984 resultados para Auger Electron Spectroscopy
Resumo:
We introduce an analytical approximation scheme to diagonalize parabolically confined two-dimensional (2D) electron systems with both the Rashba and Dresselhaus spin-orbit interactions. The starting point of our perturbative expansion is a zeroth-order Hamiltonian for an electron confined in a quantum wire with an effective spin-orbit induced magnetic field along the wire, obtained by properly rotating the usual spin-orbit Hamiltonian. We find that the spin-orbit-related transverse coupling terms can be recast into two parts W and V, which couple crossing and noncrossing adjacent transverse modes, respectively. Interestingly, the zeroth-order Hamiltonian together with W can be solved exactly, as it maps onto the Jaynes-Cummings model of quantum optics. We treat the V coupling by performing a Schrieffer-Wolff transformation. This allows us to obtain an effective Hamiltonian to third order in the coupling strength k(R)l of V, which can be straightforwardly diagonalized via an additional unitary transformation. We also apply our approach to other types of effective parabolic confinement, e. g., 2D electrons in a perpendicular magnetic field. To demonstrate the usefulness of our approximate eigensolutions, we obtain analytical expressions for the nth Landau-level g(n) factors in the presence of both Rashba and Dresselhaus couplings. For small values of the bulk g factors, we find that spin-orbit effects cancel out entirely for particular values of the spin-orbit couplings. By solving simple transcendental equations we also obtain the band minima of a Rashba-coupled quantum wire as a function of an external magnetic field. These can be used to describe Shubnikov-de Haas oscillations. This procedure makes it easier to extract the strength of the spin-orbit interaction in these systems via proper fitting of the data.
Resumo:
The thermal dependence of the zero-bias conductance for the single electron transistor is the target of two independent renormalization-group approaches, both based on the spin-degenerate Anderson impurity model. The first approach, an analytical derivation, maps the Kondo-regime conductance onto the universal conductance function for the particle-hole symmetric model. Linear, the mapping is parametrized by the Kondo temperature and the charge in the Kondo cloud. The second approach, a numerical renormalization-group computation of the conductance as a function the temperature and applied gate voltages offers a comprehensive view of zero-bias charge transport through the device. The first approach is exact in the Kondo regime; the second, essentially exact throughout the parametric space of the model. For illustrative purposes, conductance curves resulting from the two approaches are compared.
Resumo:
A recently developed dual-beam configuration that optimizes the thermal lens technique has been used to obtain the absorption spectrum of pure water from 350 to 528 nm. Our results indicate the minimum linear absorption coefficient smaller than 2 X 10(-5) cm(-1) between 360 and 400 nm. This value is lower than previous literature data, and it is blueshifted. Absorption coefficients as small as 2 X 10(-7) cm(-1) can be measured for water using 1 W of excitation power. A detection limit of similar to 6 X 10(-9) cm(-1) (P=1 W) for CCl(4) was estimated, which represents, to the best of our knowledge, the highest sensitivity obtained in small absorption measurements in liquids. (C) 2009 Optical Society of America
Resumo:
Data collected at the Pierre Auger Observatory are used to establish an upper limit on the diffuse flux of tau neutrinos in the cosmic radiation. Earth-skimming nu(tau) may interact in the Earth's crust and produce a tau lepton by means of charged-current interactions. The tau lepton may emerge from the Earth and decay in the atmosphere to produce a nearly horizontal shower with a typical signature, a persistent electromagnetic component even at very large atmospheric depths. The search procedure to select events induced by tau decays against the background of normal showers induced by cosmic rays is described. The method used to compute the exposure for a detector continuously growing with time is detailed. Systematic uncertainties in the exposure from the detector, the analysis, and the involved physics are discussed. No tau neutrino candidates have been found. For neutrinos in the energy range 2x10(17) eV < E(nu)< 2x10(19) eV, assuming a diffuse spectrum of the form E(nu)(-2), data collected between 1 January 2004 and 30 April 2008 yield a 90% confidence-level upper limit of E(nu)(2)dN(nu tau)/dE(nu)< 9x10(-8) GeV cm(-2) s(-1) sr(-1).
