Showing papers on "Free electron model published in 2007"

Mott Transition in VO2 Revealed by Infrared Spectroscopy and Nano-Imaging

Abstract: Electrons in correlated insulators are prevented from conducting by Coulomb repulsion between them. When an insulator-to-metal transition is induced in a correlated insulator by doping or heating, the resulting conducting state can be radically different from that characterized by free electrons in conventional metals. We report on the electronic properties of a prototypical correlated insulator vanadium dioxide in which the metallic state can be induced by increasing temperature. Scanning near-field infrared microscopy allows us to directly image nanoscale metallic puddles that appear at the onset of the insulator-to-metal transition. In combination with far-field infrared spectroscopy, the data reveal the Mott transition with divergent quasi-particle mass in the metallic puddles. The experimental approach used sets the stage for investigations of charge dynamics on the nanoscale in other inhomogeneous correlated electron systems.

Quantum Hall Effect in Polar Oxide Heterostructures

Abstract: We observed Shubnikov-de Haas oscillation and the quantum Hall effect in a high-mobility two-dimensional electron gas in polar ZnO/Mg(x)Zn(1-x)O heterostructures grown by laser molecular beam epitaxy. The electron density could be controlled in a range of 0.7 x 10(12) to 3.7 x 10(12) per square centimeter by tuning the magnesium content in the barriers and the growth polarity. From the temperature dependence of the oscillation amplitude, the effective mass of the two-dimensional electrons was derived as 0.32 +/- 0.03 times the free electron mass. Demonstration of the quantum Hall effect in an oxide heterostructure presents the possibility of combining quantum Hall physics with the versatile functionality of metal oxides in complex heterostructures.

Internal motions of a quasiparticle governing its ultrafast nonlinear response

Abstract: This paper reports a study of polarons in a GaAs crystal subject to a strong electric field. In addition to the overall drift motion of the polaron, an oscillatory internal motion is observed in which the electron is impulsively moved away from the centre of the surrounding lattice distortion. Such quantum coherent processes directly affect high-frequency transport in nanostructures. A charged particle modifies the structure of the surrounding medium: examples include a proton in ice1, an ion in a DNA molecule2, an electron at an interface3, or an electron in an organic4 or inorganic crystal5,6,7. In turn, the medium acts back on the particle. In a polar or ionic solid, a free electron distorts the crystal lattice, displacing the atoms from their equilibrium positions. The electron, when considered together with its surrounding lattice distortion, is a single quasiparticle5,6, known as the Frohlich polaron8,9. The basic properties of polarons and their drift motion in a weak electric field are well known10,11,12. However, their nonlinear high-field properties—relevant for transport on nanometre length and ultrashort timescales—are not understood. Here we show that a high electric field in the terahertz range drives the polaron in a GaAs crystal into a highly nonlinear regime where, in addition to the drift motion, the electron is impulsively moved away from the centre of the surrounding lattice distortion. In this way, coherent lattice vibrations (phonons) and concomitant drift velocity oscillations are induced that persist for several hundred femtoseconds. They modulate the optical response at infrared frequencies between absorption and stimulated emission. Such quantum coherent processes directly affect high-frequency transport in nanostructures and may be exploited in novel terahertz-driven optical modulators and switches.

Light quasiparticles dominate electronic transport in molecular crystal field-effect transistors

Abstract: We report on an infrared spectroscopy study of mobile holes in the accumulation layer of organic fieldeffect transistors based on rubrene single crystals Our data indicate that both transport and infrared properties of these transistors at room temperature are governed by light quasiparticles in molecular orbital bands with the effective masses m ? comparable to free electron mass Furthermore, the m ? values inferred from our experiments are in agreement with those determined from band structure calculations These findings reveal no evidence for prominent polaronic effects, which is at variance with the common beliefs of polaron formation in molecular solids

Surface waves on a quantum plasma half-space

Abstract: Surface modes are coupled electromagnetic/electrostatic excitations of free electrons near the vacuum-plasma interface and can be excited on a sufficiently dense plasma half-space. They propagate along the surface plane and decay in either sides of the boundary. In such dense plasma models, which are of interest in electronic signal transmission or in some astrophysical applications, the dynamics of the electrons is certainly affected by the quantum effects. Thus, the dispersion relation for the surface wave on a quantum electron plasma half-space is derived by employing the quantum hydrodynamical (QHD) and Maxwell–Poisson equations. The QHD include quantum forces involving the Fermi electron temperature and the quantum Bohm potential. It is found that, at room temperature, the quantum effects are mainly relevant for the electrostatic surface plasma waves in a dense gold metallic plasma.

