Showing papers on "Free electron model published in 2010"

Imaging the Fano lattice to ‘hidden order’ transition in URu 2 Si 2

Abstract: Within a Kondo lattice, the strong hybridization between electrons localized in real space (r-space) and those delocalized in momentum-space (k-space) generates exotic electronic states called ‘heavy fermions’. In URu2Si2 these effects begin at temperatures around 55 K but they are suddenly altered by an unidentified electronic phase transition at To = 17.5 K. Whether this is conventional ordering of the k-space states, or a change in the hybridization of the r-space states at each U atom, is unknown. Here we use spectroscopic imaging scanning tunnelling microscopy (SI-STM) to image the evolution of URu2Si2 electronic structure simultaneously in r-space and k-space. Above To, the ‘Fano lattice’ electronic structure predicted for Kondo screening of a magnetic lattice is revealed. Below To, a partial energy gap without any associated density-wave signatures emerges from this Fano lattice. Heavy-quasiparticle interference imaging within this gap reveals its cause as the rapid splitting below To of a light k-space band into two new heavy fermion bands. Thus, the URu2Si2 ‘hidden order’ state emerges directly from the Fano lattice electronic structure and exhibits characteristics, not of a conventional density wave, but of sudden alterations in both the hybridization at each U atom and the associated heavy fermion states. A long-standing mystery in condensed matter physics is that of the appearance of a 'hidden order' state in URu2Si2 at low temperature, an unexpected phase change that is accompanied by a sharp change in bulk properties of the material. The problem is related to the appearance of a 'heavy fermion' state (already at a higher temperature) where electron-like charge carriers propagate through the solid with an effective mass thousands of times larger than that of a free electron. Schmidt et al. have now used scanning tunnelling microscopy and spectroscopy to visualize the electronic structure of URu2Si2 with subatomic resolution. In the process, they observe the electronic structure associated with a magnetic 'Kondo' lattice, which was assumed to cause heavy fermion effects, but never observed directly. Further, the spectroscopic findings show how the hidden order state evolves with decreasing temperature from this lattice. A longstanding mystery in condensed-matter physics involves the appearance of a 'hidden order' state in URu2Si2 at low temperature — an unexpected phase change that is accompanied by a sharp change in the bulk properties of the material. The problem is related to the appearance of a 'heavy fermion' state. Here, scanning tunnelling microscopy and spectroscopy have been used to image the electronic structure of URu2Si2 at sub-atomic resolution, revealing how the hidden order state evolves with decreasing temperature.

Second- and third-harmonic generation in metal-based structures

Abstract: We present a theoretical approach to the study of second- and third-harmonic generation from metallic structures and nanocavities filled with a nonlinear material in the ultrashort pulse regime. We model the metal as a two-component medium, using the hydrodynamic model to describe free electrons and Lorentz oscillators to account for core electron contributions to both the linear dielectric constant and harmonic generation. The active nonlinear medium that may fill a metallic nanocavity, or be positioned between metallic layers in a stack, is also modeled using Lorentz oscillators and surface phenomena due to symmetry breaking are taken into account. We study the effects of incident TE- and TM-polarized fields and show that a simple reexamination of the basic equations reveals additional, exploitable dynamical features of nonlinear frequency conversion in plasmonic nanostructures.

Ingrain and grain boundary scattering effects on electron mobility of transparent conducting polycrystalline Ga-doped ZnO films

Abstract: Transparent conducting polycrystalline Ga-doped ZnO (GZO) films with different thicknesses were deposited on glass substrates at a substrate temperature of 200 °C by ion-plating deposition with direct current arc-discharge. The dependences of crystal structure, electrical, and optical properties of the GZO films on thickness have been systematically studied. Optical response due to free electrons of the GZO films was characterized in the photon energy range from 0.73 to 3.8 eV by spectroscopic ellipsometry (SE). The free electron response was expressed by the simple Drude model combined with the Tauc–Lorentz model. From the SE analysis and the results of Hall measurements, electron effective mass, m∗, and optical mobility, μopt, of the GZO films were determined, based on the assumptions that the films are homogeneous and optically isotropic. By comparing the μopt and Hall mobility, μHall, an indication on the effect of ingrain and grain boundary scattering limiting the electron mobility has been obtained....

