Showing papers on "Free electron model published in 2017"

Effective mass and Fermi surface complexity factor from ab initio band structure calculations

Abstract: The effective mass is a convenient descriptor of the electronic band structure used to characterize the density of states and electron transport based on a free electron model. While effective mass is an excellent first-order descriptor in real systems, the exact value can have several definitions, each of which describe a different aspect of electron transport. Here we use Boltzmann transport calculations applied to ab initio band structures to extract a density-of-states effective mass from the Seebeck Coefficient and an inertial mass from the electrical conductivity to characterize the band structure irrespective of the exact scattering mechanism. We identify a Fermi Surface Complexity Factor: $${N}_{{\rm{v}}}^{\ast }{K}^{\ast }$$ from the ratio of these two masses, which in simple cases depends on the number of Fermi surface pockets $$({N}_{{\rm{v}}}^{\ast })$$ and their anisotropy K *, both of which are beneficial to high thermoelectric performance as exemplified by the high values found in PbTe. The Fermi Surface Complexity factor can be used in high-throughput search of promising thermoelectric materials. A simple method for determining a material’s thermoelectric properties is developed by researchers in the United States and Belgium. Jeffrey Snyder from Northwestern University and his co-workers’ model could simplify the search for materials that efficiently generate electricity from waste heat. Even though the environment of an electron in a solid is very complex, the way an electron moves through a solid’s lattice of atoms can be treated as if it is moving in free space. However, because of the influence of its environment an effective mass, not its true mass, is used to model the movement of electrons and that material’s properties. But this effective-mass can be defined in several ways depending on which material property is being modeled. Snyder et al. determine that the ratio of two different effective masses, as computed from different electronic properties, could be a good method to identify novel thermoelectric materials and can be associated with the “complexity” of the electronic structure.

High Thermoelectric Performance in Electron-Doped AgBi3S5 with Ultralow Thermal Conductivity

Abstract: We report electron-doped AgBi3S5 as a new high-performance nontoxic thermoelectric material. This compound features exceptionally low lattice thermal conductivities of 0.5–0.3 W m–1 K–1 in the temperature range of 300–800 K, which is ascribed to its unusual vibrational properties: “double rattling” phonon modes associated with Ag and Bi atoms. Chlorine doping at anion sites acts as an efficient electron donor, significantly enhancing the electrical properties of AgBi3S5. In the carrier concentration range (5 × 1018–2 × 1019 cm–3) investigated in this study, the trends in Seebeck coefficient can be reasonably understood using a single parabolic band model with the electron effective mass of 0.22 me (me is the free electron mass). Samples of 0.33% Cl-doped AgBi3S5 prepared by spark plasma sintering show a thermoelectric figure of merit of ∼1.0 at 800 K.

From attosecond to zeptosecond coherent control of free-electron wave functions using semi-infinite light fields

Abstract: Light-electron interaction in empty space is the seminal ingredient for free-electron lasers and also for controlling electron beams to dynamically investigate materials and molecules. Pushing the coherent control of free electrons by light to unexplored timescales, below the attosecond, would enable unprecedented applications in light-assisted electron quantum circuits and diagnostics at extremely small timescales, such as those governing intramolecular electronic motion and nuclear phenomena. We experimentally demonstrate attosecond coherent manipulation of the electron wave function in a transmission electron microscope, and show that it can be pushed down to the zeptosecond regime with existing technology. We make a relativistic pulsed electron beam interact in free space with an appropriately synthesized semi-infinite light field generated by two femtosecond laser pulses reflected at the surface of a mirror and delayed by fractions of the optical cycle. The amplitude and phase of the resulting coherent oscillations of the electron states in energymomentum space are mapped via momentum-resolved ultrafast electron energy-loss spectroscopy. The experimental results are in full agreement with our theoretical framework for light-electron interaction, which predicts access to the zeptosecond timescale by combining semi-infinite X-ray fields with free electrons.

Helical Phase Structure of Radiation from an Electron in Circular Motion.

