Showing papers on "Mean free path published in 2009"

Two-dimensional normal-state quantum oscillations in a superconducting heterostructure

Abstract: Heavily doped semiconductors can exhibit superconductivity, but their performance is severely limited by extremely large electronic disorder. Similarly, the electron mean free path in low-dimensional superconducting thin films is usually limited by interface scattering or atomic-scale disorder. Kozuka et al. use niobium doping to fabricate a high-quality, two-dimensional superconducting layer within a thin-film heterostructure based on the first known superconducting semiconductor, SrTiO3. This should provide a model system in which to explore the quantum transport and interplay of both superconducting and normal electrons.

Hard x-ray photoelectron spectroscopy of buried Heusler compounds

Abstract: This work reports on high energy photoelectron spectroscopy from the valence band of buried Heusler thin films (Co2MnSi and Co2FeAl0.5Si0.5) excited by photons of about 6?keV energy. The measurements were performed on thin films covered by MgO and SiOx with different thicknesses from 1 to 20?nm of the insulating layer and additional AlOx or Ru protective layers. It is shown that the insulating layer does not affect the high energy spectra of the Heusler compound close to the Fermi energy. The high resolution measurements of the valence band close to the Fermi energy indicate a very large electron mean free path of the electrons through the insulating layer. The spectra of the buried thin films agree well with previous measurements from bulk samples. The valence band spectra of the two different Heusler compounds exhibit clear differences in the low lying s bands as well as close to the Fermi energy.

Thermal conductivity of silicon nanowire by nonequilibrium molecular dynamics simulations

Abstract: The thermal conductivity of silicon nanowires was predicted using the nonequilibrium molecular dynamics method using the Stillinger–Weber potential model and the Nose–Hoover thermostat. The dependence of the thermal conductivity on the wire length, cross-sectional area, and temperature was investigated. The surface along the longitudinal direction was set as a free boundary with potential boundaries in the other directions. The cross-sectional areas of the nanowires ranged from about 5 to 19 nm2 with lengths ranging from 6 to 54 nm. The thermal conductivity dependence on temperature agrees well with the experimental results. The reciprocal of the thermal conductivity was found to be linearly related to the nanowire length. These results quantitatively show that decreasing the cross-sectional area reduces the phonon mean free path in nanowires.

Far-infrared response of free charge carriers localized in semiconductor nanoparticles

Abstract: A Monte Carlo method is employed to calculate the dynamical conductivity in the terahertz range of free charge carriers localized in semiconductor nanoparticles. The shape of the conductivity spectrum is essentially determined by the probability of carrier transition through interparticle boundaries and by the ratio of the nanoparticle size and carrier mean free path in the bulk. It is shown that the conductivity spectrum exhibits similar features as the classical extension of the Drude conductivity of electrons proposed by Smith [Phys. Rev. B 64, 155106 (2001)]. We find and discuss the link of this model to the results of our simulations which suggests an interpretation of the phenomenological parameters of the Drude-Smith model.

Practical expressions for the mean escape depth, the information depth, and the effective attenuation length in Auger-electron spectroscopy and x-ray photoelectron spectroscopy

Abstract: The authors report new calculations of mean escape depths (MEDs), information depths (IDs), and effective attenuation lengths (EALs) for 16 photoelectron lines and 9 Auger-electron lines of five elemental solids (Si, Cu, Ag, W, and Au) and four inorganic compounds (ZrO2, ZrSiO4, HfO2, and HfSiO4). These calculations were made to update similar previous calculations with improved data for the transport mean free path (TMFP) that are now available. Ratios of averages of the new MEDs, IDs, and EALs to the inelastic mean free path for electron emission angles between 0° and 50° varied linearly with the single-scattering albedo, a simple function of the inelastic mean free path and TMFP. The slopes of the linear relations depend only weakly on the atomic potential used in calculations of differential elastic-scattering cross sections (from which TMFPs are derived). The new linear relations are simple practical expressions for determining the MED, ID, and EAL for any solid in conventional Auger electron spectro...