Resumo:
We derive a closed analytical expression for the exchange energy of the three-dimensional interacting electron gas in strong magnetic fields, which goes beyond the quantum limit (L=0) by explicitly including the effect of the second, L=1, Landau level and arbitrary spin polarization. The inclusion of the L=1 level brings the fields to which the formula applies closer to the laboratory range, as compared to previous expressions, valid only for L=0 and complete spin polarization. We identify and explain two distinct regimes separated by a critical density n(c). Below n(c), the per particle exchange energy is lowered by the contribution of L=1, whereas above n(c) it is increased. As special cases of our general equation we recover various known more limited results for higher fields, and we identify and correct a few inconsistencies in some of these earlier expressions.
Resumo:
The valence and core levels of In(2)O(3) and Sn-doped In(2)O(3) have been studied by hard x-ray photoemission spectroscopy (hv = 6000 eV) and by conventional Al K alpha (hv = 1486.6 eV) x-ray photoemission spectroscopy. The experimental spectra are compared with density-functional theory calculations. It is shown that structure deriving from electronic levels with significant In or Sn 5s character is selectively enhanced under 6000 eV excitation. This allows us to infer that conduction band states in Sn-doped samples and states at the bottom of the valence band both contain a pronounced In 5s contribution. The In 3d core line measured at hv = 1486.6 eV for both undoped and Sn-doped In(2)O(3) display an asymmetric lineshape, and may be fitted with two components associated with screened and unscreened final states. The In 3d core line spectra excited at hv = 6000 eV for the Sn-doped samples display pronounced shoulders and demand a fit with two components. The In 3d core line spectrum for the undoped sample can also be fitted with two components, although the relative intensity of the component associated with the screened final state is low, compared to excitation at 1486.6 eV. These results are consistent with a high concentration of carriers confined close to the surface of nominally undoped In(2)O(3). This conclusion is in accord with the fact that a conduction band feature observed for undoped In(2)O(3) in Al K alpha x-ray photoemission is much weaker than expected in hard x-ray photoemission.
Resumo:
Measured and calculated differential cross sections for elastic (rotationally unresolved) electron scattering from two primary alcohols, methanol (CH(3)OH) and ethanol (C(2)H(5)OH), are reported. The measurements are obtained using the relative flow method with helium as the standard gas and a thin aperture as the collimating target gas source. The relative flow method is applied without the restriction imposed by the relative flow pressure conditions on helium and the unknown gas. The experimental data were taken at incident electron energies of 1, 2, 5, 10, 15, 20, 30, 50, and 100 eV and for scattering angles of 5 degrees-130 degrees. There are no previous reports of experimental electron scattering differential cross sections for CH(3)OH and C(2)H(5)OH in the literature. The calculated differential cross sections are obtained using two different implementations of the Schwinger multichannel method, one that takes all electrons into account and is adapted for parallel computers, and another that uses pseudopotentials and considers only the valence electrons. Comparison between theory and experiment shows that theory is able to describe low-energy electron scattering from these polyatomic targets quite well.
Resumo:
The crystalline structure of transition-metals (TM) has been widely known for several decades, however, our knowledge on the atomic structure of TM clusters is still far from satisfactory, which compromises an atomistic understanding of the reactivity of TM clusters. For example, almost all density functional theory (DFT) calculations for TM clusters have been based on local (local density approximation-LDA) and semilocal (generalized gradient approximation-GGA) exchange-correlation functionals, however, it is well known that plain DFT fails to correct the self-interaction error, which affects the properties of several systems. To improve our basic understanding of the atomic and electronic properties of TM clusters, we report a DFT study within two nonlocal functionals, namely, the hybrid HSE (Heyd, Scuseria, and Ernzerhof) and GGA + U functionals, of the structural and electronic properties of the Co(13), Rh(13), and Hf(13) clusters. For Co(13) and Rh(13), we found that improved exchange-correlation functionals decrease the stability of open structures such as the hexagonal bilayer (HBL) and double simple-cubic (DSC) compared with the compact icosahedron (ICO) structure, however, DFT-GGA, DFT-GGA + U, and DFT-HSE yield very similar results for Hf(13). Thus, our results suggest that the DSC structure obtained by several plain DFT calculations for Rh(13) can be improved by the use of improved functionals. Using the sd hybridization analysis, we found that a strong hybridization favors compact structures, and hence, a correct description of the sd hybridization is crucial for the relative energy stability. For example, the sd hybridization decreases for HBL and DSC and increases for ICO in the case of Co(13) and Rh(13), while for Hf(13), the sd hybridization decreases for all configurations, and hence, it does not affect the relative stability among open and compact configurations.