Spin-orbit splitting in an anisotropic two-dimensional electron gas

Abstract: In the conventional Rashba model for an isotropic two-dimensional electron gas (2DEG), the electrons are spin-orbit split by a structural inversion asymmetry (SIA) perpendicular to the confinement plane. An additional SIA within the confinement plane leads to another contribution to the spin-orbit interaction which is investigated by means of a nearly-free electron model. The interplay of both contributions manifests itself as an enhanced splitting in the anisotropic 2DEG, as compared to the isotropic case. Further, the spin polarization of the electronic states is rotated out of the confinement plane. Both findings corroborate recent experimental and theoretical results for the ordered surface alloys $\mathrm{Bi}∕\mathrm{Ag}(111)$ and $\mathrm{Pb}∕\mathrm{Ag}(111)$.

Surface waves on a quantum plasma half-space

Abstract: Surface modes are coupled electromagnetic/electrostatic excitations of free electrons near the vacuum-plasma interface and can be excited on a sufficiently dense plasma half-space. They propagate along the surface plane and decay in either sides of the boundary. In such dense plasma models, which are of interest in electronic signal transmission or in some astrophysical applications, the dynamics of the electrons is certainly affected by the quantum effects. Thus, the dispersion relation for the surface wave on a quantum electron plasma half-space is derived by employing the quantum hydrodynamical (QHD) and Maxwell-Poison equations. The QHD include quantum forces involving the Fermi electron temperature and the quantum Bohm potential. It is found that, at room temperature, the quantum effects are mainly relevant for the electrostatic surface plasma waves in a dense gold metallic plasma.

Femtosecond field ion emission by surface optical rectification.

Abstract: We show that a model based on the surface optical rectification effect associated with the nonlinear response of free electrons may explain quantitatively, without adjustable parameters, all the observed features of the ultrafast laser-assisted field-ion emission from metal tips. Moreover, the same model provides also a plausible explanation for the low-fluence ultrafast laser ablation recently observed in metal surfaces and nanoparticles. We further test our model with experiments of ultrafast laser-assisted field-ion emission from tungsten tips in the tomographic atom probe.

Weak Localization and Electron–Electron Interaction Effects in Indium Zinc Oxide Films

Abstract: Electron weak localization (WL), electron–electron (el–el) interaction and electron–phonon (el–ph) scattering effects on the transport properties have been investigated for two to three-dimensional...

Evaluating Surface Impedance Models for Terahertz Frequencies at Room Temperature

Abstract: Commercial electromagnetic modeling software employs overly-simplifled models for the terahertz simulation of metal structures. For the flrst time, this paper gives a unique review of various modeling strategies (classical, semiclassical and quantum mechanical based) for normal metals and discusses their limitations with frequency at room temperature. High frequency CAD software packages employ overly-simplifled models for the electromagnetic simulation of metal structures; using either classical skin-efiect or classical relaxation-efiect models. At room temperatures, these models are accurate beyond the upper edges of the microwave and sub-millimeter-wave parts of the frequency spectrum, respectively. However, semiclassical models are needed to extend modeling well into the terahertz region or at signiflcantly lower temperatures. Here, issues relating to the specular or difiuse nature of electron re∞ections at the air-metal interface become apparent at very low temperatures. Within the near-infrared, visible and ultra-violet parts of the frequency spectrum, Commer- cial CAD software packages again may employ overly-simplifled empirically-fltted relaxation-efiect models, which only work over relatively narrow spectral bandwidths. However, to be accurate, an analytical model must be adopted that employs a quantum mechanical treatment, as this takes into account both energy dispersion and electron wavefunctions. The author has investigated modeling strategies for normal metals. In one study, experimental measurements that suggested the possibility of anomalous room-temperature conduction losses were examined between DC and 12.5THz (1). It was found that the classical relaxation-efiect model was still valid up to these frequencies. In another study, an elaborate semiclassical model to describe anomalous excess conduction losses at room temperature was found to be completely erroneous (2). In order to create accurate analytical models, it is important to develop semiclassical modeling strategies (3) or develop quantum mechanical treatments. To this end, and for the flrst time, this paper will review various approaches to the modeling of normal metals at room temperature. More importantly, their limitations will be discussed in detail. It will be shown that a number of well-know approaches have severe limitations to general applications. 2. CLASSICAL TREATMENT Drude's model of intraband transitions describes an ideal system of free electrons having a spherical Fermi surface. The classical relaxation-efiect model takes into account electron-phonon collisions, represented by the following expression for surface impedance, ZSR, in terms of Drude's model for intrinsic bulk conductivity, aeR:

