Quantum mechanical analysis of the elastic propagation of electrons in the Au/Si system: application to Ballistic Electron Emission Microscopy

We present a Green's function approach based on a LCAO scheme to compute the elastic propagation of electrons injected from a STM tip into a metallic film. The obtained 2D current distribution in real and reciprocal space furnish a good representation of the elastic component of Ballistic Electron Emission Microscopy (BEEM) currents. Since this component accurately approximates the total current in the near threshold region, this procedure allows --in contrast to prior analyses-- to take into account effects of the metal band structure in the modeling of these experiments. The Au band structure, and in particular its gaps appearing in the [111] and [100] directions provides a good explanation for the previously irreconcilable results of nanometric resolution and similarity of BEEM spectra on both Au/Si(111) and Au/Si(100).Comment: 12 pages, 9 postscript figures, revte

Hot electron transport in Ballistic Electron Emission Spectroscopy: band structure effects and k-space currents

Using a Green's function approach, we investigate band structure effects in the BEEM current distribution in reciprocal space. In the elastic limit, this formalism provides a 'parameter free' solution to the BEEM problem. At low temperatures, and for thin metallic layers, the elastic approximation is enough to explain the experimental I(V) curves at low voltages. At higher voltages inelastic effects are approximately taken into account by introducing an effective RPA-electron lifetime, much in similarity with LEED theory. For thick films, however, additional damping mechanisms are required to obtain agreement with experiment.Comment: 4 pages, 3 postscript figures, revte

A theoretical analysis of Ballistic Electron Emission Microscopy: k-space distributions and spectroscopy

We analyze BEEM experiments. At low temperatures and low voltages, near the threshold value of the Schottky barrier, the BEEM current is dominated by the elastic component. Elastic scattering by the lattice results in the formation of focused beams and narrow lines in real space. To obtain the current injected in the semiconductor, we compute the current distribution in reciprocal space and, assuming energy and $k_{\parallel}$ conservation. Our results show an important focalization of the injected electron beam and explain the similarity between BEEM currents for Au/Si(111) and Au/Si(100).Comment: 17 pages, 5 figures (postscript), Latex, APS, http://www.icmm.csic.es/Pandres/pedro.htm. Appl. Surf. Sci. (in press

Surface and bulk band-structure effects on CoSi₂/Si(111) ballistic-electron emission experiments

A theoretical model of ballistic-electron-emission microscopy (BEEM) based on linear combination of atomic orbitals Hamiltonians and Keldysh Green’s functions is applied to analyze experimental data obtained for CoSi2/Si(111) contacts. Hot electrons injected from a scanning tunneling microscope tip into the silicide film form a highly focused beam, which even after propagation through films of moderate thickness is narrow enough to allow the observed atomic resolution of interfacial point defects. On (2×1) reconstructed domains a certain fraction of the initial current is injected into localized surface states, leading to the reported contrast in BEEM images, reflecting the topography at the surface. These results confirm that band-structure effects, both in the bulk and at the surface of the metallic overlayer, intricately influence the interface-related information contained in BEEM data. It is found that for a careful analysis of experimental results, a theoretical model going beyond the ballistic hypotesis is required

Far Ultraviolet Spectra of B Stars near the Ecliptic

Spectra of B stars in the wavelength range of 911-1100 A have been obtained with the EURD spectrograph onboard the Spanish satellite MINISAT-01 with ~5 A spectral resolution. IUE spectra of the same stars have been used to normalize Kurucz models to the distance, reddening and spectral type of the corresponding star. The comparison of 8 main-sequence stars studied in detail (alpha Vir, epsilon Tau, lambda Tau, tau Tau, alpha Leo, zeta Lib, theta Oph, and sigma Sgr) shows agreement with Kurucz models, but observed fluxes are 10-40% higher than the models in most cases. The difference in flux between observations and models is higher in the wavelength range between Lyman alpha and Lyman beta. We suggest that Kurucz models underestimate the FUV flux of main-sequence B stars between these two Lyman lines. Computation of flux distributions of line-blanketed model atmospheres including non-LTE effects suggests that this flux underestimate could be due to departures from LTE, although other causes cannot be ruled out. We found the common assumption of solar metallicity for young disk stars should be made with care, since small deviations can have a significant impact on FUV model fluxes. Two peculiar stars (rho Leo and epsilon Aqr), and two emission line stars (epsilon Cap and pi Aqr) were also studied. Of these, only epsilon Aqr has a flux in agreement with the models. The rest have strong variability in the IUE range and/or uncertain reddening, which makes the comparison with models difficult.Comment: 25 pages, 6 figures, to be published in The Astrophysical Journa