Laser-supported solid-state absorption fronts in silica

Abstract: We develop a model based on simulation and experiment that explains the behavior of solid-state laser-supported absorption fronts generated in fused silica during high intensity (up to 5GW/cm{sup 2}) laser exposure. We find that the absorption front velocity is constant in time and is nearly linear in laser intensity. Further, this model can explain the dependence of laser damage site size on these parameters. This behavior is driven principally by the temperature-activated deep sub band-gap optical absorptivity, free electron transport and thermal diffusion in defect-free silica for temperatures up to 15,000K and pressures < 15GPa. The regime of parameter space critical to this problem spans and extends that measured by other means. It serves as a platform for understanding general laser-matter interactions in dielectrics under a variety of conditions.

The wave equation and general plane wave solutions in fractional space

Abstract: This work presents the analytical solution of vector wave equation in fractional space. General plane wave solution to the wave equation for flelds in source-free and lossless media is obtained in fractional space. The obtained solution is a generalization of wave equation from integer dimensional space to a non-integer dimensional space. The classical results are recovered when integer-dimensional space is considered.

Surface photovoltage in undoped n-type GaN

Abstract: Steady-state and transient surface photovoltage (SPV) in undoped GaN is studied in vacuum and air ambient at room temperature and 400 K with a Kelvin probe. The results are explained within a phenomenological model accounting for the accumulation of photogenerated holes at the surface, capture of free electrons from the bulk over the near-surface potential barrier, and emission of electrons from surface states into the bulk. Simple analytical expressions are obtained and compared with experimental results. In particular, the proposed model explains the logarithmic decay of the SPV after stopping illumination. Internal and external mechanisms of the SPV are discussed in detail. It is shown that an internal mechanism dominates at low illumination intensity and/or small photon energies, while external mechanisms such as charging of a surface oxide layer and photoinduced processes play a significant role for above-bandgap illumination with sufficient intensity.

A one-electron model for the aqueous electron that includes many-body electron-water polarization: Bulk equilibrium structure, vertical electron binding energy, and optical absorption spectrum.

Abstract: Previously, we reported an electron-water pseudopotential designed to be used in conjunction with a polarizable water model, in order to describe the hydrated electron [L. D. Jacobson et al., J. Chem. Phys. 130, 124115 (2009)]. Subsequently, we found this model to be inadequate for the aqueous electron in bulk water, and here we report a reparametrization of the model. Unlike the previous model, the current version is not fit directly to any observables; rather, we use an ab initio exchange-correlation potential, along with a repulsive potential that is fit to reproduce the density maximum of the excess electron’s wave function within the static-exchange approximation. The new parametrization performs at least as well as the previous model, as compared to ab initio benchmarks for (H2O)n− clusters, and also predicts reasonable values for the diffusion coefficient, radius of gyration, and absorption maximum of the bulk species. The new model predicts a vertical electron binding energy of 3.7 eV in bulk wate...

Direct acceleration of an electron in infinite vacuum by a pulsed radially-polarized laser beam

Abstract: We study the direct acceleration of a free electron in infinite vacuum along the axis of a pulsed radially-polarized laser beam. We find that net energy transfer from laser pulse to electron is maximized with the tightest focusing. We show that the net energy gain of an electron initially moving at a relativistic velocity may exceed more than half the theoretical limit of energy transfer, which is not possible with an initially stationary electron in the parameter space studied. We determine and analyze the power scaling of maximum energy gain, extending our study to include a relatively unexplored regime of low powers and revealing that substantial acceleration is already possible without the use of petawatt peak-power laser technology.

Fermi Surface and Mass Enhancement in KFe2As2 from de Haas-van Alphen Effect Measurements

Abstract: We report on a band structure calculation and de Haas–van Alphen measurements of KFe 2 As 2 . Three cylindrical Fermi surfaces are found. Effective masses of electrons range from 6 to 18 m e , m e being the free electron mass. Remarkable discrepancies between the calculated and observed Fermi surface areas and the large mass enhancement (\({\gtrsim}3\)) highlight the importance of electronic correlations in determining the electronic structures of iron pnicitide superconductors.