Resumo:
In Bohmian mechanics, a version of quantum mechanics that ascribes world lines to electrons, we can meaningfully ask about an electron's instantaneous speed relative to a given inertial frame. Interestingly, according to the relativistic version of Bohmian mechanics using the Dirac equation, a massive particle's speed is less than or equal to the speed of light, but not necessarily less. That is, there are situations in which the particle actually reaches the speed of light-a very nonclassical behavior. That leads us to the question of whether such situations can be arranged experimentally. We prove a theorem, Theorem 5, implying that for generic initial wave functions the probability that the particle ever reaches the speed of light, even if at only one point in time, is zero. We conclude that the answer to the question is no. Since a trajectory reaches the speed of light whenever the quantum probability current (psi) over bar gamma(mu)psi is a lightlike 4-vector, our analysis concerns the current vector field of a generic wave function and may thus be of interest also independently of Bohmian mechanics. The fact that the current is never spacelike has been used to argue against the possibility of faster-than-light tunneling through a barrier, a somewhat similar question. Theorem 5, as well as a more general version provided by Theorem 6, are also interesting in their own right. They concern a certain property of a function psi : R(4) -> C(4) that is crucial to the question of reaching the speed of light, namely being transverse to a certain submanifold of C(4) along a given compact subset of space-time. While it follows from the known transversality theorem of differential topology that this property is generic among smooth functions psi : R(4) -> C(4), Theorem 5 asserts that it is also generic among smooth solutions of the Dirac equation. (C) 2010 American Institute of Physics. [doi:10.1063/1.3520529]
Resumo:
This paper presents the characterization of poly(aniline) (PANI) and poly(methyl methacrylate) (PMMA) coatings obtained by mixing PANI with PMMA aqueous dispersions (latex particles). These dispersions were characterized by using dynamic light scattering for sizing, zeta-potential analysis and thermal analysis. PMMA and PANI/PMMA dispersions show negative charged particles with zeta potential greater than |40| mV, a zeta-average diameter of 64 nm for pure PMMA and a bi-modal particle-size distribution centered at 45 and 120 nm for a mixture with 25% w/w of PANI. Films obtained by casting were characterized by using scanning electron microscopy and they show a conductivity increase upon PANI content reaching a value of 1 mS cm(-1) for a film with 25% w/w of PANI. In addition, Raman spectroscopy have shown the presence of the conducting form of PANI in the films and cyclic voltammetry experiments corroborated that they are electroactive in both acid and neutral solutions.
Resumo:
In this work we present a complete characterization and magnetic study of vanadium oxide/hexadecylamine nanotubes (VO(x)/Hexa NT's) doped with Co(2)+ and Ni(2+) ions. The morphology of the NT's has been characterized by transmission electron microscopy, while the metallic elements have been quantified by the instrumental neutron activation analysis technique. The static and dynamic magnetic properties were studied by collecting data of magnetization as a function of magnetic field and temperature and by electron paramagnetic resonance. At difference of the majority reports in the literature, we do not observe magnetic dimers in vanadium oxide nanotubes. Also, we observed that the incorporation of metallic ions (Co(2+), S = 3/2 and Ni(2+), S = 1) decreases notably the amount of V(4+) ions in the system, from 14-16% (nondoped case) to 2%-4%, with respect to the total vanadium atoms (fact corroborated by XPS experiments) anyway preserving the tubular nanostructure. The method to decrease the amount of V(4+) in the nanotubes improves considerably their potential technological applications as Li-ion batteries cathodes. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3580252]
Resumo:
