption in Li-intercalated tungsten oxide

Thin films of amorphous and polycrystalline tungsten oxide were produced by reactive dc magnetron sputtering and nanocrystalline films were deposited by advanced gas evaporation. The films were submitted to electrochemical intercalation of Li ions before infrared reflectance measurements were carried out. For crystalline films, the reflectance in the wavelength region 10–30 μm increases upon intercalation, indicating an increasing free-electron contribution. On the other hand, all the films display an increased absorption at wavelengths less than 10 μm when intercalated. The thermal emittance could be varied from about 0.5 to 0.7–0.75 by intercalation in films with thicknesses in excess of 1 μm. Both absorption and interference contribute to the emittance contrast.

Fragility and mechanical moduli: do they really correlate?

The correlation between the fragility of glass-forming systems and the ratio of instantaneous bulk to shear moduli, K ∞/G ∞, of the corresponding glasses is analyzed for bulk metallic and polymeric materials. We emphasize the importance of the free electron contribution to the bulk modulus of metals and a specific intramolecular contribution to fragility in the case of polymers for some long molecules. We argue that these two contributions are the main reasons for the failure of the correlation between fragility and K ∞/G ∞ observed for these materials.

Free-electron response in reflectance anisotropy spectra

Reflectance anisotropy spectra (RAS), extended into the near infrared spectral region, of anisotropic metal islands, metal surfaces, and atomic nanowires are analyzed with respect to anisotropies in the optical response of the free-electron gas of such low dimensional metal structures. In order to distinguish conductivity and morphology effects, both a phenomenological anisotropic Drude model and an effective medium model using ellipsoidal metal inclusions are used to model the spectra. In some cases (metal islands) the RAS response is dominated by the free-electron contribution, while this contribution is significantly smaller for the metal surfaces and nanowires in the accessible spectral range.

An SFG and DFG investigation of polycrystalline Au, Au–Cu and Au–Ag–Cu electrodes in contact with aqueous solutions containing KCN

In this paper, the behaviour of polycrystalline Au, Au–Cu (Cu 25%) and Au–Ag–Cu (Ag 10%, Cu 15%) electrodes in contact with neutral aqueous solutions of KCN has been studied as a function of potential by means of in situ sum frequency generation (SFG) and difference frequency generation (DFG) spectroscopies. The potential-dependent spectra have been analysed quantitatively with a model for the second-order non-linear susceptibility accounting for vibrational and electronic effects. The potential-dependence of the CN − stretching band position and of the free-electron contribution to the real part of the non-resonant component of the second-order susceptibility have been accounted for. Spectroelectrochemical results were complemented by cyclic voltammetric measurements. The chief stress in this work has been placed on systematising and quantifying the interaction between the vibrational and electronic structures of the electrodic interfaces studied. The effects of adsorbates on the electronic structure of the adsorbing electrode, as a function of electrode alloy composition and applied potential are particularly critical for the understanding of Au-alloy electrochemistry in the presence of cyanide and cyanocomplexes. The systematic comparison of SFG and DFG spectra measured under the same electrochemical conditions for Au, Au–Cu and Au–Ag–Cu electrodes discloses a rich phenomenology related to the electronic structure of the interface.

Prediction and observation of tin and silver plasmas with index of refraction greater than one in the soft x-ray range

We present the calculated prediction and the experimental confirmation that doubly ionized Ag and Sn plasmas can have an index of refraction greater than one for soft x-ray wavelengths. Interferometry experiments conducted using a capillary discharge soft x-ray laser operating at a wavelength of confirm that in few times ionized laser-created plasmas of these elements the anomalous dispersion from bound electrons can dominate the free electron contribution, making the index of refraction greater than one. The results confirm that bound electrons can strongly influence the index of refraction of numerous plasmas over a broad range of soft x-ray wavelengths confirming recent observations. The understanding of index of refraction at short wavelengths will become even more essential during the next decade as x-ray free electron lasers will become available to probe a wider variety of plasmas at higher densities and shorter wavelengths.

