Showing papers on "Free electron model published in 2016"

The physics of x-ray free-electron lasers

Abstract: The advent of x-ray free electron lasers has made possible the study of matter at the characteristic space and time scales of atomic and molecular phenomena using intense coherent x-ray pulses. This article describes the physical principles and the theoretical models governing the interaction of charged particles, electromagnetic waves, and external magnetic fields that comprise the x-ray free-electron lasers. It also includes a discussion of existing facilities and avenues for increasing the peak power and improving the control of spectral and coherence properties to allow the exploration of an even larger range of phenomena.\hskip-0.22em

Observation of the Wigner-Huntington Transition to Solid Metallic Hydrogen

Abstract: We have studied solid hydrogen under pressure at low temperatures. With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa. We have measured the reflectance as a function of wavelength in the visible spectrum finding values as high as 0.90 from the metallic hydrogen. We have fit the reflectance using a Drude free electron model to determine the plasma frequency of 30.1 eV at T= 5.5 K, with a corresponding electron carrier density of 6.7x1023 particles/cm3, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory.

Ramsey-type phase control of free-electron beams

Abstract: Using a technique inspired by Ramsey spectroscopy it is now possible to coherently control free electrons in an electron microscope.

Determination of the electron-capture coefficients and the concentration of free electrons in GaN from time-resolved photoluminescence.

Abstract: Point defects in high-purity GaN layers grown by hydride vapor phase epitaxy are studied by steady-state and time-resolved photoluminescence (PL). The electron-capture coefficients for defects responsible for the dominant defect-related PL bands in this material are found. The capture coefficients for all the defects, except for the green luminescence (GL1) band, are independent of temperature. The electron-capture coefficient for the GL1 band significantly changes with temperature because the GL1 band is caused by an internal transition in the related defect, involving an excited state acting as a giant trap for electrons. By using the determined electron-capture coefficients, the concentration of free electrons can be found at different temperatures by a contactless method. A new classification system is suggested for defect-related PL bands in undoped GaN.

Twisted Radiation by Electrons in Spiral Motion

Abstract: We theoretically show that a single free electron in circular/spiral motion radiates an electromagnetic wave possessing helical phase structure and carrying orbital angular momentum. We experimentally demonstrate it by double-slit diffraction on radiation from relativistic electrons in spiral motion. We show that twisted photons should be created naturally by cyclotron/synchrotron radiations or Compton scatterings in various situations in cosmic space. We propose promising laboratory vortex photon sources in various wavelengths ranging from radio wave to gamma-rays.

Ion gyroradius effects on particle trapping in kinetic Alfvén waves along auroral field lines

Abstract: In this study, a 2D self-consistent hybrid gyrofluid-kinetic electron model is used to investigate Alfven wave propagation along dipolar magnetic field lines for a range of ion to electron temperature ratios The focus of the investigation is on understanding the role of these effects on electron trapping in kinetic Alfven waves sourced in the plasma sheet and the role of this trapping in contributing to the overall electron energization at the ionosphere This work also builds on our previous effort [Damiano et al, 2015] by considering a similar system in the limit of fixed initial parallel current, rather than fixed initial perpendicular electric field It is found that the effects of particle trapping are strongest in the cold ion limit and the kinetic Alfven wave is able to carry trapped electrons a large distance along the field line yielding a relatively large net energization of the trapped electron population as the phase speed of the wave is increased However, as the ion temperature is increased, the ability of the kinetic Alfven wave to carry and energize trapped electrons is reduced by more significant wave energy dispersion perpendicular to the ambient magnetic field which reduces the amplitude of the wave This reduction of wave amplitude in-turn reduces both the parallel current and the extent of the high energy tails evident in the energized electron populations at the ionospheric boundary (which may serve to explain the limited extent of the broadband electron energization seen in observations) Even in the cold ion limit, trapping effects in kinetic Alfven waves lead to only modest electron energization for the parameters considered (on the order of tens of eV) and the primary energization of electrons to keV levels coincides with the arrival of the wave at the ionospheric boundary

