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Showing papers on "Free electron model published in 2008"


Journal ArticleDOI
TL;DR: The study revealed a substantial discrepancy between the shapes of the electron trap distributions measured experimentally using charge extraction techniques and those inferred indirectly from transient current and voltage measurements, which is resolved by introduction of a numerical factor to account for non-ideal thermodynamic behavior of free electrons in the nanostructured titania.
Abstract: Dye-sensitized solar cells fabricated using ordered arrays of titania nanotubes (tube lengths 5, 10, and 20 microm) grown on titanium have been characterized by a range of experimental methods. The collection efficiency for photoinjected electrons in the cells is close to 100% under short circuit conditions, even for a 20 microm thick nanotube array. Transport, trapping, and back transfer of electrons in the nanotube cells have been studied in detail by a range of complementary experimental techniques. Analysis of the experimental results has shown that the electron diffusion length (which depends on the diffusion coefficient and lifetime of the photoinjected electrons) is of the order of 100 microm in the titania nanotube cells. This is consistent with the observation that the collection efficiency for electrons is close to 100%, even for the thickest (20 microm) nanotube films used in the study. The study revealed a substantial discrepancy between the shapes of the electron trap distributions measured experimentally using charge extraction techniques and those inferred indirectly from transient current and voltage measurements. The discrepancy is resolved by introduction of a numerical factor to account for non-ideal thermodynamic behavior of free electrons in the nanostructured titania.

773 citations


Journal ArticleDOI
TL;DR: It is established that laser-generated high-energy electron momentum spectra and high-order harmonic spectra can be used to extract accurate differential elastic scattering and photo-recombination cross sections of the target ion with free electrons, respectively.
Abstract: By analyzing accurate theoretical results from solving the time-dependent Schrodinger equation of atoms in few-cycle laser pulses, we established the general conclusion that laser-generated high-energy electron momentum spectra and high-order harmonic spectra can be used to extract accurate differential elastic scattering and photo-recombination cross sections of the target ion with free electrons, respectively. Since both electron scattering and photoionization (the inverse of photo-recombination) are the conventional means for interrogating the structure of atoms and molecules, this result implies that existing few-cycle infrared lasers can be implemented for ultrafast imaging of transient molecules with temporal resolution of a few femtoseconds.

288 citations


Journal ArticleDOI
TL;DR: In this article, an investigation into both small and large amplitude dust acoustic solitary waves in dusty plasmas with cold negative dust grains and kappa-distributed ions and/or electrons is discussed.
Abstract: An investigation into both small and large amplitude dust acoustic solitary waves in dusty plasmas with cold negative dust grains and kappa-distributed ions and/or electrons is discussed. Existence conditions for the arbitrary amplitude case are found in an appropriate parameter space, viz., an effective Mach number of the structure speed and the fraction of the charge density that resides with the free electrons, expressed in terms of the ion density. Results indicate that the kappa distribution has only a quantitative, not a qualitative effect on the existence domains and only negative potential solitons exist regardless of whether the electrons or the ions, or both, have a kappa distribution. Despite a wide-ranging search, we have not found double layers in such a plasma. In the case of positive dust, an equivalent set of results holds.

282 citations


Journal ArticleDOI
TL;DR: In this article, the formation and development of collective resonances in linear atomic chains of simple and noble metals (silver) were studied using time-dependent density-functional theory, and a longitudinal collective resonance appeared in both simple- and noble-metal chains.
Abstract: Electronic excitations in linear atomic chains of simple and noble metals (silver) have been studied using time-dependent density-functional theory. The formation and development of collective resonances in the absorption spectra were obtained as functions of the chain length. A longitudinal collective resonance appears in both simple- and noble-metal chains. Its dispersion has been deduced and is compared with that of a one-dimensional electron gas. The transverse excitation generally shows a bimodal structure, which can be assigned as the ``end and central resonances.'' The $d$ electrons of silver atoms reduce both the energies and intensities of the transverse modes but have little effect on its longitudinal resonance. This anisotropic screening is determined by the interband $(d\ensuremath{\rightarrow}p)$ transition, which is involved only in transverse oscillations. Analysis of these results yields a general picture of plasmon resonances in one-dimensional atomic structures. Implications of such atomic-scale plasmons to surface plasmons in larger dimensions are also discussed.