A twin-wave quadrature interferometer for diagnostics of pulsed processes in hydrogen and erosion plasma

A two-wave interferometer is described that is based on 0.6328-and 3.3922-μm He-Ne lasers and allows separation of the contributions of free electrons and the neutral component in partially ionized plasma to the phase shift of a probing electromagnetic wave under conditions of possible vibrations of the facility’s optical elements. Using the quadrature method for forming informative signals, phase shifts were measured in a wide range of fractions of an interference fringe to several fringes with a high homogeneous differential sensitivity. The interferometer was used to measure the dynamics of the linear electron density of both an atmospheric-pressure erosion capillary discharge in air and plasma of a hydrogen target in experiments on deceleration of heavy ions in an ionized substance.

Ultra-short pulse laser pump-probe experiments for investigation of warm dense plasmas

We describe experiments on warm dense plasmas created with an ultra-short pulse UV laser, and probed with short pulses of various frequencies. The reflected probe pulse is analyzed to give the three Stokes’ parameters which describe its elliptic polarization. From this data,—we infer the dielectric function of the warm dense plasma, taking account of its non-uniform density–temperature structure. In the experiments, we can identify an early time period when the plasma has not yet expanded significantly. In this first phase, the Fresnel formulas uniquely determine the dielectric function of hot solid-density matter from any two of the measured signals. Comparing reconstructed signals to the observed Stokes’ parameters we can determine the time (usually a few 100 fsec) after which surface expansion becomes important. In the second phase ( t > 200 – 300 fsec ) , the plasma expansion is appreciable and must be modeled with analytical or numerical hydrodynamics. Comparing to experiment, we can approximately determine the plasma dielectric function. We find the free electron contribution to the dielectric function is often much less than expected from the usual Spitzer-Drude theory. Studies of dependence of probe reflection on probe laser frequency may enable us to extract the DC electrical conductivity from measured AC optical constants. New scattered-light measurements identify the onset of condensation of the expanding plasma and the formation of droplets of liquid metal.

Quantitative oscillator analysis of IR-optical spectra on spin-cast chitosan films

Infrared optical properties of spin-cast chitosan films have been determined using spectroscopic ellipsometry. Infrared index of refraction and extinction coefficients from 750 cm–1to 4000 cm–1were determined using ellipsometric data fits to dispersion models based on Gaussian shaped oscillators. The free electron contribution was analyzed using a Drude model. This modeling determined that optical anisotropy was present over the entire infrared region. Line shape and oscillator strength analysis was performed to determine oscillator strengths, abundance, and relative bond strength.

Field-dependent charge carrier dynamics in GaN: Excitonic effects

The electric-field dependence of the charge-carrier dynamics in GaN was studied by measuring excitation spectra of the sub-band-gap (yellow) luminescence as a function of bias using a Schottky junction formed at the interface between the semiconductor and an electrolyte solution. At large bias, the contribution of free electrons and holes to the photoluminescence is significantly reduced due to the dead-layer effect. As a result, striking features are revealed in the spectra close to the fundamental absorption. These features are attributed to exciton decay via yellow luminescence centers.

Optical properties of the Au(110) surface

We have used reflectance anisotropy spectroscopy to determine the surface dielectric anisotropy of Au(110) under ultrahigh-vacuum conditions. Additional angle-resolved photoemission spectroscopy measurements were carried out to correlate surface optical with surface electronic properties. The surface dielectric anisotropy mainly results from single-particle excitations involving the near surface bulk d and $\mathrm{sp}$ bands and free-electron contributions in the spectral region below $\ensuremath{\sim}2.4 \mathrm{eV}.$ Surface-state contributions to the surface optical properties of Au(110) are discussed.