Direct-field electron acceleration with ultrafast radially polarized laser beams: scaling laws and optimization

Abstract: In the past few years, there has been a growing interest for direct-field electron acceleration with ultra-intense and ultrafast radially polarized laser beams. This particular acceleration scheme offers the possibility of producing highly collimated mono-energetic relativistic attosecond electron pulses from an initial cloud of free electrons that could be produced by ionizing a nanoparticle. In this paper, we describe how electron energy scales with laser power and we explain how the beam waist size and the pulse duration can be optimized for maximal acceleration. The main conclusion of our work is that an electron can effectively reach the high-intensity optical cycles of this particular beam and be optimally accelerated without the necessity of being released by photoionization near the pulse peak.

Effect of an interface charge density wave on surface plasmon resonance in ZnO/Ag/ZnO thin films

Abstract: The optical transmission spectra of ZnO/Ag/ZnO films showed a broad range of transmission enhancement due to a harmonic generated by surface plasmon resonance. This resonance was correlated with interface electrons between metallic Ag and dielectric ZnO layer. The interface charge density wave was calculated and compared with experimental results using Maxwell’sequations with a modified boundary condition. Finally, an observed redshift in the transmission enhancement is discussed using both the Ohmic contact model and the Drude free electron model.

Plasmon electron-hole resonance in epitaxial graphene

Abstract: The quasiparticle dynamics of the sheet plasmons in epitaxially grown graphene layers on SiC(0001) have been studied systematically as a function of temperature, intrinsic defects, influence of multilayers and carrier density. The opening of the inter-band loss channel appears as a characteristic upward shift in the plasmon dispersion and a dip in the width of the loss peak, which is explained as a resonance effect in the formation of electron-hole pairs. Despite the existence of strong electronic correlations, the plasmon dispersion can be quantitatively described within the framework of a nearly free electron gas.

Interaction of dielectrics with femtosecond laser pulses: application of kinetic approach and multiple rate equation

Abstract: We calculate the transient free-electron density in laser-irradiated dielectrics with two different approaches, both considering the energy distribution of excited electrons. The kinetic approach solves a system of complete Boltzmann collision integrals describing different excitation and relaxation processes in detail. The multiple rate equation (MRE) is an approximative way to keep track of the energy distribution of excited electrons with reduced numerical effort. Both methods are applied to trace dielectric breakdown, considering the changing optical parameters during irradiation with a high-intensity laser pulse. In the MRE approach we include also fast recombination, leading to a delay of the increase of the electronic density and to a decrease of the maximum number of free electrons.

Transient dynamics of the electronic subsystem of semiconductors irradiated with an ultrashort vacuum ultraviolet laser pulse

Abstract: The irradiation of semiconductors with ultrashort laser pulses causes excitation of electrons from bound states (the valence band and deep atomic shells) to the conduction band, which produces non-equilibrium highly energetic free electrons. We apply a Monte-Carlo simulation technique to study the ionization and excitation of the electronic subsystem of a solid silicon target irradiated with a femtosecond laser pulse, obtaining the transient distribution of electrons within the conduction band. We take into account the electronic band structure and Pauli's principle for excited electrons. Secondary excitation and ionization processes induced by electrons in the conduction band and holes in the valence band were also included and simulated event by event. The temporal distributions of the density and the energy of excited electrons are calculated and discussed. We demonstrate that, due to the energy used to overcome the ionization potential, the final kinetic energy of the free electrons is significantly less than the total energy provided by the laser pulse. We extend the concept of an 'effective energy gap' for multiple electronic excitations, which can be applied to estimate the free-electron density after high-intensity vacuum ultraviolet (VUV) laser pulse irradiation. The effective energy gap depends on both the properties of the material and the laser pulse parameters. The concept provides a fundamental understanding of the experimentally accessible pair creation energy measured in the limit of long times.