134 citations


Journal ArticleDOI
TL;DR: It is shown that anharmonic resonance in the self-generated plasma potential at a steep ion density profile may represent the leading physical absorption mechanism.
Abstract: After two decades of experiments with intense fs laser pulses the physical mechanism of collisionless absorption in overdense matter is still not understood. We show that anharmonic resonance in the self-generated plasma potential at a steep ion density profile may represent the leading physical absorption mechanism. Resonance provides for the phase shift of the free electron current which is compulsory for laser beam energy transfer to any medium and is capable of explaining the prompt generation of fast electrons with maximum energies exceeding many times their quiver energy, and the polarization dependence.

78 citations


Journal ArticleDOI
TL;DR: At high momenta the spectra show angular structure (diffraction) which is very target dependent and in good agreement with calculated differential cross sections for the scattering of free electrons from the corresponding ionic cores.
Abstract: We have measured full momentum images of electrons rescattered from Xe, Kr, and Ar following the liberation of the electrons from these atoms by short, intense laser pulses. At high momenta the spectra show angular structure (diffraction) which is very target dependent and in good agreement with calculated differential cross sections for the scattering of free electrons from the corresponding ionic cores.

78 citations


Journal ArticleDOI
TL;DR: In this paper, the surface plasmon resonance (SPR) band of Au nanoparticles embedded in an YSZ matrix was monitored at 500 °C under varying gas exposure concentrations of H2 and O2 in N2.
Abstract: The surface plasmon resonance (SPR) band of Au nanoparticles embedded in an YSZ matrix was monitored at 500 °C under varying gas exposure concentrations of H2 and O2 in N2. Because the peak position of the SPR band relies closely on the number of driven oscillating free electrons per gold nanoparticle, we were able to monitor electrochemical charge transfer from Au nanoparticles to diffusing oxygen ions by monitoring the optical properties of the Au−YSZ nanocomposite thin film, specifically the peak position of the SPR band. A direct relation was observed for the change in the equilibrium ratio, pH21/4/pO21/8, contributing to oxygen ion diffusion into and out of the YSZ matrix, and the change in the square of the SPR band peak position. Free electron theory states that this change in the square of the SPR band peak position is directly proportional to the change in conduction electrons available per Au nanoparticle; thus, our observations agree with the expected trend for charge transfer vs the redox gas ...

56 citations


Posted Content
TL;DR: The data suggest that the only mathematically rigorous route to a non-Fermi liquid is found, namely the ‘undercompensated Kondo effect’, where there are too few mobile electrons to compensate for the spins of unpaired electrons localized on impurity atoms.
Abstract: Landau Fermi liquid theory, with its pivotal assertion that electrons in metals can be simply understood as independent particles with effective masses replacing the free electron mass, has been astonishingly successful. This is true despite the Coulomb interactions an electron experiences from the host crystal lattice, its defects, and the other ~1022/cm3 electrons. An important extension to the theory accounts for the behaviour of doped semiconductors1,2. Because little in the vast literature on materials contradicts Fermi liquid theory and its extensions, exceptions have attracted great attention, and they include the high temperature superconductors3, silicon-based field effect transistors which host two-dimensional metals4, and certain rare earth compounds at the threshold of magnetism5-8. The origin of the non-Fermi liquid behaviour in all of these systems remains controversial. Here we report that an entirely different and exceedingly simple class of materials - doped small gap semiconductors near a metal-insulator transition - can also display a non-Fermi liquid state. Remarkably, a modest magnetic field functions as a switch which restores the ordinary disordered Fermi liquid. Our data suggest that we have finally found a physical realization of the only mathematically rigourous route to a non-Fermi liquid, namely the 'undercompensated Kondo effect', where there are too few mobile electrons to compensate for the spins of unpaired electrons localized on impurity atoms9-12.