Energetics of native defects in anatase TiO2: a hybrid density functional study

Abstract: The energetics and electronic structures of native defects in anatase TiO2 are comprehensively studied using hybrid density functional calculations. We demonstrate that oxygen vacancies (VO) and titanium interstitials (Tii) act as shallow donors, and can form at substantial concentrations, giving rise to free electrons with carrier densities from 1011 to 1019 cm-3 under oxygen-rich and oxygen-poor conditions, respectively. The titanium vacancies (VTi), identified as deep acceptors and induced hole carriers, are incapable of fully compensating for the free electrons originating from the donor-type defects at any oxygen chemical potential. Even under extreme oxygen-rich conditions, the Fermi level, which is determined from the charge neutrality condition among charge defects, electron and hole carriers, is located 2.34 eV above the valence band maximum, indicating that p-type conductivity can never be realized under any growth conditions without external doping. This is consistent with common observations of intrinsic n-type conductivity of TiO2. At a typical annealing temperature and under a typical oxygen partial pressure, the carrier concentration is found to be approximately 5 × 1013 cm-3.

Laser control of electron matter waves

Abstract: In recent years laser light has been used to control the motion of electron waves. Electrons can now be diffracted by standing waves of light. Laser light in the vicinity of nanostructures is used to affect free electrons, for example, femto-second and atto-second laser-induced electrons are emitted from nanotips delivering coherent fast electron sources. Optical control of dispersion of the emitted electron waves, and optically controlled femto-second switches for ultrafast electron detection are proposed. The first steps towards electron accelerators and matter optics on-a-chip are now being taken. New research fields are driven by these new technologies. One example is the optical generation of electron pulses on-demand and quantum degenerate pulses. Another is the emerging development of interaction free electron microscopy. This review will focus on the field of free electron quantum optics with technologies at the interplay of lasers, electron matter waves, and nanostructures. Questions that motivate their development will also be addressed. This review will focus on the field of free electron quantum optics (FEQO) with technologies at the interplay of lasers, electron matter waves, and nanostructures. Recent developments that include laser generation, acceleration, coherent beam splitting, compression and detection of electron pulses with femto-second resolution are discussed. Some of the motivations for these developments concerning societal impact, finding solutions to long standing scientific problems, and speculative issues are indicated.

Comparative study of gyrokinetic, hybrid-kinetic and fully kinetic wave physics for space plasmas

Abstract: A set of numerical solvers for the linear dispersion relations of the gyrokinetic (GK), the hybrid-kinetic (HK), and the fully kinetic (FK) model is employed to study the physics of the KAW and the fast magnetosonic mode in these models In particular, we focus on parameters that are relevant for solar wind oriented applications (using a homogeneous, isotropic background), which are characterized by wave propagation angles averaging close to 90° It is found that the GK model, while lacking high-frequency solutions and cyclotron effects, faithfully reproduces the FK wave physics close to, and sometimes significantly beyond, the boundaries of its range of validity The HK model, on the other hand, is much more complete in terms of high-frequency waves, but owing to its simple electron model it is found to severely underpredict wave damping rates even on ion spatial scales across a large range of parameters, despite containing full kinetic ion physics

The Formation of Gas Bubbles by Processing of Liquid n-Heptane in the Microwave Discharge

Abstract: Numerical modeling of the process of formation of gas bubbles during initiation of the microwave discharge in liquid n-heptane at atmospheric pressure has been performed. The developed model has an axial symmetry. The model is based on joint solution of the Maxwell equations, Navier–Stokes equation, heat equation, continuity equations for electrons (written in the ambipolar diffusion approximation) and the n-heptane concentration (including its thermal decomposition and dissociation by electron impact) and the Boltzmann equation for free electrons of the plasma. The calculations allowed to describe the dynamics of the formation of gas bubbles in the liquid, to evaluate the role of electron impact in the decomposition of n-heptane, and to estimate the characteristic times of various processes in the system. The results of new experiments are compared with the simulation results. On the basis of this comparison one could explain the presence in the spectra of the discharge only bands of C2.