Multiband Mobility in Semiconducting Carbon Nanotubes

Abstract: We present new data and a compact mobility model for semiconducting single-wall carbon nanotubes, with only two adjustable parameters, the elastic and inelastic collision mean free paths at 300 K. The mobility increases with diameter, decreases with temperature, and has a more complex dependence on charge density. The model and data suggest that the room temperature mobility does not exceed 10 000 cm2/Vmiddots at high carrier density (n > 0.5 nm-1) for typical single-wall nanotube diameters, due to the strong scattering effect of the second subband.

Analytical description of nonlinear cosmic ray scattering: isotropic and quasilinear regimes of pitch-angle diffusion

Abstract: Aims. We investigate pitch-angle scattering, which is a fundamental process in the physics of cosmic rays. Methods. By employing the second-order quasilinear theory, the pitch-angle Fokker-Planck coefficient is calculated analytically for the first time. Results. We demonstrate that for sufficiently strong turbulence the pitch-angle Fokker-Planck coefficient is isotropic. The derived results can be used to compute the parallel mean free path for all forms of the turbulence spectrum. We also consider applications, namely the transport of solar energetic particles and the propagation of cosmic rays in the Galaxy. Conclusions. The previously used assumption of isotropic pitch-angle diffusion is indeed correct for sufficiently strong turbulence. An analytical description of nonlinear particle scattering is possible.

Mean free path limitation of thermoelectric properties of bismuth nanowire

Abstract: A limiting mean free path was considered in order to better understand the temperature and wire diameter dependence of the resistivity and Seebeck coefficient of bismuth microwire and nanowire samples. The mean free path limited mobility was numerically calculated from experimentally measured mobility in a bulk bismuth sample, and the electron and hole mobilities were dramatically decreased to a 10 μm mean free path. Therefore, the temperature dependence of resistivity in very thin wire was quite different from that of a bulk sample, which had a positive temperature coefficient. The calculations showed that the temperature coefficient decreased gradually with decreasing mean free path, and the coefficient became negative for a mean free path of less than 1 μm at about 150 K. The Seebeck coefficient was also calculated, but showed only a weak dependence on mean free path compared with the resistivity. Experimental comparisons were made to previous measurements of bismuth microwire or nanowire samples, and ...

Generation and accretion of electrons in complex plasmas with cylindrical particles

Abstract: This paper presents an analytical model for the physical understanding of the charging of cylindrical dust particles in an open complex plasma system. Two different mechanisms, viz., thermionic emission and photoelectric emission have been considered for the electron generation from the charged cylindrical dust particles; the corresponding expressions for the rate of emission of electrons and their mean energy have been derived. A simple approach has been adopted to derive the expression for the rate of electron accretion to the dust particle. Further a new expression for the mean energy associated with the accreted electrons due to cylindrical dust particle has been derived and presented. An interesting comparison of results obtained in the case of spherical and cylindrical dust particles has also been made. Using these expressions, a formalism has been developed for the electronic processes in an illuminated dust cloud with cylindrical particles, on the basis of charge neutrality condition and number and energy balance of electrons; the charge carried by the cylindrical dust particles, electron temperature, and electron density corresponding to a given situation have been determined. The limitation of the applicability of the theory, viz., that the mean free path of an electron for accretion by dust particles be less than the dimension of the dust cloud has been pointed out.

Testing transport theories with solar energetic particles

Abstract: The detailed modeling of solar particle events offers the possibility of deriving coefficients describing the propagation of energetic particles in the inner heliosphere such as scattering mean free paths and thus to test the validity of different theories for the interaction of the particles with magnetic field fluctuations. In addition, information about the three-dimensional structure and the dynamical properties of the fluctuations can be obtained and compared with results from direct magnetic field observations. We apply different methods to numerically solve the focused transport equation for pitch angle diffusion coefficients calculated from standard and dynamical quasi-linear theory, and investigate the resulting pitch angle distributions for 100 keV electrons and for MeV protons. We find that pitch angle distributions predicted for electrons from a model comprising dynamical quasi-linear theory and the assumption that the fluctuations are composed of a 20% slab and an 80% two-dimensional component differ significantly from those predicted for protons. A comparison with particle observations from the solar event of 2000 February 18 reveals that these predictions are also in strong disagreement with the observed electron pitch angle distributions. Our findings indicate that the above model, inspite of its recent success in making quantitatively correct predictions for the particle's scattering mean free path parallel to the average magnetic field from observations of solar wind turbulence, is still not complete.