51 citations


Journal ArticleDOI
TL;DR: In this article, a very early plasma dynamics (first 100 ns) using direct plasma imaging, light scattering, and transmission measurements from a synchronized 532-nm probe laser pulse was studied.

50 citations


Journal ArticleDOI
TL;DR: In this paper, the properties of free electrons and neutral particles in condensed helium are described by the electron (atomic) bubble model, whereas for the positive ions a snowball structure is considered.

49 citations


Journal ArticleDOI
TL;DR: In this article, the stability of metallic thin films is studied with a free electron model, which is popularly known as the model of ''particle in a box.'' A detailed theoretical framework is presented, along with discussion on typical metals, such as Pb, Al, Ag, Na, and Be.
Abstract: The stability of metallic thin films is studied with a free electron model, which is popularly known as the model of ``particle in a box.'' A detailed theoretical framework is presented, along with discussion on typical metals, such as Pb, Al, Ag, Na, and Be. This simple model is found to be able to describe well the oscillation pattern of stability for continuous metallic films. In particular, it yields even-odd oscillations in the stability of Pb(111) film, consistent with both experimental observation and ab initio results. However, the free electron model is too crude to predict at what thickness a film is stable. Film stability is further examined with a model of ``particle in a corrugated box,'' where a lattice potential is added along the vertical direction of a film. The effect of lattice potential is found not substantial.

Journal ArticleDOI
TL;DR: In this article, a self-consistent model based on a nonrelativistic screened-hydrogenic model with l-splitting was presented to calculate the equation of state of matter in local thermodynamic equilibrium.

Journal ArticleDOI
TL;DR: In this article, a reduced dimensionality model was proposed for double ionization of atoms in strong laser pulses using a reduced-dimensional model, which confine each electron to move along the lines that point towards the twoparticle Stark saddle in the presence of a field.
Abstract: We discuss double ionization of atoms in strong laser pulses using a reduced dimensionality model. Following the insight obtained from an analysis of the classical mechanics of the process, we confine each electron to move along the lines that point towards the two-particle Stark saddle in the presence of a field. The resulting effective two-dimensional model is similar to the aligned electron model, but it enables correlated escape of electrons with equal momenta, as observed experimentally. The time-dependent solution of the Schr\"odinger equation allows us to discuss in detail the time dynamics of the ionization process, the formation of electronic wave packets, and the development of the momentum distribution of the outgoing electrons. In particular, we are able to identify the rescattering process, simultaneous direct double ionization during the same field cycle, as well as other double ionization processes. We also use the model to study the phase dependence of the ionization process.

Journal ArticleDOI
TL;DR: In this article, a theoretical model based on the rate equation for free electron density is proposed to investigate transient progression of plasma formation in soft biological tissues during laser shock processing, where the laser focusing region around the focus point is considered to be one-dimensional along the direction of the incident beam, and is discretized into numerous thin control volumes.
Abstract: A theoretical model based on the rate equation for free electron density is proposed to investigate transient progression of plasma formation in soft biological tissues during laser shock processing. The laser focusing region around the focus point is considered to be one-dimensional along the direction of the incident beam, and is discretized into numerous thin control volumes. In simulation of the transient plasma progression, the laser intensity distribution and the temporal evolution of the free electron density are calculated sequentially for each control volume using a fourth-order Runge–Kutta method with adaptive time step control. The rate-equation formalism is first validated with previously published theoretical and experimental results. Simulation of the dynamics of plasma formation is then performed. The results include temporal evolution and spatial distribution of the free electron density as well as the growth of the plasma. It is shown that the threshold laser intensity for optical breakdown in water and the maximum length of the resulting plasma obtained from the present model are in good agreement with existing experimental data.