High Strength Stainless Austenitic CrMnN steels - Part III: Electronic Properties

Abstract: Some results of ab initio calculations and experimental studies of the C, N and C+N effect on the electron structure of austenitic CrMn steels are presented. It is shown that the total electron energy per atom decreases due to alloying in the sequence of carbon→nitrogen→carbon+nitrogen, which suggests a corresponding increase in the thermodynamic stability of the austenite. Alloying with nitrogen and nitrogen+carbon increases the density of electron states at the Fermi level in comparison with interstitial-free and carbon-alloyed steel. Measurements of conduction electron spin resonance (CESR) allow separating the contributions from free electrons which are responsible for the metallic character of interatomic bonds and from localized electrons involved in the covalent bonds. It is shown that, in contrast to carbon, nitrogen enhances the metallic character of atomic interactions with a maximum of the concentration of free electrons at some critical content of nitrogen (about 2 at.%). The combined alloying with carbon+nitrogen leads to two effects: (i) a larger concentration of free electrons and (ii) a shift of the critical content of interstitials towards higher values. The obtained results of the theoretical and experimental studies of the electron structure were used as a physical background for the development of super-high‐strength stainless austenitic steels.

Temperature-driven transition from the Wigner crystal to the bond-charge-density wave in the quasi-one-dimensional quarter-filled band

Abstract: It is known that within the interacting electron model Hamiltonian for the one-dimensional $\frac{1}{4}$-filled band, the singlet ground state is a Wigner crystal only if the nearest-neighbor electron-electron repulsion is larger than a critical value. We show that this critical nearest-neighbor Coulomb interaction is different for each spin subspace, with the critical value decreasing with increasing spin. As a consequence, with the lowering of temperature, there can occur a transition from a Wigner crystal charge-ordered state to a spin-Peierls state that is a bond-charge-density wave with charge occupancies different from the Wigner crystal. This transition is possible because spin excitations from the spin-Peierls state in the $\frac{1}{4}$-filled band are necessarily accompanied by changes in site charge densities. We apply our theory to the $\frac{1}{4}$-filled band quasi-one-dimensional organic charge-transfer solids, in general, and to 2:1 tetramethyltetrathiafulvalene (TMTTF) and tetramethyltetraselenafulvalene cationic salts, in particular. We believe that many recent experiments strongly indicate the Wigner crystal to bond-charge-density Wave transition in several members of the TMTTF family. We explain the occurrence of two different antiferromagnetic phases but a single spin-Peierls state in the generic phase diagram for the 2:1 cationic solids. The antiferromagnetic phases can have either the Wigner crystal or the bond-charge-spin-density wave charge occupancies. The spin-Peierls state is always a bond-charge-density wave.

Electromagnetic global gyrokinetic simulation of shear Alfven wave dynamics in tokamak plasmas

Abstract: Electromagnetic gyrokinetic simulation in toroidal geometry is developed based on a fluid-kinetic hybrid electron model. The Alfven wave propagation in a fully global gyrokinetic particle simulation is investigated. In the long-wavelength magnetohydrodynamic limit, shear Alfven wave oscillations, continuum damping, and the appearance of the frequency gap in toroidal geometries are demonstrated. Wave propagation across the magnetic field (kinetic Alfven wave) is examined by comparing the simulation results with the theoretical dispersion relation. Furthermore, finite-beta stabilization of the ion temperature gradient mode and the onset of the kinetic ballooning mode are demonstrated.

Energy dissipation via electron energization in standing shear Alfven waves

Abstract: A two-dimensional hybrid magnetohydrodynamic-kinetic electron model in dipolar coordinates is used to study the case of a fundamental mode toroidal field line resonance (FLR) centered on an L=10 closed dipolar magnetic field line. The model is initialized via a perturbation of the azimuthal shear Alfven velocity so that only upward field aligned currents (corresponding to downwelling electrons) are present at the ionospheric boundaries during the first half wave period. It is found that the acceleration of the electrons to carry the field aligned currents can be a significant sink of Alfven wave energy depending on the width of the flux tube. For a FLR with an equatorial perpendicular wavelength of 0.25 RE about 20% of the wave energy is dissipated over a half cycle. This varies inversely with the width of the flux tube increasing to 40% by a width of 0.15 RE, which, unless the system is driven, can completely damp the resonance in about 2–3cycles.

