Free Electron Model Research Articles

Electronic Structure of Liquid Mercury Using Compton Scattering Technique

The isotropic Compton profile of mercury has been measured, using 661.65 keV gamma-rays from a 20 Ci 137Cs source. To extract the true experimental Compton line shape, besides the usual systematic corrections we have incorporated for the first time the background correction due to bremsstrahlung radiation generated by photo and Compton electrons. Theoretical computations have been carried out, using the renormalised-free-atom (RFA) for the electron configuration 4f145d106s2 and free electron models. It is found that the present experimental data with bremsstrahlung background correction are in better agreement with the RFA calculations. This work suggests the incorporation of the bremsstrahlung background correction in Compton scattering experiments of heavy materials using high-energy gamma-ray sources.

THz-field-induced electronic transmission step-structure for a quantum wire

We study theoretically the low-temperature electronic transport property of a straight quantum wire under the irradiation of a finite-range transversely polarized external terahertz (THz) electromagnetic (EM) field. Using the free-electron model and the scattering matrix approach, we show an unusual behaviour of the electronic transmission of this system. A sharp step-structure appears in the electronic transmission probability as the EM field strength increases to a threshold value when a coherent EM field is applied. We demonstrate that this effect physically comes from the inelastic scattering of electrons with lateral photons through intersubband transitions.

Effect of the inclusion of small metallic components in a two-dimensional dielectric photonic crystal with large full band gap

In this study, we investigate the effect of metallic inclusion modeled as perfect conductor on a dielectric photonic crystal (silicon/air) with large full band gap. The dielectric crystal consists of a hexagonal array of circular dielectric columns, each connected to its nearest neighbors by slender rectangular rods. It is found that inclusion of small metallic components inside the circular dielectrics sharply ``turns off`` the full band gap of the dielectric photonic crystal. By increasing the radius of metallic inclusion above a threshold value, the full band gap (of the metallodielectric photonic crystal) makes its appearance again and continues to grow in size. On the other hand, metallic inclusion in the air region shows an opposite trend that the full band gap is not turned off, and its size diminishes gradually to zero with increasing the radius of inclusion. These peculiar behaviors can be explained on a unified basis by examining different types of boundary conditions for TM and TE modes, and employing variational arguments based on Rayleigh's quotients. Moreover, the free-electron model for metallic components is also considered for TM modes. At large plasma frequencies, these modes show very close band structures to those described above for the case of perfect conductors.

Electromagnetic-field-induced cohesive force enhancement in a metallic nanowire

We theoretically study the conducting electronic contribution to the cohesive force in ametallic nanowire irradiated under a transversely polarized external electromagnetic field atlow temperatures and in the ballistic regime. In the framework of the free-electron model,we have obtained a time-dependent two-level electronic wavefunction by means of a unitarytransformation. Using a thermodynamic statistical approach with this wavefunction, wehave calculated the cohesive force in the nanowire. We show that the cohesive force can bedivided into two components, one of which is independent of the electromagnetic field(static component), which is consistent with the existing results in the literature. Themagnitude of the other component is proportional to the electromagnetic field strength.This extra component of the cohesive force is originally from the coherent couplingbetween the two lateral energy levels of the wire and the electromagnetic field.

Determination of the electron effective band mass in amorphous carbon from density-functional theory calculations

We generate a series of amorphous carbon materials of densities varying between 2.0 and $3.5\phantom{\rule{0.3em}{0ex}}\mathrm{g}∕{\mathrm{cm}}^{3}$. Their dielectric functions are calculated within the density-functional theory approach, and the low-loss electron energy-loss spectra are evaluated. The dependence of the $\ensuremath{\pi}+\ensuremath{\sigma}$ plasmon energy ${E}_{p}$ on the valence electron density $({n}_{e})$ is determined. We find that the free-electron model in which ${E}_{p}\ensuremath{\propto}{n}_{e}^{0.5}$ is valid. The coefficient of proportionality leads to a ratio of the electron-effective band mass ${m}^{*}$ to the free-electron mass $m$ of ${m}^{*}∕m=0.87$, which is the value obtained using measurements based on the grazing-angle x-ray reflectivity and electron energy-loss spectroscopy [see A. C. Ferrari et al., Phys. Rev. B 62, 11089 (2000)]. This result supports the reliability of amorphous carbon mass densities obtained from EELS data when an electron-effective mass of $0.87\phantom{\rule{0.3em}{0ex}}m$ is used.