An experimental method for calibration of the plasmon mean free path.

Abstract: Transmission electron microscopy specimens in the form of elongated, conical needles were made using a dual-beam focused ion beam system, allowing the specimen thickness to be geometrically determined for a range of thickness values. From the same samples electron energy loss maps were acquired and the plasmon mean free path (lambda) for inelastic scattering was determined experimentally from the measured values of specimen thickness. To test the method lambda was determined for Ni (174 +/- 17 nm), alpha-Al(2)O(3) (143 +/- 14 nm), Si (199 +/- 20 nm) and amorphous SiO(2) (238 +/- 12 nm), and compared both to experimental values of lambda taken from the literature and to calculated values. The calculated values of lambda significantly underestimate the true sample thickness for high accelerating voltages (300 kV) and large collection angles. A linear dependence of lambda on thickness was confirmed for t/lambda 0.6). The experimental method proposed in this contribution offers a means to calibrate lambda for any type of material or phase that can be milled using a focused ion beam system.

Strong dispersive effects in the light-scattering mean free path in photonic gaps

Abstract: We have performed measurements of the scattering mean free path s in photonic crystals with different and controlled amounts of disorder. In the most perfect crystals, 1 order of magnitude chromatic variation in s for just 3% shift around the band gap 27 nm in wavelength is obtained. It is argued that the s dispersion is governed by both the total density of states and the group index in the incident direction, with this last quantity being responsible for the large dispersion of s. The ability to modify and enhance light scattering in disordered systems is much sought after. Artificially engineered materials allow the control of light transport and density of light states N through interference in the internal nanostructure rather than on the refraction in the body boundaries, engendering new materials properties. Photonic crystals in which the dielectric constant is periodically modulated control fundamental aspects of light-matter interaction such as light emission 1 and light transport, 2 much like semiconductors controlling electrons. Redistribution and inhibition of the emission from photonic crystals were proven, 3 but unconventional light transport in partially disordered photonic crystals has only been hinted at by pioneering experiments. 4 Other topologies, such as random media, 5,6 correlated disordered, 7 or fractal, 8 employ the aperiodic subwavelength

Contribution of Ballistic Electron Transport to Energy Transfer During Electron-Phonon Nonequilibrium in Thin Metal Films

Abstract: With the ever decreasing characteristic lengths of nanomaterials, nonequilibrium electron-phonon scattering can be affected by additional scattering processes at the interface of two materials. Electron-interface scattering would lead to another path of energy flow for the high-energy electrons other than electron-phonon coupling in a single material. Traditionally, electron-phonon coupling in transport is analyzed with a diffusion (Fourier) based model, such as the two temperature model (TTM). However, in thin films with thicknesses less than the electron mean free path, ballistic electron transport could lead to electron-interface scattering, which is not taken into account in the TTM. The ballistic component of electron transport, leading to electron-interface scattering during ultrashort pulsed laser heating, is studied here by a ballistic-diffusive approximation of the Boltzmann transport equation. The results for electron-phonon equilibration times are compared with calculations with TTM based approximations and experimental data on Au thin films.

When do particles follow field lines

Abstract: [1] We examine charged particle transport perpendicular to the large scale magnetic field. We find that the limit of an infinite parallel mean free path of particles diffusing along the large scale magnetic field is a necessary condition for which the diffusive spread of the magnetic field lines leads to a proportional spread of the particles. When it occurs this requires that parallel mean free path is well in excess of the smaller of the system size and the turbulence ultrascale. However, there are alternative situations in which particles may diffuse, but field lines do not. In the latter cases the asymptotic behavior is that which persists after the parallel mean free path exceeds some multiple of the correlation scales. This phenomenon of diffusing particles/non-diffusing field lines is typically determined by the 2D turbulence spectrum, where the diffusion coefficient of the magnetic field due to 2D turbulence can diverge if the spectrum of the 2D fluctuations is not well behaved at small wave numbers. We also show that the classical relation between parallel and perpendicular diffusion for high energy particles is consistent with the field line random walk description of particle diffusion.