Direct observation of photocarrier electron dynamics in C60 films on graphite by time-resolved two-photon photoemission

Abstract: Time-resolved two-photon photoemission (TR-2PPE) spectroscopy is employed to probe the electronic states of a C60 fullerene film formed on highly oriented pyrolytic graphite (HOPG), acting as a model two-dimensional (2D) material for multi-layered graphene. Owing to the in-plane sp2-hybridized nature of the HOPG, the TR-2PPE spectra reveal the energetics and dynamics of photocarriers in the C60 film: after hot excitons are nascently formed in C60 via intramolecular excitation by a pump photon, they dissociate into photocarriers of free electrons and the corresponding holes, and the electrons are subsequently detected by a probe photon as photoelectrons. The decay rate of photocarriers from the C60 film into the HOPG is evaluated to be 1.31 × 1012 s-1, suggesting a weak van der Waals interaction at the interface, where the photocarriers tentatively occupy the lowest unoccupied molecular orbital (LUMO) of C60. The photocarrier electron dynamics following the hot exciton dissociation in the organic thin films has not been realized for any metallic substrates exhibiting strong interactions with the overlayer. Furthermore, the thickness dependence of the electron lifetime in the LUMO reveals that the electron hopping rate in C60 layers is 3.3 ± 1.2 × 1013 s-1.

Plasmons in Finite Spherical Electrolyte Systems: RPA Effective Jellium Model for Ionic Plasma Excitations.

Abstract: Plasmons are fundamental collective excitations in many particle charged systems like in free electron liquid in metals, high energy nuclear plasma in solar core or in fusion devices, in ion gas in ionosphere or in intra- and inter-galactic gas clouds. Plasmons play a central role also in small systems, in particular in metallic nanoparticles and in their arrays allowing for subdiffraction light manipulation. In analogy to metallic nanoparticles, we have developed description of the soft plasmonics in finite electrolyte systems confined in micrometer scale by insulating membranes. Plasmon-type excitations in such finite ionic systems are determined via originally formulated theoretical model allowing to describe surface and volume plasmons in confined geometry of the ion liquid. Size-effect for attenuation of surface plasmons in the finite electrolyte system is described and its various regimes are identified. The cross-over in the plasmon damping system-size-dependence is demonstrated including scattering of ions and their energy losses via irradiation. The plasmon resonances in ion systems replicate the metal cluster plasmon phenomena, though in distinct energy and size scale related to larger ion mass and lower ion concentration (in low energy plasma) in comparison to electrons in metals. The possibility for tuning plasmon resonances in finite ionic systems in a wide range by changing system size, ion, and electrolyte parameters is demonstrated.

Nondestructive measurement of orbital angular momentum for an electron beam

Abstract: Free electrons with a helical phase front, referred to as ``twisted'' electrons, possess an orbital angular momentum (OAM) and, hence, a quantized magnetic dipole moment along their propagation direction. This intrinsic magnetic moment can be used to probe material properties. Twisted electrons thus have numerous potential applications in materials science. Measuring this quantity often relies on a series of projective measurements that subsequently change the OAM carried by the electrons. In this Letter, we propose a nondestructive way of measuring an electron beam's OAM through the interaction of this associated magnetic dipole with a conductive loop. Such an interaction results in the generation of induced currents within the loop, which are found to be directly proportional to the electron's OAM value. Moreover, the electron experiences no OAM variations and only minimal energy losses upon the measurement, and, hence, the nondestructive nature of the proposed technique.

Four-wave-mixing experiments with seeded free electron lasers

Abstract: The development of free electron laser (FEL) sources has provided an unprecedented bridge between the scientific communities working with ultrafast lasers and extreme ultraviolet (XUV) and X-ray radiation. Indeed, in recent years an increasing number of FEL-based applications have exploited methods and concepts typical of advanced optical approaches. In this context, we recently used a seeded FEL to demonstrate a four-wave-mixing (FWM) process stimulated by coherent XUV radiation, namely the XUV transient grating (X-TG). We hereby report on X-TG measurements carried out on a sample of silicon nitride (Si3N4). The recorded data bears evidence for two distinct signal decay mechanisms: one occurring on a sub-ps timescale and one following slower dynamics extending throughout and beyond the probed timescale range (100 ps). The latter is compatible with a slower relaxation (time decay > ns), that may be interpreted as the signature of thermal diffusion modes. From the peak intensity of the X-TG signal we could estimate a value of the effective third-order susceptibility which is substantially larger than that found in SiO2, so far the only sample with available X-TG data. Furthermore, the intensity of the time-coincidence peak shows a linear dependence on the intensity of the three input beams, indicating that the measurements were performed in the weak field regime. However, the timescale of the ultrafast relaxation exhibits a dependence on the intensity of the XUV radiation. We interpreted the observed behaviour as the generation of a population grating of free-electrons and holes that, on the sub-ps timescale, relaxes to generate lattice excitations. The background free detection inherent to the X-TG approach allowed the determination of FEL-induced electron dynamics with a sensitivity largely exceeding that of transient reflectivity and transmissivity measurements, usually employed for this purpose.