Theory of the electronic structure of the alloys of the actinides

An earlier theory of the electronic structure of the actinides is improved, generalized to alloys of actinides, and applied to plutonium and its alloys with other trivalent metals. The theory combines the s and d electrons as free electrons, treating them to second order in an empty-core pseudopotential. The f levels produce bands in this theory, which are actually broadened by electron correlations, but their role in the bonding is decreased. The effects both of the coupling between neighboring f shells and of the correlation are included in a second moment in terms of which the total energy is estimated using a Friedel model. This allows all contributions to be rewritten in terms of near-neighbor interactions and on-site terms. It is found that f-shell contribution alone favors a much smaller spacing than the free-electron contribution alone, and the competition makes the resulting structure very sensitive to the parameters of the theory. Adjustment of the parameters seems essential. An f-shell radius is adjusted, within the range of previous estimates, so that the minimum in the f-shell contributions for plutonium comes at a spacing of 2.5 \AA{}, and the total is scaled back such that the minimum is of the same depth. The same scaling factors are applied to other actinides. Then the pseudopotential core radius is adjusted for each actinide and trivalent simple metal in the face-centered-cubic structure to yield the observed atomic volume (that for the delta structure in the case of plutonium). The resulting electronic structure is tested successfully by using it to predict the bulk modulus of all of these metals. Using the same parameters for plutonium, the energy is calculated in the \ensuremath{\alpha} structure, found to have higher energy than the fcc structure at the atomic volume of \ensuremath{\delta} plutonium, but lower energy at a reduced volume, near that of the observed low-temperature \ensuremath{\alpha} structure. The same formulas and parameters are applicable to random alloys of any of these metals, evaluating each term in an alloy of x atomic fraction of B in A with the appropriate weighting of x, $x(1\ensuremath{-}x),$ or $(1\ensuremath{-}{x)}^{2}.$ This does not predict correct variations with concentration x of the lattice spacings unless we introduce the interaction between simple-metal core d states and actinide valence d states, absent in the pure materials. A moderate scaling of previously predicted d-state radii brings the dilute-alloy spacings into accord with experiment for plutonium alloys with gallium, indium, and thallium. Aluminum, without core d states, was initially in accord and is not affected. The ordered alloys, ${\mathrm{Pu}}_{3}\mathrm{Ga},$ ${\mathrm{Pu}}_{3}\mathrm{In},$ and ${\mathrm{Pu}}_{3}\mathrm{Al},$ are found to have larger spacings than the random alloys, in accord with experiment, though ${\mathrm{Pu}}_{3}\mathrm{Al}$ is not found to be the stable structure. For alloys of two actinides, or with rare earths, the effects of d-state couplings are expected to be negligible, but additional terms in the f-state energy arise from the contribution of f levels of different energy to the global f bands. With this effect included, cerium, americium, and curium are found to increase the spacing of \ensuremath{\delta} plutonium, in accord with experiment, but the lighter actinides, as well as the type-B simple metals, Al, Ga, In, and Tl decrease the spacing, also in accord with experiment. Alloys with the type A metals Sc, Y, and La are found to increase the spacing. Initial studies of the displacement of plutonium neighbors to substituted gallium atoms showed \ensuremath{\alpha}-like reconstructions, as in pure plutonium.

Broadening of resonances in yttrium nanoparticle optical spectra

The dielectric function of yttrium in the range between 0.2 μm and 2 μm is composed of a harmonic oscillator contribution due to a discrete interband transition and the contribution of free electrons. Hence, it is possible to discuss surface plasmon polaritons as well as other electronic resonances in the optical extinction spectra of yttrium nanoparticles. For the latter, we discuss the broadening of the resonance caused by the aggregation of particles. When particles are lumped into aggregates, the color of the particle system also changes. Aggregation also affects the surface plasmon resonance in yttrium nanoparticles in a way comparable to silver or gold nanoparticle aggregates. Comparison is made with the first experimental results on yttriumnanoparticles, showing that aggregation is the dominant effect for the broad resonance in the measured extinction spectra.