Abstract: We theoretically show that a single free electron in circular motion radiates an electromagnetic wave possessing helical phase structure, which is closely related to orbital angular momentum carried by it. We experimentally demonstrate it by interference and double-slit diffraction experiments on radiation from relativistic electrons in spiral motion. Our results indicate that photons carrying orbital angular momentum 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.

Hot electrons in water: injection and ponderomotive acceleration by means of plasmonic nanoelectrodes.

Abstract: We present a theoretical and experimental study of a plasmonic nanoelectrode architecture that is able to inject bunches of hot electrons into an aqueous environment. In this approach, electrons are accelerated in water by ponderomotive forces up to energies capable of exciting or ionizing water molecules. This ability is enabled by the nanoelectrode structure (extruding out of a metal baseplate), which allows for the production of an intense plasmonic hot spot at the apex of the structure while maintaining the electrical connection to a virtually unlimited charge reservoir. The electron injection is experimentally monitored by recording the current transmitted through the water medium, whereas the electron acceleration is confirmed by observation of the bubble generation for a laser power exceeding a proper threshold. An understanding of the complex physics involved is obtained via a numerical approach that explicitly models the electromagnetic hot spot generation, electron-by-electron injection via multiphoton absorption, acceleration by ponderomotive forces and electron-water interaction through random elastic and inelastic scattering. The model predicts a critical electron density for bubble nucleation that nicely matches the experimental findings and reveals that the efficiency of energy transfer from the plasmonic hot spot to the free electron cloud is much more efficient (17 times higher) in water than in a vacuum. Because of their high kinetic energy and large reduction potential, these proposed wet hot electrons may provide new opportunities in photocatalysis, electrochemical processes and hot-electron driven chemistry.

Mid-infrared dispersive wave generation in gas-filled photonic crystal fibre by transient ionization-driven changes in dispersion.

Abstract: Gas-filled hollow-core photonic crystal fibre is being used to generate ever wider supercontinuum spectra, in particular via dispersive wave emission in the deep and vacuum ultraviolet, with a multitude of applications. Dispersive waves are the result of nonlinear transfer of energy from a self-compressed soliton, a process that relies crucially on phase-matching. It was recently predicted that, in the strong-field regime, the additional transient anomalous dispersion introduced by gas ionization would allow phase-matched dispersive wave generation in the mid-infrared—something that is forbidden in the absence of free electrons. Here we report the experimental observation of such mid-infrared dispersive waves, embedded in a 4.7-octave-wide supercontinuum that uniquely reaches simultaneously to the vacuum ultraviolet, with up to 1.7 W of total average power. Dispersive wave emission in gas-filled hollow-core photonic crystal fibres has been possible in the visible and ultraviolet via the optical Kerr effect. Here, Kottig et al. demonstrate dispersive waves generated by an additional transient anomalous dispersion from gas ionization in the mid-infrared.

Electron effective mass and mobility limits in degenerate perovskite stannate BaSnO 3

Abstract: The high room temperature mobility and the electron effective mass in ${\mathrm{BaSnO}}_{3}$ are investigated in depth by evaluation of the free carrier absorption observed in infrared spectra for epitaxial films with free electron concentrations from $8.3\ifmmode\times\else\texttimes\fi{}{10}^{18}$ to $7.6\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Both the optical band gap widening by conduction band filling and the carrier scattering mechanisms in the low- and high-doping regimes are consistently described employing parameters solely based on the intrinsic physical properties of ${\mathrm{BaSnO}}_{3}$. The results explain the current mobility limits in epitaxial films and demonstrate the potential of ${\mathrm{BaSnO}}_{3}$ to outperform established wide-band gap semiconductors also in the moderate doping regime.

Electronic Stopping of Slow Protons in Transition and Rare Earth Metals: Breakdown of the Free Electron Gas Concept.

Abstract: Measurements of how slow protons stop in tantalum and gadolinium show that the process is dependent on the density of states.