Neutron irradiation of SmFeAsO1?xFx

Abstract: SmFeAsO1−x Fx was irradiated in a fission reactor by a fast (E > 0.1 MeV) neutron fluence of 4 × 10 21 m −2 . The introduced defects increased the normal state resistivity due to a reduction in the mean free path of the charge carriers. This leads to an enhancement of the upper critical field at low temperatures. The critical current density within the grains, Jc, increases upon irradiation. The second maximum in the field dependence of Jc disappears and the critical current density becomes a monotonically decreasing function of the applied magnetic field. (Some figures in this article are in colour only in the electronic version)

Flow conductance of a single nanohole.

Abstract: The mass flow conductance of single nanoholes with a diameter ranging from 75 to 100 nm was measured using mass spectrometry. For all nanoholes, a smooth crossover is observed between single-particle statistical flow (effusion) and the collective viscous flow emanating from the formation of a continuum. This crossover is shown to occur when the gas mean free path matches the size of the nanohole diameter. As a consequence of the pinhole geometry, the breakdown of the Poiseuille approximation is observed in the power-law temperature exponent of the measured conductance.

Simple parametrization of photon mass energy absorption coefficients of H-, C-, N- and O-based samples of biological interest in the energy range 200–1500 keV

Abstract: In this paper, we provide polynomial coefficients and a semi-empirical relation using which one can derive photon mass energy absorption coefficient of any H-, C-, N-, O-based sample of biological interest containing any other elements in the atomic number range 2–40 and energy range 200–1500 keV. More interestingly, it has been observed in the present work that in this energy range, both the mass attenuation coefficients and the mass energy absorption coefficients for such samples vary only with respect to energy. Hence it was possible to represent the photon interaction properties of such samples by a mean value of these coefficients. By an independent study of the variation of the mean mass attenuation coefficient as well as mass energy absorption coefficient with energy, two simple semi-empirical relations for the photon mass energy absorption coefficients and one relation for the mass attenuation coefficient have been obtained in the energy range 200–1500 keV. It is felt that these semi-empirical relations can be very handy and convenient in biomedical and other applications. One possible significant conclusion based on the results of the present work is that in the energy region 200–1500 keV, the photon interaction characteristics of any H-, C-, N-, O-based sample of biological interest which may or may not contain any other elements in the atomic number range 2–40 can be represented by a sample-independent (single) but energy-dependent mass attenuation coefficient and mass energy absorption coefficient.

Radiation hydrodynamic theory of double ablation fronts in direct-drive inertial confinement fusion

Abstract: The one-dimensional theory of double ablation fronts is developed for direct-drive inertial confinement fusion targets. The theory is based on the subsonic ablation front approximation and includes the effects of both radiation and electron heat fluxes. It is found that the structure of the ablation front is determined by two dimensionless parameters: the Boltzmann number and the effective mean free path. The Boltzmann number represents the ratio of the convective thermal and radiation energy fluxes, while the effective mean free path is the ratio between the characteristic plasma temperature gradient conduction scale length and the radiation mean free path. The development of a double ablation front is determined based on the range of the above dimensionless parameters. Temperature and density profiles in double ablation fronts are derived from a simplified analytic model and compared with the results of numerical simulations.

Resonant optical scattering in nanoparticle-doped polymer photonic crystals

Abstract: A broadband hyperspectral technique is used to measure the coherent optical backscatter across a wide spectral bandwidth, showing the resonant suppression of the photon transport mean free path around the photonic bandgap of a shear-assembled polymer photonic crystal. By doping with carbon nanoscale scatterers that reside at specific points within the photonic crystal lattice, the ratio between photon mean free path and optical penetration is tuned from 10 to 1, enhancing forward scatter at the expense of back-scatter. The back-scattering strength of different polarisations is not explained by any current theory.

On the reliability of the Bhatnagar–Gross–Krook collision model in weakly ionized plasmas

Abstract: The ion velocity distribution in weakly ionized plasma with mobility-limited ion flow driven by an electric field is studied numerically, with the assumption that the charge exchange is the dominant mechanism of ion scattering. The calculations are performed using the realistic constant mean free path model and the results are compared with those given by the constant mean free time (Bhatnagar–Gross–Krook) model. The comparison shows that in the regime where the flow velocity is less than or comparable with the thermal velocity of neutrals the two models yield quite similar velocity distributions. Nevertheless, even in this regime, there are quantitative differences that might be important for certain cases. The implications for theoretical investigations in dusty plasmas are discussed.