Concentration-Dependent Hole Mobility and Recombination Coefficient in Bulk Heterojunctions Determined from Transient Absorption Spectroscopy

Abstract: A simple analytical function based on the multiple trapping model, is used to describe the biomolecular recombination of charge carriers in a bulk heterojunction (BHJ) film in the presence of an exponential energetic tail of localized hole "trap" states. The function is used to fit charge carrier decay data from an unannealed P3HT/PCBM film measured by transient absorption. The analysis assumes that only free holes participate in recombination and transport. This implies an effective recombination rate coefficient which varies with the ratio of free to trapped holes. The fit parameters yield a bimolecular recombination constant for free holes with free electrons (k(o) = 3.4 x 10(-12) cm(3) s(-1)) and information about the distribution of trap states (trap distribution parameter beta = 0.29) Assuming the Langevin recombination limit, the analysis yields a concentration dependent effective hole mobility saturating at mu(o) approximate to 7 x 10(-2) cm(2) V-1 s(-1). This approach should be useful to compare BHJs in a consistent and meaningful manner.

Femtosecond laser structuring and optical properties of a silver and zinc phosphate glass

Abstract: We report on the micro- and nano-structuring of a silver-containing zinc phosphate glass under high repetition rate femtosecond near-infrared laser exposure. Luminescent silver clusters are locally formed thanks to multi-photon absorption. The excitation mechanisms in the glass are investigated with a transient absorption pump-probe experiment. The free electron density of the femtosecond-laser-induced ionized material for irradiation conditions leading to structural modifications is measured. We show that the involved photo-excitation process in the laser–glass interaction is a four-photon absorption and the measured free electron density is on the order of 1017 cm− 3, four orders of magnitude below the critical electron density. The luminescence properties of these resulting structures have been investigated. Emission spectra are compared with those collected after different irradiations (γ and electron beams). The migration of silver species has been assigned to be responsible for local modifications and selective acid etching behavior of the structure.

Transmitting hertzian optical nanoantenna with free-electron feed.

Abstract: A pair of coupled gold nanorods excited by a beam of free electrons acts as a transmitting Hertzian antenna in the optical part of the spectrum. Significantly enhanced resonant emission is observed from the antenna when the electron beam is injected around the junction between the rods, where the local density of electromagnetic states is elevated.

Field-Induced Free-Electron Carriers in Graphite

Abstract: First-principles calculations show that free-electron carriers are induced in a few-layer graphite by applying an external electric field normal to the graphene plane. The electric field substantially shifts the nearly free electron (NFE) states downward, and the states are not distributed at atomic sites but in spacious regions in graphite. The NFE state finally crosses the Fermi level under an electric field of about 0.5 V/A. It is found that the threshold voltage resulting in free-electron carriers monotonically decreases with increasing number of layers. This indicates the possibility of field-induced superconductivity in all carbon materials even without chemical doping.

Microscopic model for the higher-order nonlinearity in optical filaments

Abstract: Using an exactly soluble one-dimensional atomic model, we explore the idea that the recently observed high-order nonlinearity in optical filaments is due to virtual transitions involving the continuum states. We show that the model's behavior is qualitatively comparable with the experimentally observed crossover from self-focusing to defocusing at high intensities, and only occurs at intensities which result in significant ionization. Based on these observations, we conjecture that this continuum electron nonlinear refraction exhibits strong memory effects and, most importantly, the change of its sign is effectively masked by the defocusing due to free electrons.

Ion acoustic solitary waves in plasma with nonisothermal electron, positron, and ion components

Abstract: Ion acoustic solitary waves in three-component plasmas with nonisothermal electrons, positrons and ions are studied. The Sagdeev pseudo-potential method has been used in this work. The trapping parameter (β), the ratio of the free electron temperature to the trapped electron temperature, is treated as a variable. We study the effect of β on the speed of solitons.