Roles of Free Electrons and H2O2 in the Optical Breakdown-Induced Photochemical Reduction of Aqueous [AuCl4]−

Abstract: Free electrons and H2O2 formed in an optical breakdown plasma are found to directly control the kinetics of [AuCl4]− reduction to form Au nanoparticles (AuNPs) during femtosecond laser-assisted synthesis of AuNPs. The formation rates of both free electrons and H2O2 strongly depend on the energy and duration of the 800 nm laser pulses over the ranges of 10–2400 μJ and 30–1500 fs. By monitoring the conversion of [AuCl4]− to AuNPs using in situ UV–vis spectroscopy during laser irradiation, the first- and second-order rate constants in the autocatalytic rate law, k1 and k2, were extracted and compared to the computed free electron densities and experimentally measured H2O2 formation rates. For laser pulse energies of 600 μJ and lower at all pulse durations, the first-order rate constant, k1, was found to be directly proportional to the theoretically calculated plasma volume, in which the electron density exceeds the threshold value of 1.8 × 1020 cm–3. The second-order rate constant, k2, was found to correlate...

Determination of n-Type Doping Level in Single GaAs Nanowires by Cathodoluminescence

Abstract: We present an effective method of determining the doping level in n-type III–V semiconductors at the nanoscale. Low-temperature and room-temperature cathodoluminescence (CL) measurements are carried out on single Si-doped GaAs nanowires. The spectral shift to higher energy (Burstein–Moss shift) and the broadening of luminescence spectra are signatures of increased electron densities. They are compared to the CL spectra of calibrated Si-doped GaAs layers, whose doping levels are determined by Hall measurements. We apply the generalized Planck’s law to fit the whole spectra, taking into account the electron occupation in the conduction band, the bandgap narrowing, and band tails. The electron Fermi levels are used to determine the free electron concentrations, and we infer nanowire doping of 6 × 1017 to 1 × 1018 cm–3. These results show that cathodoluminescence provides a robust way to probe carrier concentrations in semiconductors with the possibility of mapping spatial inhomogeneities at the nanoscale.

Novel mid-infrared dispersive wave generation in gas-filled PCF by transient ionization-driven changes in dispersion

Abstract: Gas-filled hollow-core photonic crystal fibre (PCF) is being used to generate ever wider supercontinuum spectra, in particular via dispersive wave (DW) emission in the deep and vacuum ultraviolet, with a multitude of applications. DWs are the result of the resonant transfer of energy from a self-compressed soliton, a process which relies crucially on phase matching. It was recently predicted that, in the strong-field regime, the additional transient anomalous dispersion introduced by gas ionization would allow phase-matched DW generation in the mid-infrared (MIR)-something that is forbidden in the absence of free electrons. Here we report for the first time the experimental observation of such MIR DWs, embedded in a 4.7-octave-wide supercontinuum that uniquely reaches simultaneously to the vacuum ultraviolet, with up to 1.7 W of total average power.

Spin-Polarizing Interferometric Beam Splitter for Free Electrons

Abstract: A spin-polarizing electron beam splitter is described that relies on an arrangement of linearly polarized laser waves of nonrelativistic intensity. An incident electron beam is first coherently scattered off a bichromatic laser field, splitting the beam into two portions, with electron spin and momentum being entangled. Afterwards, the partial beams are coherently superposed in an interferometric setup formed by standing laser waves. As a result, the outgoing electron beam is separated into its spin components along the laser magnetic field, which is shown by both analytical and numerical solutions of Pauli's equation. The proposed laser field configuration thus exerts the same effect on free electrons as an ordinary Stern-Gerlach magnet does on atoms.

Conduction-band effective mass and bandgap of ZnSnN2 earth-abundant solar absorber

Abstract: Pseudo III-V nitride ZnSnN2 is an earth-abundant semiconductor with a high optical absorption coefficient in the solar spectrum. Its bandgap can be tuned by controlling the cation sublattice disorder. Thus, it is a potential candidate for photovoltaic absorber materials. However, its important basic properties such as the intrinsic bandgap and effective mass have not yet been quantitatively determined. This paper presents a detailed optical absorption analysis of disordered ZnSnN2 degenerately doped with oxygen (ZnSnN2−x O x ) in the ultraviolet to infrared region to determine the conduction-band effective mass (m c *) and intrinsic bandgap (E g). ZnSnN2−x O x epilayers are n-type degenerate semiconductors, which exhibit clear free-electron absorption in the infrared region. By analysing the free-electron absorption using the Drude model, m c * was determined to be (0.37 ± 0.05)m 0 (m 0 denotes the free electron mass). The fundamental absorption edge in the visible to ultraviolet region shows a blue shift with increasing electron density. The analysis of the blue shift in the framework of the Burstein-Moss effect gives the E g value of 0.94 ± 0.02 eV. We believe that the findings of this study will provide important information to establish this material as a photovoltaic absorber.