Doping effects on optical properties of epitaxial ZnO layers determined by spectroscopic ellipsometry

Optical properties of Al-and Ga-doped ZnO layers have been studied in the spectral range from 1.5 to 5.4eV using a four-zone null spectroscopic ellipsometer and in the spectral range from 0.5 to 6.5eV using near-normal incidence reflectivity measurements. The layers were prepared by RF magnetron sputtering onto (112̄0) oriented single-crystal sapphire substrates. Al-and Ga-doping gives rise to a shift of the fundamental absorption edge from 3.4 to 3.7eV. The model dielectric function (MDF) based on an excitonic structure derived by Tanguy [Phys. Rev. B 60 (1999) 10660] was completed by the Sellmeier and Drude terms. The Drude term describes a free-electron contribution originating from presence of the dopant. Spectroscopic ellipsometry and reflectometry are very sensitive to a surface roughness. The surface roughness was modeled by a surface layer of the Bruggeman effective medium and by diffraction theory.

Visible-infrared sum and difference frequency generation at adsorbate-covered Au

We develop a model for sum and difference frequency generation (SFG/DFG) at the interface of an isotropic centrosymmetric nonsimple metal covered by a noncentrosymmetric adsorbate. We separate the nonlinear response of the surface into a free-electron contribution that characterizes the intraband transitions, a bound-electron contribution that characterizes the interband transitions, and a cross term due to the interaction among free and bound electrons. For the former we introduce an adjustable parameter consistent with semi-infinite jellium calculations, whereas for the others we apply a screened dipollium model that gives the nonlinear response in terms of the bound- and free-electron contributions to the substrate's bulk dielectric function. The nonlinear response of the adsorbate is described through the Raman and IR response of its stretch mode. We calculate SFG/DFG spectra for a ${\mathrm{CN}}^{\ensuremath{-}}$ covered Au-electrolyte interface choosing the direction of the dipole moment of the adsorbate either toward or away from the surface. The interference between the substrate's and overlayer's responses allows the extraction of orientational information from a comparison of our model calculations to recently obtained experimental spectra.

Study of Quantum Well States in Ultrathin Overlayer Films by Magnetization-induced Second Harmonic Generation

We report a detailed study of the nonlinear magneto-optical response from wedge-shaped (111)-oriented films of noble metals Cu, Au and Ag grown on top of an ultra thin Co film. The spectral dependencies of the oscillatory part of the total optical second harmonic response indicate a dominating role of the free-electron contribution in the optical response from quantum well states.

On measurements of stopping power in explosively driven plasma targets

A new method to generate plasma targets with electron densities n e ≥10 20 cm −3 behind strong shock waves to study the energy loss of protons and heavy ions is discussed. Estimates on the stopping power in hydrogen, xenon and argon were carried out. It is shown that shock velocities of more than 20 km/s in xenon and argon and of more than 60 km/s in hydrogen are needed to get a major contribution of free electrons. The problems of matching of large scale accelerator facility and explosive technique are considered. It is suggested to use a small (<150 g TNT) vacuum pumped explosive metallic chambers with fast valves in such experiments. Details on the construction and performance of small-sized explosively driven generators of strong shock waves are presented. The experimental setup including the proton accelerator ISTRA-36 and the explosive chamber, which have been installed in ITEP, is also presented.

Electron momentum spectroscopy studies on the oxidation of aluminium

Electron momentum spectroscopy has been used to measure the electronic structure of aluminium after exposure to increasing amounts of oxygen. The experimental valence band results showed the structural features of aluminium oxide beginning to dominate the free electron contribution of metallic aluminium as the exposure increased. The interpretation of this is that the aluminium oxide layer increases in thickness as the exposure increases, eventually saturating at a thickness of about 15 Å. After subtracting the intensity of the underlying Al metal contribution from the oxidised states the valence band features of aluminium oxide were resolved even after exposure to only 600 L of oxygen. It is estimated that only about 5 Å of aluminium oxide is formed at this stage. A quantitative comparison was made to a linear muffin-tin orbital (LMTO) calculation for spherically averaged α-Al 2O 3. Best agreement was obtained for the results of the lowest oxygen exposure (600 L). The chemical shifts of the Al 2p core level were also measured in separate experiments for two intermediate oxygen exposures between the aluminium and aluminium oxide cases for further characterisation. These measurements show that electron momentum spectroscopy is able to obtain the electron bands and the momentum densities of very thin disordered surface layers.