Numerical simulations of the polariton kinetic energy distribution in GaAs quantum-well microcavity structures

Abstract: Recent experiments have shown that polaritons in microcavities behave like a Bose-Einstein condensate above a critical density threshold and at low temperature. The polaritons are not in full equilibrium, however, due to particle decay and streaming in of generated particles from high-energy states. In this paper we present a full simulation of the thermalization of polariton and exciton populations, including polariton-polariton and exciton-exciton scattering, phonon emission and absorption, and polariton scattering with free electrons. We find that we can obtain good fits over a wide range of polariton densities. The fits imply that the enhancement of the polariton-polariton scattering due to Bose-Einstein statistics does indeed play the major role in the particles piling up in low-energy states, which is the precursor to Bose condensation. Our fits also indicate that at low density, scattering of the polaritons with free electrons plays a more important role than polaritonphonon scattering.

Transient interference implications on the subpicosecond laser damage of multidielectrics

Abstract: Laser-induced damage in optical thin films with subpicosecond pulses is investigated. A model dedicated to optical interference coatings and based on the rate equation for free electron generation is introduced. It takes into account the transient interference effects induced by changes in the dielectric function during the laser pulse and its feedback effect on the electron density distribution in the multilayer stack. Simulations are compared to experiments on HfO2 and Ta2O5 films with pulses ranging from 45 fs to 1 ps. It is shown that this approach can improve the interpretation of femtosecond and picosecond laser induced damage in thin films.

Optical detection of tunneling ionization.

Abstract: We have experimentally detected optical harmonics that are generated due to a tunneling-ionization-induced modulation of the electron density. The optical signature of electron tunneling can be isolated from concomitant optical responses by using a noncollinear pump-probe setup. Whereas previously demonstrated tools for attosecond metrology of gases, plasmas, and surfaces rely on direct detection of charged particles, detection of the background-free time-resolved optical signal, which uniquely originates from electron tunneling, offers an interesting alternative that is especially suited for systems in which free electrons cannot be directly measured.

Streak investigations of the initial phase of a subnanosecond pulsed electrical breakdown of high-pressure gas gaps

Abstract: The initiation and the development of a breakdown of highly overvoltaged high-pressure (from 4 to 40 atm) gas gaps by voltage pulses having the risetime of 1 ns or shorter are studied experimentally and in theoretical terms. The study revealed that ionization processes leading to the breakdown start in the gas volume and not from the surface of the electrodes. The gap flashover is followed by ionization wave processes initiating in the gas volume and playing the decisive role at the first phase of the breakdown. The dynamics of the ionization waves strongly depends on the initial distribution of free electrons over the gas gap. The distribution of ionization waves is analysed when the initial electrons are distributed uniformly and nonuniformly over the gap. The calculation results are in qualitative agreement with the relevant experimental data. It is shown that the propagation of the ionization waves at the initial stage of subnanosecond pulsed electrical breakdown of gas leads to a redistribution of the electric field in the discharge gap and a region of a strong field, whose intensity is sufficient for the onset of emission processes and the generation of a short beam of fast electrons near the cathode, is formed at the cathode for a very short (up to 100 ps) time.

Rotons in a hybrid Bose-Fermi system.

Abstract: We calculate the spectrum of elementary excitations in a two-dimensional exciton condensate in the vicinity of a two-dimensional electron gas. We show that attraction of excitons due to their scattering with free electrons may lead to formation of a roton minimum. The energy of this minimum may go below the ground state energy which manifests breaking of the superfluidity. The Berezinsky-Kosterlitz-Thouless phase transition temperature decreases due to the exciton-exciton attraction mediated by electrons.

Diamagnetic shift of the A free exciton in CuGaSe2 single crystals

Abstract: Single crystals of CuGaSe2 were studied using magnetophotoluminescence in magnetic fields up to 20 T at 4.2 K. The rate of the diamagnetic shift in the A free exciton peak was determined to be 9.82×10−6 eV/T2. This rate was used to calculate the reduced mass as 0.115m0, the binding energy as 12.9 meV, the Bohr radius as 5.1 nm and an effective hole mass of 0.64m0 (m0 is the free electron mass) of the free A exciton using a low-field perturbation approach and the hydrogenic model.

Downsizing the Hydrated Electron's Lair

Abstract: A new model for free electrons in water suggests that they are localized in regions of enhanced rather than depleted water density.