Design of a compact objective for SWIR applications

Lately the short-wave infrared (SWIR) has become very important due to the recent appearance on the market of the small detectors with a large focal plane array. Military applications for SWIR cameras include handheld and airborne systems with long range detection requirements, but where volume and weight restrictions must be considered. In this paper we present three different designs of telephoto objectives that have been designed according to three different methods. Firstly the conventional method where the starting point of the design is an existing design. Secondly we will face design starting from the design of an aplanatic system. And finally the simultaneous multiple surfaces (SMS) method, where the starting point is the input wavefronts that we choose. The designs are compared in terms of optical performance, volume, weight and manufacturability. Because the objectives have been designed for the SWIR waveband, the color correction has important implications in the choice of glass that will be discussed in detai

Ballistic Electron Emission Microscopy on CoSi2{}_2/Si(111) interfaces: band structure induced atomic-scale resolution and role of localized surface states

Applying a Keldysh Green`s function method it is shown that hot electrons injected from a STM-tip into a CoSi${}_2$/Si(111) system form a highly focused beam due to the silicide band structure. This explains the atomic resolution obtained in recent Ballistic Electron Emission Microscopy (BEEM) experiments. Localized surface states in the $(2 \times 1)$-reconstruction are found to be responsible for the also reported anticorrugation of the BEEM current. These results clearly demonstrate the importance of bulk and surface band structure effects for a detailed understanding of BEEM data.Comment: 5 pages, RevTex, 4 postscript figures, http://www.icmm.csic.es/Pandres/pedro.ht

Fate of nutrients during hydrothermal treatment of food waste

Hydrothermal carbonization was evaluated as a food waste valorization strategy to obtain hydrochar and recover nutrients. In the hydrothermal treatment, the temperature (170–230 °C), reaction time (5–60 min), and addition of HCl (0.1–0.5 M) during the reaction were analyzed. Compared to the feedstock, hydrochar showed an increase in fixed carbon (greater than 45%) and a decrease in ash content (<7%), along with a higher heating value (18.6–26.2 MJ/kg), which would allow for its application as a biofuel for industry according to ISO/TS 17225–8. The hydrochar obtained using plain carbonization showed 75% P and 40% N of the feedstock content, whereas the HCl-mediated treatment (0.5 M) solubilized most of the P, K, and N in the process water (98% P as PO4-P, 98% K, and the total N content as NH4-N (16%) and organic-N) operating at 170 °C for 60 min.The authors greatly appreciate funding from Spanish MICINN (Project PID2019-108445RB-I00) and Madrid Regional Government (Project S2018/EMT-4344). A. Sarrion wishes to thank the Spanish MICINN and ESF for a research grant (BES-2017-081515). The authors thank Silvia Rodríguez for her valuable hel

Electron energy relaxation times from ballistic-electron-emission spectroscopy

Using a Green’s-function approach that incorporates band-structure effects, and a complementary k-space Monte-Carlo analysis, we show how to get a theoretically consistent determination of the inelastic mean free path λee(E) due to electron-electron interaction from ballistic electron emission spectroscopy. Exploiting experimental data taken at T=77K on a thin-Au film (ee(E) predicted by the standard Fermi-liquid theory provides excellent agreement between theoretical and experimental I(V) spectra. In agreement with theories for real metals, an enhancement of λee(E) by a factor of two with respect to its electron-gas value is found

Manejo da cultura da soja em terras baixas em safras com El-Niño.

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