Spin-dependent electron dynamics and recombination in GaAs 1−x N x alloys at room temperature

Abstract: Conduction-electron spin polarization dynamics achieved by pulsed optical pumping at room temperature in GaAs1−x N x alloys with a small nitrogen content (x = 21, 27, and 34%) is studied both experimentally and theoretically It is found that the photoluminescence circular polarization determined by the mean spin of free electrons reaches 40–45% and this giant value persists within 2 ns Simultaneously, the total free-electron spin decays rapidly with the characteristic time ≈ 150 ps The results are explained by spin-dependent capture of free conduction electrons on deep paramagnetic centers resulting in the dynamical polarization of bound electrons A nonlinear theory of spin dynamics in the coupled system of spin-polarized free and localized carriers has been developed which describes the experimental dependencies, in particular, the electron spin quantum beats observed in a transverse magnetic field

Nanostructuring Borosilicate Glass With Near-Field Enhanced Energy Using a Femtosecond Laser Pulse

Abstract: A model based on the evolution of electron density derived from the Fokker-Planck equation has been built to describe ablation of dielectrics during femtosecond laser pulses. The model is verified against an experimental investigation of borosilicate glass with a 200 fs laser pulse centered at 780 nm wavelength in a range of laser energies. The ablation mechanisms in dielectrics include multi-photon ionization (MPI) and avalanche ionization. MPI dominates the ionization process during the first stages of the laser pulse, contributing seed electrons which supply avalanche ionization. The avalanche process initiates and becomes responsible for the majority of free-electron generation. The overall material removal is shown to be highly dependent upon the optical response of the dielectric as plasma is formed. The ablation model is employed to predict the response of borosilicate glass to an enhanced electromagnetic field due to the presence of microspheres on the substrate surface. It is shown that the diffraction limit can be broken, creating nanoscale surface modification. An experimental study accompanies the model, with AFM and SEM characterizations that are consistent with the predicted surface modifications. DOI: 10.1115/1.2360595

Many-electron model for multiple ionization in atomic collisions

Abstract: We have developed a many-electron model for multiple ionization of heavy atoms bombarded by bare ions. It is based on the transport equation for an ion in an inhomogeneous electronic density. Ionization probabilities are obtained by employing the shell-to-shell local plasma approximation with the Levine and Louie dielectric function to take into account the binding energy of each shell. Post-collisional contributions due to Auger-like processes are taken into account by employing recent photoemission data. Results for single-to-quadruple ionization of Ne, Ar, Kr and Xe by protons are presented showing a very good agreement with experimental data.

Zitterbewegung of nearly-free and tightly-bound electrons in semiconductors

Abstract: We show theoretically that non-relativistic nearly-free electrons in solids should experience a trembling motion (Zitterbewegung, ZB) in the absence of external fields, similarly to relativistic electrons in a vacuum The ZB is directly related to the influence of the periodic potential on the free electron motion The frequency of the ZB is , where Eg is the energy gap The amplitude of the ZB is determined by the strength of periodic potential and the lattice period, and it can be of the order of nanometres We show that the amplitude of the ZB does not depend much on the width of the wavepacket representing an electron in real space An analogue of the Foldy–Wouthuysen transformation, known from relativistic quantum mechanics, is introduced in order to decouple electron states in various bands We demonstrate that after the bands are decoupled electrons should be treated as particles of a finite size In contrast to nearly-free electrons we consider a two-band model of tightly-bound electrons We show that in this case also the electrons should experience the trembling motion It is concluded that the phenomenon of ZB of electrons in crystalline solids is the rule rather than the exception

Sound-Wave-Like Collective Electronic Excitations in Au Atom Chains

Abstract: Sound-wave-like collective excitation in Au atom chains on the Si(557) surface is investigated. Electron scattering spectroscopy using a highly collimated slow electron beam has detected a characteristic low-energy one-dimensional (1D) plasmon (wire plasmon) that is confined in the atom chains. Theoretical analysis adopting a quantum-mechanical scheme beyond the free-electron model indicates a significant dynamic exchange-correlation effect due to strong 1D confinement. By cooling to below 100 K, we have detected for the first time a significant change in the plasmon dispersion in a tiny momentum and energy region, which definitely reflects a gap opening due to a metal-to-insulator transition of the atom chains.