Transition energies, transition probabilities and weighted oscillator strengths of He-like Al in hot and dense plasmas

Abstract: We present a detailed investigation of the transition properties for He-like Al embedded in hot and dense plasma environments. The correlation effect between the bound electrons is treated with the configuration interaction method. Plasma screening on the nucleus is described using the self-consistent-field ion sphere model. Transition energies, transition probabilities and weighted oscillator strengths decrease quickly with the increase of free electron densities, but increase slightly with the electron temperature, and approach those of the uniform electron density ion sphere model at a given average free electron density. The results reported in this work are useful for plasma diagnostics.

First-Principles Prediction of Electronic, Magnetic, and Optical Properties of Co 2 MnAs Full-Heusler Half-Metallic Compound

Abstract: Electronic, magnetic, and optical properties of Co2MnAs full-Heusler compound have been calculated using a first-principles approach with the full-potential linearized augmented plane-wave (FP-LAPW) method and generalized gradient approximation plus U (GGA + U). The results are compared with various properties of Co2MnZ (Z = Si, Ge, Al, Ga, Sn) full-Heusler compounds. The results of our calculations show that Co2MnAs is a half-metallic ferromagnetic compound with 100% spin polarization at the Fermi level. The total magnetic moment and half-metallic gap of Co2MnAs compound are found to be 6.00μB and 0.43 eV, respectively. It is also predicted that the spin-wave stiffness constant and Curie temperature of Co2MnAs compound are about 3.99 meV nm2 and 1109 K, respectively. The optical results show that the dominant behavior, at energy below 2 eV, is due to interactions of free electrons in the system. Interband optical transitions have been calculated based on the imaginary part of the dielectric function and analysis of critical points in the second energy derivative of the dielectric function. The results show that there is more than one plasmon energy for Co2MnAs compound, with the highest occurring at 25 eV. Also, the refractive index variations and optical reflectivity for radiation at normal incidence are calculated for Co2MnAs. Because of its high magnetic moment, high Curie temperature, and 100% spin polarization at the Fermi level as well as its optical properties, Co2MnAs is a good candidate for use in spintronic components and magnetooptical devices.

Determination of Hydrogen Density by Swift Heavy Ions.

Abstract: A novel method to determine the total hydrogen density and, accordingly, a precise plasma temperature in a lowly ionized hydrogen plasma is described. The key to the method is to analyze the energy loss of swift heavy ions interacting with the respective bound and free electrons of the plasma. A slowly developing and lowly ionized hydrogen theta-pinch plasma is prepared. A Boltzmann plot of the hydrogen Balmer series and the Stark broadening of the H_{β} line preliminarily defines the plasma with a free electron density of (1.9±0.1)×10^{16} cm^{-3} and a free electron temperature of 0.8-1.3 eV. The temperature uncertainty results in a wide hydrogen density, ranging from 2.3×10^{16} to 7.8×10^{18} cm^{-3}. A 108 MHz pulsed beam of ^{48}Ca^{10+} with a velocity of 3.652 MeV/u is used as a probe to measure the total energy loss of the beam ions. Subtracting the calculated energy loss due to free electrons, the energy loss due to bound electrons is obtained, which linearly depends on the bound electron density. The total hydrogen density is thus determined as (1.9±0.7)×10^{17} cm^{-3}, and the free electron temperature can be precisely derived as 1.01±0.04 eV. This method should prove useful in many studies, e.g., inertial confinement fusion or warm dense matter.