Journal ArticleDOI
TL;DR: In this article, the dispersion relation of two-stream free electron laser (FEL) is derived employing linear fluid theory and analyzed by numerical solutions. But the results show that the growth rate is considerably enhanced on suitable ranges of normalized axial guiding magnetic field and normalized wave number.
Abstract: The linearized theory of two-stream free electron lasers (FELs) consisting of a relativistic electron beam transported along the axis of a planar wiggler in the presence of an axial guiding magnetic field is proposed and investigated. The dispersion relation is derived employing linear fluid theory. The characteristics of the dispersion relation are analyzed by numerical solutions. The results show that the growth rate is considerably enhanced on suitable ranges of normalized axial guiding magnetic field and normalized wave number. The effect of the difference between the velocities of the two beams, Δv=v1−v2, in this configuration of FELs is also considered and found that the growth rate is remarkably affected by velocity differences. It is also shown that the effect on the electrostatic mode is stronger than electromagnetic.

Journal ArticleDOI
TL;DR: It is shown that the alkali-doped BNNT can generate an emission current 2 orders of magnitude larger than the carbon nanotube and proposed that it should be an excellent electron emitter in terms of the large emission current as well as its chemical and mechanical stability.
Abstract: We investigate the electronic structures and electron emission properties of alkali-doped boron-nitride nanotubes (BNNTs) using density-functional theory calculations. We find that the nearly free-electron (NFE) state of the BNNT couples with the alkali atom states, giving rise to metallic states near the Fermi level. Unlike the cases of potassium-doped carbon nanotubes, not only the s but the d orbital state substantially takes part in the hybridization, and the resulting metallic states preserve the free-electron-like energy dispersion. Through first-principles electron dynamic simulations under applied fields, it is shown that the alkali-doped BNNT can generate an emission current 2 orders of magnitude larger than the carbon nanotube. The nodeless wave function at the Fermi level, together with the lowered work function, constitutes the major advantage of the alkali-doped BNNT in electron emission. We propose that the alkali-doped BNNT should be an excellent electron emitter in terms of the large emission current as well as its chemical and mechanical stability.

Journal ArticleDOI
TL;DR: In this article, the BKS potential is applied and modified to describe molecular interactions and the effect of free electrons in laser ablation of fused silica, and the smooth particle mesh of the Ewald method (SPME) is adopted to calculate the Coulomb force.
Abstract: Ultrafast laser ablation of fused silica is studied using molecular dynamics simulations. Ionization and generation of free electrons, absorption of the laser energy by free electrons and energy coupling between free electrons and ions are considered. The BKS potential is applied and modified to describe molecular interactions and the effect of free electrons. Smooth particle mesh of the Ewald method (SPME) is adopted to calculate the Coulomb force. It is found that the electrostatic Coulomb force, which is caused by the ionization, plays an important role in the laser ablation process.

Journal ArticleDOI
TL;DR: In this paper, the authors used spectroscopic photoemission and low-energy electron microscopy to measure two-dimensional (2D) emission patterns of secondary electrons (SEs) emitted from graphene layers formed on SiC(0.0, 0.0, 0.1 ) to prove that electrons are partially confined in thin graphene layers even above the vacuum level.

Journal ArticleDOI
TL;DR: In this paper, an integrated theoretical model has been developed to predict the entire range of emission from thermionic to field emission, including the mixed emission regime, assuming a Sommerfeld free electron model supply function, for which the Fermi Dirac distribution applies with a nonzero temperature.
Abstract: An integrated theoretical model has been developed to predict the entire range of emission from thermionic to field emission, including the mixed emission regime. The model assumes a Sommerfeld free electron model supply function, for which the Fermi-Dirac distribution applies with a nonzero temperature. The electron transmission coefficient is calculated in one dimension using a transfer matrix method (TMM) to solve the steady-state Schrodinger equation. Emission current densities have been measured for a periodic copper knife-edge cathode to compare with the TMM model result. It is shown that the computational result utilizing this model provides good agreement with the experimental data. Unambiguous and reliable estimates of the effective field enhancement factor βeff (βeff=Es∕Eg, where Es is the cathode surface electric field and Eg is the gap electric field between the cathode and anode) and the effective work function ϕeff are obtained from experimental measurements using this model by simultaneousl...