Reducing thermal conductivity of thermoelectric materials by using a narrow wire geometry

Abstract: The dependence of the thermal conductivity of narrow wires made from bismuth and covalently bonded materials on wire diameter was numerically calculated by considering contributions of mean free paths of carriers and phonons. The results suggest that a reduction in the thermal conductivity should be observable in a bismuth wire having a diameter of less than 1 μm sample. A reduction of nearly 20% in the temperature range of 150–300 K is expected due to the use of a narrow wire geometry. Such a geometry reduces the mobility and the thermal conductivity of the carriers, which is the dominant component, while the thermal conductivity due to phonons was dramatically reduced by using narrow wires at temperatures under 50 K due to the longer mean free path phonons. The thermal conductivity of materials with covalent bonding such as silicon was also estimated, and it is expected that the thermal conductivity of a silicon wire could be reduced due to the mean free path of phonons being longer than that of the car...

Experimental observation of second-harmonic generation and diffusion inside random media

Abstract: We have experimentally measured the distribution of the second-harmonic intensity that is generated inside a highly scattering slab of porous gallium phosphide. Two complementary techniques for determining the distribution are used. First, the spatial distribution of second-harmonic light intensity at the side of a cleaved slab has been recorded. Second, the total second-harmonic radiation at each side of the slab has been measured for several samples at various wavelengths. By combining these measurements with a diffusion model for second-harmonic generation that incorporates extrapolated boundary conditions, we present a consistent picture of the distribution of the second-harmonic intensity inside the slab. We find that the ratio l2ω/Lc of the mean free path at the second-harmonic frequency to the coherence length, which was suggested by some earlier calculations, cannot describe the second-harmonic yield in our samples. For describing the total second-harmonic yield, our experiments show that the scattering parameter at the fundamental frequency k1ωl1ω is the most relevant parameter in our type of samples.

Multipoint observations of ions in the 30–160 keV energy range upstream of the Earth's bow shock

Abstract: [1] We use multipoint observation data by Cluster during time periods when the interspacecraft separation distance was between 1 and 1.5 Earth radii in order to study the physical processes related to diffuse ions at <200 keV/e. For our analysis we use data from the Research with Adaptive Particle Imaging Detectors (RAPID) experiment onboard Cluster SC1 and SC3. We determine spatial ion density gradients by using proton intensities in the 27.7–159.7 keV energy range and helium intensities in the 137.8–235.1 keV energy range as a function of distance from the bow shock along the magnetic field. Our results show that the diffuse ions are subject to diffusive transport and the ion partial densities decrease exponentially with increasing distance from the bow shock. By complementing RAPID data with Cluster Ion Spectrometry measurements at lower energies (from 10 to 32 keV) from the same upstream ion event we find that the e-folding distance of energetic ion density increases almost linearly with energy. This effect is also seen in the hardening of the particle spectra with increasing distance from the bow shock. We determine the spatial diffusion mean free path and the diffusion coefficient as a function of ion energy by assuming that upstream diffusion is balanced by downstream convection.

Effective spin-flip scattering in diffusive superconducting proximity systems with magnetic disorder

Abstract: We revisit the problem of diffusive proximity systems involving superconductors and normal metals (or ferromagnets) with magnetic disorder. On the length scales much larger than its correlation length, the effect of sufficiently weak magnetic disorder may be incorporated as a local spin-flip term in the Usadel equations. We derive this spin-flip term in the general case of a three-dimensional disordered Zeeman-type field with an arbitrary correlation length. Three different regimes may be distinguished: pointlike impurities (the correlation length is shorter than the Fermi wavelength), medium-range disorder (the correlation length between the Fermi wavelength and the mean free path), and long-range disorder (the correlation length longer than the mean free path). We discuss the relations between these three regimes by using the three overlapping approaches: the Usadel equations, the nonlinear sigma model, and the diagrammatic expansion. The expressions for the spin-flip rate agree with the existing results obtained in less general situations.