Predicting point defect equilibria across oxide hetero-interfaces: model system of ZrO2/Cr2O3

Abstract: We present a multi-scale approach to predict equilibrium defect concentrations across oxide/oxide hetero-interfaces. There are three factors that need to be taken into account simultaneously for computing defect redistribution around the hetero-interfaces: the variation of local bonding environment at the interface as epitomized in defect segregation energies, the band offset at the interface, and the equilibration of the chemical potentials of species and electrons via ionic and electronic drift-diffusion fluxes. By including these three factors from the level of first principles calculation, we build a continuum model for defect redistribution by concurrent solution of Poisson's equation for the electrostatic potential and the steady-state equilibrium drift-diffusion equation for each defect. This model solves for and preserves the continuity of the electric displacement field throughout the interfacial core zone and the extended space charge zones. We implement this computational framework to a model hetero-interface between the monoclinic zirconium oxide, m-ZrO2, and the chromium oxide Cr2O3. This interface forms upon the oxidation of zirconium alloys containing chromium secondary phase particles. The model explains the beneficial effect of the oxidized Cr particles on the corrosion and hydrogen resistance of Zr alloys. Under oxygen rich conditions, the ZrO2/Cr2O3 heterojunction depletes the oxygen vacancies and the sum of electrons and holes in the extended space charge zone in ZrO2. This reduces the transport of oxygen and electrons thorough ZrO2 and slows down the metal oxidation rate. The enrichment of free electrons in the space charge zone is expected to decrease the hydrogen uptake through ZrO2. Moreover, our analysis provides a clear anatomy of the components of interfacial electric properties; a zero-Kelvin defect-free contribution and a finite temperature defect contribution. The thorough analytical and numerical treatment presented here quantifies the rich coupling between defect chemistry, thermodynamics and electrostatics which can be used to design and control oxide hetero-interfaces.

Controlling electron spin dynamics in bichromatic Kapitza-Dirac scattering by the laser field polarization

Abstract: Electron spin dynamics in counterpropagating, bichromatic laser fields are studied by solving the Dirac equation with different laser polarization configurations. It is found that the spin-dependent Kapitza-Dirac effect in the dynamical processes may be used to design spin filters for free electrons.

Vibrational kinetics of electronically excited states in H2 discharges

Abstract: The evolution of atmospheric pressure hydrogen plasma under the action of repetitively ns electrical pulse has been investigated using a 0D state-to-state kinetic model that self-consistently couples the master equation of heavy particles and the Boltzmann equation for free electrons. The kinetic model includes, together with atomic hydrogen states and the vibrational kinetics of H2 ground state, vibrational levels of singlet states, accounting for the collisional quenching, having a relevant role because of the high pressure. The mechanisms of excitations, radiative decay and collisional quenching involving the excited H2 states and the corresponding cross sections, integrated over the non-equilibrium electron energy distribution function (EEDF) to obtain kinetic rates, are discussed in the light of the kinetic simulation results, i.e. the time evolution during the pulse of the plasma composition, of the EEDF and of the vibrational distributions of ground and singlet excited states.

Reduction of Intrinsic Electron Emittance from Photocathodes Using Ordered Crystalline Surfaces.

Abstract: The generation of intense electron beams with low emittance is key to both the production of coherent x rays from free electron lasers, and electron pulses with large transverse coherence length used in ultrafast electron diffraction. These beams are generated today by photoemission from disordered polycrystalline surfaces. We show that the use of single crystal surfaces with appropriate electronic structures allows us to effectively utilize the physics of photoemission to generate highly directed electron emission, thus reducing the emittance of the electron beam being generated.

Tuning the Competitive Recombination of Free Carriers and Bound Excitons in Perovskite CH3NH3PbBr3 Single Crystal

Abstract: Excitation of perovskite CH3NH3PbBr3 crystal generates long-lived carriers, which radiatively recombine via free electrons and holes or localized excitons. Here, we investigate the competitive recombination of free carriers and bound excitons by tuning two-photon excitation wavelength. At excitation far from resonance, the CH3NH3PbBr3 perovskite single crystal shows the free carrier up-conversion emission because the exciton binding energy is comparable with the thermal energy at room temperature. At excitation near resonance (1.97 eV < hνexc < 2.16 eV), the anti-Stokes process with a consistent phonon energy of ∼186 meV is reported for the first time. Furthermore, when the anti-Stokes transition is resonant with localized exciton level below the band, highly efficient up-converted luminescence from the bound exciton recombination is observed. The finding that the excited state recombination kinetics vary versus excitation wavelength, in the present work, is helpful for developing the high-performance opt...