Low-height sputter-deposited magnesium oxide tunnel barriers: experimental report and free electron modeling

Magnetic tunnel junctions with a barrier of magnesium oxide were prepared by plasma oxidation of sputter-deposited magnesium. They show magnetoresistance ratios up to 4.5% at room temperature and 5.5% at low temperatures for barrier thickness of 1.6 nm. The material exhibits low barrier heights of around 0.7 eV. These junctions follows the predictions of the free electron model which contrast with the predictions of band structure calculations and experimental results on epitaxial MgO based tunnel junctions.

FRACTIONAL CONDUCTANCE IN MULTIWALLED CARBON NANOTUBES: A SEMI-CLASSICAL THEORY

An analytical formulation to interpret the quantized electrical conductance in multiwalled carbon nanotubes with defects is presented. In this formulation, free-electron theory is utilized in order to calculate conductance which is found to be fractional according to an inverse proportionality law with respect to the number of layers involved in a given multiwalled tube. Our results agree well with experimental observations.

Spin structure of the Shockley surface state onAu(111)

The free-electron like surface state on the (111) surface of gold shows a splitting into two parabolic subbands induced by the spin orbit interaction. Spin-resolved high-resolution photoemission experiments performed with a full three-dimensional spin polarimeter provide a detailed image of the resulting spin structure. In particular, spin-resolved momentum distribution maps show that the spin vector lies in the surface plane and is perpendicular to the momentum of the electrons as expected in a free-electron model. This method of measuring the spin structure of a two-dimensional electron gas allows the observation of the direction of electric fields as probed by the electrons. Although the energy splitting can only be understood as a consequence of strong atomic electric fields, no modulation of the spin direction due to these fields is detected.

Electronic shell effects and the stability of alkali nanowires

Experimental conductance histograms for Na nanowires are analyzed in detail and compared to recent theoretical results on the stability of cylindrical and elliptical nanowires, using the free-electron model. We find a one-to-one correspondence between the peaks in the histograms and the most stable nanowire geometries, indicating that several of the commonly observed nanowires have elliptical cross sections.

Alkanethiol-Induced Structural Rearrangements in Silica−Gold Core−Shell-type Nanoparticle Clusters: An Opportunity for Chemical Sensor Engineering

Electrostatically bonded SiO2.Au nanoparticle clusters form by reaction of 3-aminopropylsilane-modified SiO2 spheres (470 nm) with citrate-coated gold nanoparticles (9.7 nm) in water. Reaction of the clusters with 0.01 M KBr or HCl solution induces desorption of the gold nanoparticles within minutes. Reaction of the clusters with alkanethiols CnH2n+1SH (n = 2-18) at 80 degrees C causes the gold nanoparticles to form stringlike gold nanoparticle structures for thiols with short alkane groups (n = 2, 3, 4) and hexagonally packed arrays of gold nanoparticles for thiols with long alkane groups (n = 5-18) on the silica surfaces. The structural changes indicate that the bonding between Au and SiO2 nanoparticles has changed from electrostatic to van der Waals. Elemental analyses show that the reaction with hexanethiol does not affect the Au/Si/O composition of the SiO2.Au cluster, and Raman spectra on the hexanethiol-reacted cluster indicate the formation of a thiol SAM on the gold nanoparticles. The thiol-reacted SiO2.Au clusters display characteristic shifts of the absorption maxima in the visible spectra, and there is an inverse relation between these shifts and the lengths of the alkyl groups in the thiols. This relationship can be understood in terms of the free electron model for metals. The use of SiO2.Au nanoparticle clusters as coulometric sensors for the qualitative detection of thiols is discussed.