Journal ArticleDOI
TL;DR: In order to achieve quantum interference of free electrons inside a solid, we have modified the geometry of the solid so that de Broglie waves interfere destructively inside the solid as discussed by the authors.
Abstract: In order to achieve quantum interference of free electrons inside a solid, we have modified the geometry of the solid so that de Broglie waves interfere destructively inside the solid. Quantum interference of de Broglie waves leads to a reduction in the density of possible quantum states of electrons inside the solid and increases the Fermi energy level. This effect was studied theoretically within the limit of the quantum theory of free electrons inside the metal. It has been shown that if a metal surface is modified with patterned indents, the Fermi energy level will increase and consequently the electron work function will decrease. This effect was studied experimentally in both Au and SiO2 thin films of special geometry and structure. Work function reductions of 0.5 eV in Au films and 0.2 eV in SiO2 films were observed. Comparative measurements of work function were made using the Kelvin Probe method based on compensation of internal contact potential difference. Electron emission from the same thin films was studied by two independent research groups using Photoelectron Emission Microscopy (PEEM).

Journal ArticleDOI
TL;DR: In this paper, electron density measurements in the plasma were presented using an ultrafast microwave resonance technique (time resolution below 1 μs), which allows studying the charge and charging kinetics of nanoparticles within the initial milliseconds of their growth.
Abstract: Plasma-produced nanoparticles are of interest for many applications. They have very specific properties that can vary greatly from those in the atomic and bulk materials, including thermodynamic properties, such as a reduced melting temperature, and optical properties, such as blue-shifted blackbody radiation. Since a plasma is dominated by free electrons, charge related properties, like a reduced work function for electron attachment and increased work function for photo ionization, are of major importance. In situdetection of nanoparticles inside a plasma is difficult. Hence, indirect methods including changes in optical emission, plasma voltage and currents are used to study particle growth. Here, electron density measurements in the plasma are presented using an ultrafast microwave resonance technique (time resolution below 1 μs). This technique allows studying the charge and charging kinetics of nanoparticles within the initial milliseconds of their growth.

Journal ArticleDOI
TL;DR: In this paper, a two-dimensional numerical model is proposed to take into account the effects of free electrons on nanoparticle charging in a negative direct current wire-tube corona charger.
Abstract: A two-dimensional numerical model is proposed in this letter to take into account the effects of free electrons on nanoparticle charging in a negative direct current wire-tube corona charger. Numerical results are in excellent agreement with the experimental data by using a capturing probability of electrons onto nanoparticles with a value of 0.013. These free electrons contribute greatly to the charging efficiency at high products of mean ion concentration and mean residence time, which explains very well the large discrepancy found in earlier models that considered only negative ions.

Journal ArticleDOI
TL;DR: In this article, the dynamics of fs-laser ablation of monocrystalline copper has been investigated by molecular dynamics simulation and experimental method, and a simulation model was developed by taking the laser energy absorption, the thermal transport of free electrons as well as the energy exchange between electrons and lattice into account.



Journal ArticleDOI
TL;DR: In this paper, a theory of electromagnetic-wave wiggler with ion-channel guiding is presented, where steady-state electron trajectories are obtained by solving the equations of motion for a single electron in a free-electron laser with electromagnetic-wiggler and ion-Channel guiding field.
Abstract: A theory of electromagnetic-wave wiggler with ion-channel guiding is presented. Steady-state electron trajectories are obtained by solving the equations of motion for a single electron in a free-electron laser with electromagnetic-wave wiggler and ion-channel guiding field. The eighth-degree polynomial dispersion equation for electromagnetic and space charge waves in this configuration is derived. The characteristics of dispersion relation are analyzed in detail by numerical solutions. It is found that the growth rate for group I and II orbits has been affected by the presence of the ion-channel guiding and the growth rate for group II orbits is more affected.