We have systematically studied the magnetic properties of ferrite nanoparticles with 3, 7, and 11 nm of diameter with very narrow grain size distributions. Samples were prepared by the thermal decomposition of Fe (acac)(3) in the presence of surfactants giving nanoparticles covered by oleic acid. High resolution transmission electron microscopy (HRTEM) images and XRD diffraction patterns confirms that all samples are composed by crystalline nanoparticles with the spinel structure expected for the iron ferrite. ac and dc magnetization measurements, as well in-field Mossbauer spectroscopy, indicate that the magnetic properties of nanoparticles with 11 and 7 nm are close to those expected for a monodomain, presenting large M(S) (close to the magnetite bulk). Despite the crystalline structure observed in HRTEM images, the nanoparticles with 3 nm are composed by a magnetically ordered region (core) and a surface region that presents a different magnetic order and it contains about 66% of Fe atoms. The high saturation and irreversibility fields in the M(H) loops of the particles with 3 nm together with the misalignment at 120 kOe in the in-field Mossbauer spectrum of surface component indicate a high surface anisotropy for the surface atoms, which is not observed for the core. For T < 10 K, we observe an increase in the susceptibility and of the magnetization for former sample, indicating that surface moments tend to align with applied field increasing the magnetic core size. (C) 2010 American Institute of Physics. [doi:10.1063/1.3514585]
Resumo:
The behavior of Au nanorods and Ag nanocubes as analytical sensors was evaluated for three different classes of herbicides. The use of such anisotropic nanoparticles in surface-enhanced Raman scattering (SERS) experiments allows the one to obtain the spectrum of crystal violet dye in the single molecule regime, as well as the pesticides dichlorophenoxyacetic acid (2,4-D), trichlorfon and ametryn. Such metallic substrates show high SERS performance at low analyte concentrations making them adequate for use as analytical sensors. Density functional theory (DFT) calculations of the geometries and vibrational wavenumbers of the adsorbates in the presence of silver or gold atoms were used to elucidate the nature of adsorbate-nanostructure bonding in each case and support the enhancement patterns observed in each SERS spectrum.
Resumo:
We have investigated the stability, electronic properties, Rayleigh (elastic), and Raman (inelastic) depolarization ratios, infrared and Raman absorption vibrational spectra of fullerenols [C(60)(OH)(n)] with different degrees of hydroxylation by using all-electron density-functional-theory (DFT) methods. Stable arrangements of these molecules were found by means of full geometry optimizations using Becke's three-parameter exchange functional with the Lee, Yang, and Parr correlation functional. This DFT level has been combined with the 6-31G(d,p) Gaussian-type basis set, as a compromise between accuracy and capability to treat highly hydroxylated fullerenes, e.g., C(60)(OH)(36). Thus, the molecular properties of fullerenols were systematically analyzed for structures with n=1, 2, 3, 4, 8, 10, 16, 18, 24, 32, and 36. From the electronic structure analysis of these molecules, we have evidenced an important effect related to the weak chemical reactivity of a possible C(60)(OH)(24) isomer. To investigate Raman scattering and the vibrational spectra of the different fullerenols, frequency calculations are carried out within the harmonic approximation. In this case a systematic study is only performed for n=1-4, 8, 10, 16, 18, and 24. Our results give good agreements with the expected changes in the spectral absorptions due to the hydroxylation of fullerenes.
Resumo:
Early reports stated that Au was a catalyst of choice for the BOR because it would yield a near complete faradaic efficiency. However, it has recently been suggested that gold could yield to some extent the heterogeneous hydrolysis of BH(4)(-),therefore lowering the electron count per BH(4)(-), especially at low potential. Actually, the blur will exist regarding the BOR mechanism on Au as long as no physical proof regarding the reaction intermediates is not put forward. In that frame, in situ physical techniques like FTIR exhibit some interest to study the BOR. Consequently, in situ infrared reflectance spectroscopy measurements (SPAIRS technique) have been performed in 1 M NaOH/1 M NaBH(4) on a gold electrode with the aim to detect the intermediate species. We monitored several bands in B-H ((nu) over bar similar to 1180,1080 and 972 cm(-1)) and B-O bond regions ((nu) over bar =1325 and similar to 1425cm(-1)), which appear sequentially as a function of the electrode polarization. These absorption bands are assigned to BH(3), BH(2) and BO(2)(-) species. At the light of the experimental results, possible initial elementary steps of the BOR on gold electrode have been proposed and discussed according to the relevant literature data.