Extraction of emission area from Fowler–Nordheim plots

The process of deriving emission-area estimates from Fowler–Nordheim (FN) plots is investigated, using emission-area extraction functions and an iterative procedure suggested earlier [R. G. Forbes, J. Vac. Sci. Technol. B 17, 526 (1999)]. Simulated FN-plot data have been prepared using free-electron theory and three different tunneling-barrier models (elementary triangular barrier, image-rounded “Schottky–Nordheim” barrier, and a quadratically enhanced barrier). These have been analyzed using two area-extraction spreadsheets with different models (elementary triangular barrier and Schottky–Nordheim barrier) embedded in them. It is confirmed that significant errors in the area estimate may occur if the emission model used to analyze the FN-plot data does not correspond well with the model/physics responsible for generating the data. It is also concluded that parameters used in the area-extraction process should correspond to emission variables in the midrange of the FN-plot data.

Direct femtosecond laser writing of waveguides in As2S3 thin films.

Single-channel waveguides and Y couplers were fabricated in chalcogenide thin films by use of femtosecond laser pulses from a 25-MHz repetition rate Ti:sapphire laser. Refractive-index differentials (delta n > 10(-2)) were measured through interferometric microscopy and are higher than the typical values reported for oxide glasses. The dependence of the index differential on the peak intensity reveals the nonlinear nature of the photosensitivity in arsenic trisulfide below its bandgap energy, and the refractive-index change is correlated to the photoinduced structural changes inferred by Raman spectroscopy data. A free-electron model to predict the parametric dependence of delta n is proposed.

Mesoscopic mechanics

This article is concerned with the existence, status and description of the so-called emergent phenomena believed to occur in certain principally planar electronic systems. In fact, two distinctly different if inseparable tasks are accomplished. First, a rigorous mathematical model is proposed of emergent character, which is conceptually bonded with quantum mechanics, while apparently nonderivable from the many-body Schrödinger equation. I call the resulting conceptual framework the mesoscopic mechanics (MeM). Its formulation is space-independent and comprises a nonlinear and holistic extension of the free electron model. Secondly, the question of relevancy of the proposed ‘emergent mechanics’ to the actually observed phenomena is discussed. In particular, I postulate a probabilistic interpretation, and indicate how the theory could be applied and verified by experiment. The MeM proposed here has been deduced from the nonlinear Maxwell theory (NMT)—a classical in character nonlinear field theory. This latter theory has already been shown to provide a consistent phenomenological model of such phenomena as superconductivity, charge stripes, magnetic vortex lattice, and magnetic oscillations. The NMT, which arose from geometric considerations, has long been awaiting an explanation as to its ties with the fundamental principles. I believe the MeM provides at least a partial explanation to this effect.

Analytical model for the Fermi energy and the work function of thin metallic films

An analytical theory of size-dependent energetic characteristics of a metal film is developed in a free-electron model and a finite square potential well. A regular expansion of energetic characteristics in terms of 1/ L is performed. The errors occurring at each step of the expansion are analyzed. This simple model allows to calculate size oscillations of the Fermi energy and the work function.

Absolute electron detachment cross sections of atomic anions of the second and third periods incident on noble gases

Anions of the second and third periods\char22{}${\mathrm{B}}^{\ensuremath{-}},$ ${\mathrm{O}}^{\ensuremath{-}},$ ${\mathrm{F}}^{\ensuremath{-}},$ ${\mathrm{Al}}^{\ensuremath{-}},$ ${\mathrm{S}}^{\ensuremath{-}},$ and ${\mathrm{Cl}}^{\ensuremath{-}}$\char22{}had their total electron detachment cross sections measured, in the 0.3\char21{}1.5 a.u. velocity range, for He, Ne, and Ar targets. It is observed that, for any given target, the cross sections present similar velocity dependence and a conspicuous maximum at the same velocity, which differs for distinct targets. It was also obtained that these cross sections scale with target-independent factors, and the ratio of factors within the same group of the periodic table is also nearly group independent. Explanations for these behaviors are presented considering the presence of low- and high-velocity regions, and describing the latter by a free-electron model. The roles of the double ionization and the strong electron correlation in the detachment process are also discussed.