Journal ArticleDOI
TL;DR: In this paper, the effects of two-stream on electromagnetic wiggler free electron lasers (TSEMWFEL) with an axial guiding magnetic field are studied and the dispersion relation is derived employing linear fluid theory.
Abstract: The effects of two-stream on electromagnetic wiggler free electron lasers (TSEMWFEL) with an axial guiding magnetic field are studied. An analysis of the two-stream steady-state electron trajectories is given by solving the equation of motion in the axial guiding magnetic field and the electromagnetic wiggler. Numerical calculations are made to illustrate the effects of the dual electron beam on the trajectories. The dispersion relation is derived employing linear fluid theory. The characteristics of the dispersion relation are analyzed numerically. The result shows that the growth rate is considerably enhanced in comparison with single-stream. The maximum growth rate is studied numerically as a function of axial guiding magnetic field for multiple electron trajectories. It is shown that the maximum growth rate of TSEMWFEL increases and decreases with respect to the axial guiding field for different trajectories.

Journal ArticleDOI
TL;DR: In this paper, the second-order nonlinear polarizability of sector-shaped metallic nanoparticles was analyzed using free electron theory and the dependence of the ground state electron density distribution and polarizabilities on various shape parameters of sector are analyzed.
Abstract: In this paper, we present results of calculations of linear and second-order nonlinear polarizabilities of sector-shaped metallic nanoparticles hemisphere is a special case using free electron theory. The dependences of the ground state electron density distribution and polarizabilities on various shape parameters of sector are analyzed. The ground state electron densities near the corners and edges of sector-shaped nanoparticle are very low and do not contribute to the linear and second-order polarizabilities. The second-order polarizability is found to depend strongly on the angle of the sector and is shown to be proportional to the product of an appropriately defined asymmetric volume of the particle and the third power of the electron cloud length.

Journal ArticleDOI
TL;DR: In this paper, the authors considered the stimulated emission of an electron beam in CNT bundles and showed that the modification of electron wavefunction in a CNT bundle as compared with isolated CNT can result in a significant change of the electron beam propagation in nanotubes.
Abstract: Recently, a hypothetical nanoscale lasing device exploiting the emission of electromagnetic waves by electron beam in an isolated carbon nanotube (CNT) has been proposed [K.G. Batrakov, P.P. Kuzhir, S.A. Maksimenko, in: A. Lakhtakia, S.A. Maksimenko (Eds.), Proceedings of the SPIE, vol. 6328, 2006, p. 63280Z]. The present work considers the stimulated emission of an electron beam in CNT bundles. It is shown that the modification of electron wavefunction in CNT bundle as compared with isolated CNT can result in a significant change of the electron beam propagation in nanotubes. Two cases of the CNT collection arrangement—a “square” lattice and a densely packed bundle of CNTs—are discussed. The distribution of the electron density corresponding to four- and six-wave diffraction in the CNT collection is presented. The ranges where the electron scattering is suppressed are found to be preferable for lasing. The proposed way to increase the generation length extends substantially the potentiality of CNT bundle as a basic element of the nanoscale analog of the traveling wave tube (TWT), backward oscillator (BWO) and free-electron laser (FEL).

Journal ArticleDOI
TL;DR: In this article, it was shown that due to destructive interference of de Broglie waves, some quantum states inside the low-dimensional metal become quantum mechanically forbidden for free electrons.
Abstract: Changes in the metal properties, caused by periodic indents in the metal surface, have been studied within the limit of quantum theory of free electrons. It was shown that due to destructive interference of de Broglie waves, some quantum states inside the low-dimensional metal become quantum mechanically forbidden for free electrons. Wave vector density in k space, reduce dramatically. At the same time, number of free electrons does not change, as metal remains electrically neutral. Because of Pauli exclusion principle some free electrons have to occupy quantum states with higher wave numbers. Fermi vector and Fermi energy of low-dimensional metal increase and consequently its work function decrease. In experiment, magnitude of the effect is limited by the roughness of metal surface. Rough surface causes scattering of the de Broglie waves and compromise their interference. Recent experiments demonstrated reduction of work function in thin metal films, having periodic indents in the surface. Experimental results are in good qualitative agreement with the theory. This effect could exist in any quantum system comprising fermions inside a potential energy box of special geometry.