Structure-Dependent Optical Properties of Self-Organized Bi2Se3 Nanostructures: From Nanocrystals to Nanoflakes.

Abstract: Bismuth selenide (Bi2Se3), with a wide bulk band gap and single massless Dirac cone at the surface, is a promising three-dimensional topological insulator. Bi2Se3 possesses gapless surface states and an insulator-like bulk band gap as a new type of quantum matter. Different Bi2Se3 nanostructures were prepared using electron beam evaporation with high production efficiency. Structural investigations by energy-dispersive X-ray analysis, scanning electron microscopy, and X-ray diffraction revealed the sample stoichiometries and the structural transition mechanism from nanocrystals to nanoflakes. The optical properties systematically probed and analyzed by spectroscopic ellipsometry showed strong dependence on the nanostructures and were also predicted to have structure-modifiable technological prospects. The optical parameters, plasma frequencies, scattering rates of the free electrons, and optical band gaps were related to the topological properties of the Bi2Se3 nanostructures via light–matter interactions...

Spin in the extended electron model

Abstract: It has been found that a model of extended electrons is more suited to describe theoretical simulations and experimental results obtained via scanning tunnelling microscopes, but while the dynamic properties are easily incorporated, magnetic properties, and in particular electron spin properties pose a problem due to their conceived isotropy in the absence of measurement. The spin of an electron reacts with a magnetic field and thus has the properties of a vector. However, electron spin is also isotropic, suggesting that it does not have the properties of a vector. This central conflict in the description of an electron’s spin, we believe, is the root of many of the paradoxical properties measured and postulated for quantum spin particles. Exploiting a model in which the electron spin is described consistently in real three-dimensional space–an extended electron model–we demonstrate that spin may be described by a vector and still maintain its isotropy. In this framework, we re-evaluate the Stern–Gerlach experiments, the Einstein–Podolsky–Rosen experiments, and the effect of consecutive measurements and find in all cases a fairly intuitive explanation.

Ground- and excited-state scattering potentials for the stopping of protons in an electron gas

Abstract: The self-consistent electron–ion potential V(r) is calculated for H+ ions in an electron gas system as a function of the projectile energy to model the electronic stopping power for conduction-band electrons. The results show different self-consistent potentials at low projectile-energies, related to different degrees of excitation of the electron cloud surrounding the intruder ion. This behavior can explain the abrupt change of velocity dependent screening-length of the potential found by the use of the extended Friedel sum rule and the possible breakdown of the standard free electron gas model for the electronic stopping at low projectile energies. A dynamical interpolation of V(r) is proposed and used to calculate the stopping power for H+ interacting with the valence electrons of Al. The results are in good agreement with the TDDFT benchmark calculations as well as with experimental data.

Elementary Processes and Kinetic Modeling for Hydrogen and Helium Plasmas

Abstract: We report cross-sections and rate coefficients for excited states colliding with electrons, heavy particles and walls useful for the description of H 2 /He plasma kinetics under different conditions. In particular, the role of the rotational states in resonant vibrational excitations of the H 2 molecule by electron impact and the calculation of the related cross-sections are illustrated. The theoretical determination of the cross-section for the rovibrational energy exchange and dissociation of H 2 molecule, induced by He atom impact, by using the quasi-classical trajectory method is discussed. Recombination probabilities of H atoms on tungsten and graphite, relevant for the determination of the nascent vibrational distribution, are also presented. An example of a state-to-state plasma kinetic model for the description of shock waves operating in H 2 and He-H 2 mixtures is presented, emphasizing also the role of electronically-excited states in affecting the electron energy distribution function of free electrons. Finally, the thermodynamic properties and the electrical conductivity of non-ideal, high-density hydrogen plasma are finally discussed, in particular focusing on the pressure ionization phenomenon in high-pressure high-temperature plasmas.