Geometric diode, applications and method

Abstract: A geometric diode, method and device applications are described. The geometric diode is produced including a device body formed from an electrically conductive material having an equilibrium mobile charge density, and having a device surface configuration. The material has a charge carrier mean free path with a mean free path length and the device body size is selected based on said the free path length to serve as an electrically conductive path between first and second electrodes delimited by the device surface configuration that is asymmetric with respect to a forward flow of current in a forward direction from the first electrode to the second electrode as compared to a reverse current flow in an reverse direction from the second electrode to the first electrode. A system includes an antenna for receiving electromagnetic radiation coupled with the geometric diode antenna to receive the electromagnetic radiation to produce an electrical response.

Nonlinear propagation, confinement, and anisotropy of ultrahigh-energy cosmic rays in the Galaxy

Abstract: We explore the problems of propagation, confinement, and anisotropy of ultrahigh-energy cosmic rays. Previous investigations of plasma-particle interactions are based on a first-order perturbation theory also known as quasilinear theory. It is the subject of this article to employ a nonlinear diffusion theory, namely, the second-order quasilinear theory. We compute the parallel mean free path for slab and for isotropic turbulence for a wave spectrum with small-wave-number cutoff. It is demonstrated that we obtain a finite parallel mean free path even if the Larmor radius of the charged particle is larger than the largest scale of turbulence. Therefore, charged cosmic rays of all energies can be scattered and confined to the Galaxy by the turbulent magnetic fields.

A Comparative Study of Two-Temperature and Boltzmann Transport Models for Electron-Phonon Nonequilibrium

Abstract: Ultrashort-pulsed laser irradiation on metals creates a thermal nonequilibrium between electrons and the phonons. Previous computational studies used the two-temperature model and its variants to model this nonequilibrium. However, when the laser pulse duration is smaller than the relaxation time of the energy carriers or when the carriers mean free path is larger than the material dimension, these macroscopic models fail to capture the physics accurately. In this article, the nonequilibrium between energy carriers caused by a laser pulse interaction with a metal film is modeled via a numerical solution of the Boltzmann transport model (BTM) for electrons and phonons. A comparative assessment of the two-temperature model and its variants is carried out relative to the BTM. The higher order Runge-Kutta discontinuous Galerkin (RKDG) method is used for numerical discretization of the models. In this study, the gold film thickness is varied between 2–2000 nm, and the laser pulse duration and fluence are varie...

Modeling Electron-Phonon Nonequilibrium in Gold Films Using Boltzmann Transport Model

Abstract: Ultrashort-pulsed laser irradiation on metals creates a thermal nonequilibrium between electrons and the phonons. Previous computational studies used the two-temperature model and its variants to model this nonequilibrium. However, when the laser pulse duration is smaller than the relaxation time of the energy carriers or when the carriers' mean free path is larger than the material dimension, these macroscopic models fail to capture the physics accurately. In this paper, the nonequilibrium between energy carriers is modeled via a numerical solution of the Boltzmann transport model (BTM) for electrons and phonons, which is applicable over a wide range of lengths and time scales. The BTM is solved using the discontinuous Galerkin finite element method for spatial discretization and the three-step Runge―Kutta temporal discretization. Temperature dependent electron-phonon coupling factor and electron heat capacity are used due to the strong electron-phonon nonequilibrium considered in this study. The results from the proposed model are compared with existing experimental studies on laser heating of macroscale materials. The model is then used to study laser heating of gold films, by varying parameters such as the film thickness, laser fluence, and pulse duration. It is found that the temporal evolution of electron and phonon temperatures in nanometer size gold fzlms is very different from the macroscale films. For a given laser fluence and pulse duration, the peak electron temperature increases with a decrease in the thickness of the gold film. Both film thickness and laser fluence significantly affect the melting time. For a fluence of 1000 J/m 2 , and a pulse duration of 75 fs, gold films of thickness smaller than 100 nm melt before reaching electron-phonon equilibrium. However, for the film thickness of 2000 nm, even with the highest laser fluence examined, the electrons and phonons reach equilibrium and the gold film does not melt.