Monte-Carlo modelling of nano-material photocatalysis: bridging photocatalytic activity and microscopic charge kinetics.

Abstract: In photocatalysis, it is known that light intensity, organic concentration, and temperature affect the photocatalytic activity by changing the microscopic kinetics of holes and electrons. However, how the microscopic kinetics of holes and electrons relates to the photocatalytic activity was not well known. In the present research, we developed a Monte-Carlo random walking model that involved all of the charge kinetics, including the photo-generation, the recombination, the transport, and the interfacial transfer of holes and electrons, to simulate the overall photocatalytic reaction, which we called a “computer experiment” of photocatalysis. By using this model, we simulated the effect of light intensity, temperature, and organic surface coverage on the photocatalytic activity and the density of the free electrons that accumulate in the simulated system. It was seen that the increase of light intensity increases the electron density and its mobility, which increases the probability for a hole/electron to find an electron/hole for recombination, and consequently led to an apparent kinetics that the quantum yield (QY) decreases with the increase of light intensity. It was also seen that the increase of organic surface coverage could increase the rate of hole interfacial transfer and result in the decrease of the probability for an electron to recombine with a hole. Moreover, the increase of organic coverage on the nano-material surface can also increase the accumulation of electrons, which enhances the mobility for electrons to undergo interfacial transfer, and finally leads to the increase of photocatalytic activity. The simulation showed that the temperature had a more complicated effect, as it can simultaneously change the activation of electrons, the interfacial transfer of holes, and the interfacial transfer of electrons. It was shown that the interfacial transfer of holes might play a main role at low temperature, with the temperature-dependence of QY conforming to the Arrhenius model. The activation of electrons from the traps to the conduction band might become important at high temperature, which accelerates the electron movement for recombination and leads to a temperature dependence of QY that deviates from the Arrhenius model.

First-principles investigation of the electronic and optical properties of Al-doped FeS 2 pyrite for photovoltaic applications

Abstract: In this study, we use density functional theory with the generalized gradient approximation proposed by Perdew–Burke–Ernzerhof (GGA-PBE) to investigate the effect of Al-doping on the structural, electronic and optical properties of the pyrite FeS2 structure It shows an indirect band gap of 107 eV, which is in reasonable agreement with the experimental data When Al atom occupies the Fe site, one of the most effects occurred in the electronic structure is to create shallow donor states partially occupied at the bottom of the conduction band, which could be considered as the primary induced origin of the electrical conductivity in the Al doped FeS2 system Then, the injection of free electrons into the conduction band could be carried out only from the low excitation energies as well as the photon absorptions could be enhanced in the visible range Therefore, all these features may be making of Al-doped FeS2 a useful material for cell photovoltaic applications with a reasonable cost

Methodology for determining the electronic thermal conductivity of metals via direct nonequilibrium ab initio molecular dynamics

Abstract: Many physical properties of metals can be understood in terms of the free electron model, as proven by the Wiedemann-Franz law. According to this model, electronic thermal conductivity can be inferred from the Boltzmann transport equation (BTE). However, the BTE does not perform well for some complex metals, such as Cu. Moreover, the BTE cannot clearly describe the origin of the thermal energy carried by electrons or how this energy is transported in metals. The charge distribution of conduction electrons in metals is known to reflect the electrostatic potential of the ion cores. Based on this premise, we develop a new methodology for evaluating electronic thermal conductivity of metals by combining the free electron model and non-equilibrium ab initio molecular dynamics simulations. We confirm that the kinetic energy of thermally excited electrons originates from the energy of the spatial electrostatic potential oscillation, which is induced by the thermal motion of ion cores. This method directly predicts the electronic thermal conductivity of pure metals with a high degree of accuracy, without explicitly addressing any complicated scattering processes of free electrons. Our methodology offers a new route to understand the physics of heat transfer by electrons at the atomistic level. The methodology can be further extended to study similar electron involved problems in materials, such as electron-phonon coupling, which is underway currently.

Photoelectron angular distributions and correlations in sequential double and triple atomic ionization by free electron lasers

Abstract: We present a review of theoretical studies of the simplest nonlinear photoprocesses detected in the XUV range with the use of free electron lasers: sequential double and triple ionization of atoms by two and three XUV photons. Photoelectron angular distributions and angular correlations between emitted electrons are considered. A comparison of the calculated results with recent angle-resolved photoelectron spectroscopy experiments is discussed.