Free‐Electron Generation in Laser‐Irradiated Dielectrics

Abstract: When transparent solids are irradiated with laser or particle beams an electron-hole plasma may be generated, which consists of free electrons in the conduction band of the solid and holes in the valence band. If the electron density reaches values close to the critical plasma density (at which the plasma frequency of the free electron gas equals the laser frequency), the dielectric becomes highly absorbing. The large energy transfer to the solid leads to dielectric breakdown and further to phase transitions and ablation. We compare different models describing the temporal evolution of the free-electron density under laser irradiation. The role of different ionization processes is investigated in dependence on laser pulse duration and intensity. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamic Spin-Polarized Resonant Tunneling in Magnetic Tunnel Junctions

Abstract: Precisely engineered tunnel junctions exhibit a long sought effect that occurs when the energy of the electron is comparable to the potential energy of the tunneling barrier. The resistance of metal-insulator-metal tunnel junctions oscillates with an applied voltage when electrons that tunnel directly into the barrier's conduction band interfere upon reflection at the classical turning points: the insulator-metal interface and the dynamic point where the incident electron energy equals the potential barrier inside the insulator. A model of tunneling between free electron bands using the exact solution of the Schrodinger equation for a trapezoidal tunnel barrier qualitatively agrees with experiment.

Energy of the quasi-free electron in argon, krypton and xenon

Abstract: Recent field ionization measurements of various high-n molecular Rydberg states doped into argon, krypton and xenon perturbers are presented as a function of perturber number density up to the density of the triple point liquid. These data are modeled to within ± 0.3 % of experiment on both critical and noncritical isotherms using a new theoretical treatment that includes: (i) the polarization of the perturber by the dopant cation, (ii) the polarization of the perturber by the quasi-free electron that arises from field ionization of the dopant, and (iii) the kinetic energy of the quasi-free electron. The polarization terms are determined by a standard statistical mechanical treatment. However, the kinetic energy of the quasi-free electron is calculated within a new local Wigner–Seitz model that contains only one adjustable parameter. This treatment provides an accurate model of the energy of the bottom of the conduction band ( V 0 ) in argon, krypton and xenon from the dilute gas up to the density of the triple point liquid, on both critical and noncritical isotherms.

Quasifree electron transport under electric field in nonpolar simple-structured condensed matters

Abstract: Drift behaviours of quasifree electrons in nonpolar condensed matters (dense gases, liquids and solids) under uniform electric fields are reported in this review. Measurements have shown that, in some sorts of nonpolar fluid, zero-field electron mobility depends strongly on the number density of atom or molecule comprising the matters from dilute gas to solid phase. The dependence was explained by the fact that in the condensed phase new energy levels of the valence electrons are formed through a regular spatial arrangement of long-range order, the crystal lattice, due to the Pauli exclusion principle, and that excess electrons move in the conduction band. The electron drift motion under the fields in the conduction band is also described.

Enhancement of the electron spin memory by localization on donors in a CdTe quantum well

Abstract: We present easily reproducible experimental conditions giving rise to a long electron spin memory at low temperature. The proposed system consists of an electron localized by a donor potential, and immerged in a quantum well. We have measured, by using photoinduced Faraday rotation technique, the spin relaxation time of electrons localized on iodine donors placed at the middle of a 80 A CdTe quantum well, and we have obtained 20 ns; this spin relaxation time is two orders of magnitude longer than for free electrons in a similar CdTe quantum well [J. Tribollet , Phys. Rev. B 68, 235316 (2003)].

Modeling of electron dynamics in laser-irradiated solids: progress achieved through a continuum approach and future prospects

Abstract: A continuum model based on a drift-diffusion approach is applied to describe the dynamics of electronic excitation, heating and charge-carrier transport in metals and dielectrics under near-infrared femtosecond laser irradiation. The dependence of laser-induced charging of the targets on laser fluence and pulse duration is investigated. Various aspects concerning the mechanism of Coulomb Explosion (CE) are discussed. The CE threshold as a function of pulse duration is evaluated numerically for dielectric materials (sapphire and ULE glass). A special attention is paid to studies of interconnection between the electron emission yield and surface charging dynamics. It has been found that in dielectrics the photoemission yield saturates with increasing laser fluence as a result of self-regulation of the free-electron population. By contrast in metals, due to effective supply of electrons to the charging zone on the target surface, electron emission becomes unwarrantably high for short laser pulses and high fluences. However, photo- and thermionic emissions can be suppressed by the generated electric field whose amplitude is a function of pulse duration and laser fluence. The question on self-consistency of electron emission and surface charging is analyzed with outlining further studies.