Optical absorption of nanoclusters by sequentially implanting into SiO2 glass and subsequently annealing in a selected atmosphere

Ag and Cu ions were implanted sequentially into SiO2glass by using a metal vapor vacuum arc (MEVVA) source in the room temperature. Both of the doses were 5×1016 ions/cm2The accelerated voltage for Ag was 43kV and for Cu 30kV with the ion flux density～2mA/cm2The valence states of composite sa mples were observed in details by x-ray photoelectron spectroscopy (XPS). The results showed that there were metallic Ag and Cu and CuO in the sample. The formation of alloy nanoclusters have been evidenced by selected area electron diffraction (SAED), energy dispersive xray spectra (EDS) and optical absorption spectra which fit the result of theoretical simulation well. After annealed at different temperatures for 1h in either oxidizing (air) or reducing(70%N2+30%H2 gas mixture, gas pressure 20Pa) atmosphere, the absorption peaks shifted to blue and appeared a new peak during annealing in reducing atmosphere. Applying the free electron model of metal particles, we have calculated the average size of nanoclusters annealed at different temperatures, the results showed that the size of nanoclusters increased gradually with the enhancement of temperature. AgCu alloy nanoclusters were decomposed after annealing in oxidizing atmosphere. Cu atoms migrated to the surface of the sample and are oxidized; Ag nanoclusters were formed due to low oxygensilver interaction. For the sample annealed in reducing atmosphere, the alloy decomposed to Ag and Cu nanoclusters.

金蒸着層の分光放射率の非線形解析

The relation between the thickness of the other material on the base metal surface and the spectral emissivity hasn't yet been clarified.The authors choose SUS304 as the base metal and examined as follows. Gold is evaporated on the SUS304, whose wavelength dependency of spectral emissivity in the visible region and neighboring (0.38∼0.90 μm) is totally different. As the result, the spectral emissivity changes from the value of SUS304's emissivity as the layer thickness increases from 0 μm. When the layer thickness reaches to 0.05∼0.07 μm, the emissivity gradually shows asymptotic convergence to the emissive value of the gold. Generally the spectral Emissivity depends on gold layer thickness. However, the spectral emissivity is constant (0.40) at the wavelength 0.53 μm, which is independent of the layer thickness and shows that is a fixed point. The above result (fixed point) does not comply if it is applied to optical dispassion formula of linear analysis. It means the relation of spectral emissivity between a layer thickness and a wavelength is not always linear.This study considers the spectral emissivity of evaporated gold layer, where the emissivity depends on layer thickness, is analyzed by the nonlinear dispersion model of optical interference phenomenon. The wavelength dependency can be explained by nonlinear behavior of free electron model and analyzed the data calculated from nonlinear dispersion formula, which can justify the fixed point.

Second-harmonic generation in the scattering of light by an infinite cylinder

We study the generation of second-harmonic radiation in the interaction of light with an infinite cylinder illuminated perpendicular to its axis by an s- or p-polarized monochromatic electromagnetic wave. With the assumption of a homogeneous and isotropic medium, we derive analytical expressions for the second-harmonic field radiated by the cylinder. The theoretical results are illustrated through calculations of the total scattered power and its angular distribution by employment of a free-electron model for the linear and nonlinear susceptibilities. The scattering cross sections at the harmonic frequency display peaks due to the excitation of resonances.

Interplay of Rayleigh and Peierls instabilities in metallic nanowires.

A quantum-mechanical stability analysis of metallic nanowires within the free-electron model is presented. The stability is determined by an interplay of electron-shell effects, the Rayleigh instability due to surface tension, and the Peierls instability. Although the latter effect limits the maximum length also for wires with "magic radii," it is found that nanowires in the micrometer range can be stable at room temperature.

Spin blockade in ferromagnetic nanocontacts

Using a free-electron model and a linear response theory we investigate spin-dependent electronic transport in magnetic nanocontacts in the ballistic regime of conduction. We emphasize the fact that in atomic-size ferromagnetic contacts it is possible to achieve the conductance value of e2/h, which implies a fully spin-polarized electric current. We explore some consequences of this phenomenon. In particular, we show that the presence of a nonmagnetic region in the nanocontact separating two ferromagnetic electrodes can lead to a spin blockade resulting in very large values of magnetoresistance.