Characterization of local thermodynamic equilibrium in a laser-induced aluminum alloy plasma.

Abstract: The electron temperature was evaluated using the line-to-continuum ratio method, and whether the plasma was close to the local thermodynamic equilibrium (LTE) state was investigated in detail. The results showed that approximately 5 μs after the plasma formed, the changes in the electron and excitation temperatures, which were determined using a Boltzmann plot, overlapped in the 15% error range, which indicated that the LTE state was reached. The recombination of electrons and ions and the free electron expansion process led to the deviation from the LTE state. The plasma's expansion rate slowed over time, and when the expansion time was close to the ionization equilibrium time, the LTE state was almost reached. The McWhirter criterion was adopted to calculate the threshold electron density for different species, and the results showed that experimental electron density was greater than the threshold electron density, which meant that the LTE state may have existed. However, for the nonmetal element N, the threshold electron density was greater than the value experimental value approximately 0.8 μs after the plasma formed, which meant that LTE state did not exist for N.

Coherent Cherenkov radiation and laser oscillation in a photonic crystal

Abstract: We demonstrate that photonic crystals can be used to generate powerful and highly coherent Cherenkov radiation that is excited by the injection of a beam of free electrons. Using theoretical and numerical investigations we present the startup dynamics and coherence properties of such a laser, in which gain is provided by matching the optical phase velocity in the photonic crystal to the velocity of the electron beam. The operating frequency can be varied by changing the electron beam energy and scaled to different ranges by varying the lattice constant of the photonic crystal.

A linear dispersion relation for the hybrid kinetic-ion/fluid-electron model of plasma physics

Abstract: A dispersion relation for a commonly used hybrid model of plasma physics is developed, which combines fully kinetic ions and a massless-electron fluid description. Although this model and variations of it have been used to describe plasma phenomena for about 40 years, to date there exists no general dispersion relation to describe the linear wave physics contained in the model. Previous efforts along these lines are extended here to retain arbitrary wave propagation angles, temperature anisotropy effects, as well as additional terms in the generalized Ohm's law which determines the electric field. A numerical solver for the dispersion relation is developed, and linear wave physics is benchmarked against solutions of a full Vlasov–Maxwell dispersion relation solver. This work opens the door to a more accurate interpretation of existing and future wave and turbulence simulations using this type of hybrid model.

Equation of state of the relativistic free electron gas at arbitrary degeneracy

Abstract: We study the problem of the relativistic free electron gas at arbitrary degeneracy. The specific heat at constant volume and particle number CV and the specific heat at constant pressure and particle number CP are calculated. The question of equation of state is also studied. Non degenerate and degenerate limits are considered. We generalize the formulas obtained in the non-relativistic and ultra-relativistic regimes.

Electron energy transfer effect in Au NS/CH 3 NH 3 PbI 3-x Cl x heterostructures via localized surface plasmon resonance coupling.

Abstract: Localized surface plasmon resonance coupling effects (LSPR) have attracted much attention due to their interesting properties. This Letter demonstrates significant photoluminescence (PL) enhancement in the Au NS/CH3NH3PbI3−xClx heterostructures via the LSPR coupling. The observed PL emission enhancement is mainly attributed to the hot electron energy transfer effect related to the LSPR coupling. For the energy transfer effect, photo-generated electrons will be directly extracted into Au SPs, rather than relaxed into exciton states. This energy transfer process is much faster than the diffusion and relaxation time of free electrons, and may provide new ideas on the design of high-efficiency solar cells and ultrafast response photodetectors.

A model dielectric function for low and very high momentum transfer

Abstract: A model dielectric function is derived for TiO2 based on reflection electron energy loss spectroscopy data and photoabsorption cross sections. The model is based on a set of Mermin oscillators. The input data is dominated by excitations at low momentum transfer, i.e. near the optical limit. Surprisingly the dielectric function derived at low momentum transfer describes the Compton profile quite well, while approaches based on Drude oscillators fail dramatically. The link between the dielectric function in the high-momentum transfer limit and a Compton profile is discussed. The underlying reason why the Mermin approach, which is based on a free electron model, is successful in describing the Compton profile is tentatively discussed.